When should one use slashes or square brackets when transcribing in IPA?

Question: When should one use /fubar/ and when [fubar] when transcribing in IPA? What are the differences?
Comment: Related: [What's the difference between Phonetics and Phonology?](http://linguistics.stackexchange.com/questions/180/whats-the-difference-between-phonetics-and-phonology)
Comment: Note that when people are considering both the answer is pretty clear, but in certain contexts one form of bracketing will be used irregardless. I've definitely seen `[ ]` in dictionaries pretty often but general purpose dictionaries are always at the phonemic level. Also Wikipedia seems to use `[ ]` for all IPA currently from what I can see, whether phonetic, narrow, broad, or phonemic.
Answer: Square brackets ([fubar]) are generally used for what is known as narrow transcription - this includes as much detail as the transcriber feels is necessary. Slashes (/fubar/) represent the broad transcription, which does not include "predictable" information.
For example, in English, voiceless plosives are aspirated word-initially and in stressed onsets. Thus, a narrow transcription of "cool" might be [kʰul], while a broad transcription would be /kul/. Similarly, "lack" could be represented as [læk] and /læk/ - note that broad /k/ can become narrow [k] or [kʰ], depending on its position in the word and surrounding sounds. Because this information is predictable by the above rule, the aspiration is left out of the broad transcription.
In linguistic description, the "broad" and "narrow" designations are defined somewhat loosely, generally according to whatever convention the linguist reporting the language feels is most useful. Under the Generative Phonology framework, however, these are assumed to represent two distinct stages of phonological processing, each with a psychological reality. Square brackets denote the final stage of processing (which is sent to the articulators), called "phonetic transcription", while slashes denote the form stored in the mental lexicon (stripped of all predictable information), called "phonemic transcription".
To perform this kind of analysis, first you must determine which sounds are contrastive in the output. For example, in English, [pʰ] and [p] are not contrastive, because [pʰæt] and [pæt] "pat" are judged to be "the same" word by (most) native speakers (even though [pæt] would be ill-formed). However, in Hindi, [kapʰi] "coffee" and [kapi] "copy" are two separate words. The generative hypothesis is that each set of non-contrastive sounds is stored as a single unit in the brain, called a phoneme (in slashes), which is transformed into a final form (in brackets) passed to the articulators by a series of serially-ordered rules or simultaneous constraints on the possible output forms.
Comment: Your starting sentence seems to be a bit misleading; [brackets] are used for what is known as ***phonetic* transcription** which can either be broad or narrow.
Comment: Regarding your [pʰæt] and [pæt]. What you say seems to hold for when we speak normally, but when whispering, I think [pæt] is how the word *bat* is realised.
Comment: I think the broad/narrow and slashes/brackets distinction depends on the context and which framework you're operating from. Your definition is correct in many cases, but the asker didn't specify either context or framework, so I took the most neutral position at first, and delved into generativity halfway through.
Answer: Yes, /fubar/ is typically used for phonemic transcriptions, and [fubar] for phonetic transcriptions. But, just to clarify the terminology, phonemic vs. phonetic is not necessarily the same thing as broad vs. narrow transcription.
Many linguists talk about using both broad and narrow phonetic transcriptions, which just refers to the level of detail used in representing the actual speech sounds.
A narrow phonetic transcription would represent every tiny little characteristic of the speech sounds as they were produced in an utterance of Language X, while a broad phonetic transcription would indicate some of the most salient characteristics of the transcribed phones, without being exhaustive but also without necessarily making claims about which segments are phonemically/phonologically contrastive.
Phonemic transcription, on the other hand, can really only be broad, in that it only represents the sounds that are purported to be contrastive in the given language, without any detail that is not directly relevant to forming these contrasts. Phonemic transcription does not describe how an utterance actually sounds when produced by a particular speaker speaking in a particular style in a particular situation - phonemic transcription is the 'idealised' representation of the speech sounds, and supposed to represent the underlying contrasts that are meaningful to speakers.
You can only do phonemic transcription when you have already done quite a bit of work on Language X (collected a range of lexical items, found some minimal pairs and/or worked out the patterns of allophonic variation, etc etc) because phonemic transcription implies that you/someone has decided which phones relate to contrastive phonemes (different sounds that can occur in exactly the same environment) and which phones occur as the result of free/conditioned variation.
The examples given by @Alek Storm above illustrate the differences between [phonetic] and /phonemic/ transcription - just remember that phonetic can be broad and narrow.
Comment: @Cerberus I'm not a linguist, but it seems that "atoms" of phonemic and phonetic descriptions are different (even though IPA is often used for both cases). E.g., Toki Pona (a minimalistic conlang) doesn't discriminate [p] from [b], so the phonemic description of `pona` is only /pona/ (if we agree that both [p] and [b] map to /p/, not /b/ or some /B/) while phonetic descriptions may include [pona] and [bona] (more broad), ['pona] and [po(pause)na] (less broad), ['po(pause)na], etc. So: (1) `p` in /p/ and [p] are from different sets; (2) different levels of description may be regarded as broad.
Comment: +1 So can a certain variant of broad phonetic transcription be identical to the phonetic description of a word, such that all words are transcribed like /yxy/ and [yxy], where yxy is the same in both description for each word? Or does this never happen in practice, and will linguistics always choose a variant of broad phonetic description that is different from the phonemic description?
Answer: In addition to slashes and square brackets, sometimes also used are double-slashes, pipes, and angle brackets. Their uses are:

Angle brackets — ⟨cats⟩ or cats or "cats" or cats — orthography
Indicates a linguistic entity, like a word or grapheme, written according to a language's orthography. Alternatively, the orthography is often given in italics or quotes, or simply not indicated.
Square brackets — [ˈkʰæʔt͡s] — phonetic transcription
Indicates a transcription that records the phones (speech sounds) that are spoken, without attempting to classify them into phonemes. The amount of phonetic detail transcribed can vary; the example here is a fairly narrow (detailed) transcription. A broader transcription might be [ˈkʰæts].
Slashes — /ˈkæts/ — phonemic transcription
Indicates a transcription that records only information asserted to be contrastive. This records a sequence of phonemes (as well as any phonemic suprasegmental features like stress or tone), ignoring allophonic differences. In this example, the fact that the initial /k/ is pronounced aspirated ([kʰ]) is ignored, because /k/ and /kʰ/ do not contrast in English.
Double-slashes or pipes — //ˈkæt z// or |ˈkæt z| — morphophonemic transcription
Indicates a transcription that attempts to record the underlying sounds of morphemes, before they are combined to form words. This ignores sound changes conditioned on location, which may result in multiple phonemes, that get applied in actual speech. To illustrate, here, the final sound is written ⟨z⟩ (rather than ⟨s⟩ as in its pronunciation) because it represents the English morphophoneme //z//, used as the plural marker. //z// can be realized as /z/ (as in pigs or kangaroos), /s/ (as in cats), or /ɪz/ (as in horses). The posited morphophoneme cannot be an actual phoneme, because English distinguishes /z/ /s/ /ɪz/ in other contexts (like whores, horse, horiz(ontal)). (The choice of ⟨z⟩ for the transcription of this morphophoneme is arbitrary, but makes sense because it is realized as /z/ in the absence of the effect of an adjacent voiceless segment.)

More examples

Morphophonological processes are especially prominent in French. Example phrase: On a laissé la fenêtre ouverte. 'We left the window open.':

orthographic ⟨On a laissé la fenêtre ouverte.⟩
phonetic [ɔ̃.na.le.se.laf.nɛː.tχu.vɛχt]
phonemic /ɔ̃naleselafnɛːtʁuvɛʁt/
morphophonemic //ɔn a les e la fənɛːtʁ uvɛʁt ə//

French petit 'small': orthographic ⟨petit⟩, phonetic [pɵt̪ʲi], phonemic /pəti/, morphophonemic //pətit//
Standard Russian vowel reduction
In the standard (Moscow-based) Russian accent, five vowels /i e a o u/ are distinguished in stressed syllables, but at most three /ɪ~ɨ ɐ~ə ʊ/ in unstressed syllables. /i e/ reduce to /ɪ/, /a o/ to /ɐ/, and /u/ to /ʊ/.  Russian also has word-final obstruent devoicing and assimilation of voicing and palatalization. Russian orthography is roughly morphophonemic. Examples:

meaning: poppy, mage, poppies, mages, earth (nom. pl.), earth (gen. sg.), transport (infinitive), transports (present, 3sg)
orthographic ⟨ма́к⟩ ⟨ма́г⟩ ⟨ма́ки⟩ ⟨ма́ги⟩ ⟨зе́мли⟩ ⟨земли́⟩ ⟨вози́ть⟩ ⟨во́зит⟩
phonetic [ˈmak] [ˈmak] [ˈmakʲɪ] [ˈmaɡʲɪ] [ˈzʲe.mʲlʲɪ] [zʲɪˈmʲlʲi] [vɐˈzʲitʲ] [ˈvo.zʲɪt]
phonemic /ˈmak/ /ˈmak/ /ˈmakʲɪ/ /ˈmaɡʲɪ/ /ˈzʲe.mʲlʲɪ/ /zʲɪˈmʲlʲi/ /vɐˈzʲitʲ/ /ˈvo.zʲɪt/
morphophonemic //ˈmak// //ˈmaɡ// //ˈmak‿ʲi// //ˈmaɡ‿ʲi// //ˈzʲe.mlʲi// //zʲeˈmlʲi// //voˈzʲitʲ// //ˈvo.zʲit//
Comment: Excellent answer. Is it possible that silent ⟨gh⟩ in English is still morphophonemically significant? This would account for the different sound in _expedite → expeditious_ versus _right → righteous_.
Answer: [fubar] is used for phonetic transcriptions (emphasizing what is actually articulated) and /fubar/ for phonological transcriptions (emphasizing what is phonologically contrastive)1.
For example [pʰɛʔts] vs /pets/ (or even /petz/) with the implication that former is predictable from the latter using phonological rules.

NOTES:

By "phonologically contrastive", I mean that in English, there are no cases where alternating [pʰ] and [p] changes the meaning of an utterance. Which one is articulated is entirely predictable from the context and so many theories models this as a single /p/. /p/ and /b/ contrast because, for example, /pat/ and /bat/ are different words. [pʰ] and [p] do not contrast at this level.
Comment: @LouisRhys in English, there are no cases where alternating [pʰ] and [p] changes the meaning of an utterance. Which one is articulated is entirely predictable from the context and so many theories models this as a single /p/. /p/ and /b/ contrast because, for example, /pat/ and /bat/ are different words. [pʰ] and [p] do not contrast at this level.
Comment: what do you mean by something that is "phonologically contrastive"?
Answer: A complement to the answers above:
The alphabet used for transcribing and the level of transcription are frequently confused. IPA can, just as any other alphabet, be used for both phonetic and phonemic transcription (the difference has already been neatly explained), or anything in between. One might argue IPA is richer, poorer, more or less readable, flexible, regular, whatever, but in principle there is nothing in it, or any other alphabet, that forces you to use it at any particular level of abstraction. Alphabet and level of abstraction are two independent variables.
Answer: This answer from Prof. Kevin Russell (BSc Manitoba; MA, PhD USC) should help.

There is no such thing as the transcription of a word.
Strictly speaking, you can only transcribe a single utterance -- for example, how Kevin Russell pronounced the word cat at 12:58:03 pm on February 4, 2004. You can transcribe this utterance as exactly as possible, within the limits of your hearing and the conventions provided by the IPA.
If you want to go beyond that, to try to describe how Kevin Russell pronounces the word in general, or further still to how English speakers pronounce it in general, then you have to start making abstractions -- you have to decide which details to include and which to ignore.
It's common to distinguish between two kinds of transcription, based on how many details the transcribers decide to ignore:

Narrow transcription: captures as many aspects of a specific pronunciation as possible and ignores as few details as possible. Using the diacritics provided by the IPA, it's possible to make very subtle distinctions between sounds.
Broad transcription (or phonemic transcription): ignores as many details as possible, capturing only enough aspects of a pronunciation to show how that word differs from other words in the language.

The key factor in a broad transcription is meaning -- if a pronunciation detail can change the meaning of words in a language, it must be included in a broad transcription of that language.
For example, consider the difference between the vowels in [liv] and [lɪv].

For Canadian English, a narrow transcription would note the difference between the [i] and the [ɪ]. So would a broad transcription, since leave and live mean different things.
For Canadian French, a narrow transcription would note the difference between [i] and [ɪ]. But a broad transcription would not.  [liv] and [lɪv] do not mean different things in Canadian French -- they're both ways of saying 'book'. Both [i] and [ɪ] occur in the language, but they never contrast, that is, they never cause a difference in meaning. So a broad transcription would ignore the difference and write both as [liv].

With the symbols covered in this section, we are able to make broad transcriptions of Canadian English.
Some frequently asked questions about broad and narrow transcription.
Answer: Professional phonologists need a notation that distinguishes a theory or a perception of pronunciation, for which they often use slashes, from actual facts about the pronunciations, for which they often used square brackets.  In part, this is because a phonologist will usually be aware of several different theories about a given interesting pronunciation.  Without a systematic way of distinguishing fact from theory, a phonologist's world would be even more confusing than it is.
Answer: Typically, use slashes for broad [phonemic] transcriptions and brackets for narrow [phonetic] transcriptions.
I hope that I got that right.
Comment: can you explain what is a phonemic and a phonetic transcription?
Comment: Phonetics (where you would use a phonetic transcription) deals with the actual audio properties of spoken language. Phonology (where you would use a phonemic transcription) deals with how those sounds are perceived in a given language.
Answer: It is important to note that many dictionaries use the square brackets instead of slashes, but none of these is actually giving a close or broad phonetic transcription. A dictionary entry is (necessarily?) phonemic. Thus their use of square brackets is (for those who know the difference) incorrect, if not to say annoying.


If $f$ a holomorphic function is defined on a connnected open set $U$ and $f$ is locally zero near some $z\in U$

Question: Does this imply $f$ is zero on the whole of $U$? If $U$ is a disc and $f$ is zero near the center then from the Taylor series I see that $f$ is constant. If $U$ is any connected opens set, then since it is open, it must be path connected, so my intuition is that for any $w\in U$ I can draw a path from $z$ to $w$ and since the path is compact I should be able to cover it with finitely many discs and I just have to make sure that the centre of the $n$th disc lies in the $n-1$th disc and use induction to show that $f(w)=0$.
I think my claim is true by the above "intuition" but it would be to cumbersome to write down properly as a proof. Does anyone know how to prove this "properly" or disprove it?
Comment: Your claim is certainly true. This is called the "principle of analytic continuation." I think your proof is just fine, but another way is to note that the set of points $z\in U$ such that all the derivatives $f^{(n)}(z)$ for $n\geq 0$ vanish is both open and closed. If this set is nonempty, it must be all of $U$, and hence $f$ must be $\equiv 0$.
Answer: There is a well-known result which states that if a holomorphic map $f: \mathbb{C} \to \mathbb{C}$ is constant on a neighbourhood of a point then it is constant on all of $\mathbb{C}$.
To see this, assume that $f(z)=w$ for all $z \in U \subset \mathbb{C}$. The set $f^{-1}(w)$ of points $z \in \mathbb{C}$ for which $f(z)=w$ is closed because holomorphic functions are continuous and the set $\{w\} \subset \mathbb{C}$ is closed.
Let $V$ denote the interior of $U$, and assume that there exists a point $p$ on the boundary of $V$. It follows that $f$ is not constant in a neighbourhood of $p$. However, we can easily construct a sequence of points $(p_k)$, with each $p_i \in U$ such that $p_k \to p$ as $k \to \infty$. Since $p_k \in U$ for all $k$ we must have $f(p_k) = w$ for all $k$. This is a contradiction, and so the boundary of $V$ must be empty.
Since $U$ is closed without boundary it must be both open and closed and hence $U = \mathbb{C}$.


Is philosophy constrained by a philosopher's language?

Question: Since a philosopher exercises his philosophy speaking and writing, is philosophy constrained by the limits of the language?
Comment: do you mean like Whorfianism (http://www.dictionary.com/browse/sapir-whorf-hypothesis ) or something else?
Answer: No, a philosophy is constrained by the limit of thinking. A philosophy is based on meditating, asking questions to yourself, wondering, finding explanations(or at least look for them).
And if the language restricts you to write your thoughts, only then, yes, the language becomes a limit.
Here you can find what is the philosophy and mainly, the only limit, is you.
Comment: `if the language restricts you to write your thoughts, only then, yes`.  then yes because it very obviously does.
Comment: +1 This implies that language does not limit thought and meditation or create contemplation to use Plotinus's term may go beyond language.
Answer: The reply needs to be nuanced. Probably more nuanced than I can manage but I will try to walk carefully through a minefield. In a word, philosophy is constrained by a philosopher's language.
▻ A COMMON WORLD : CO-REFERENTIALITY
Not totally. There are respects in which all language-speakers share a common world to which at least some of their assertions refer. The associations (beliefs about the referents : the 'sense' as I shall call it later) may be different but all languages have words which refer to the sun, the moon, to shadows, to distance, to space, to time, to succession and simultaneity, to past, present and future. In the lingo the words are co-referential.
▻ SENSE VERSUS REFERENCE
But other examples unsettle the simple picture. 'The Shoah' and 'the Holocaust' are co-referential - they refer to the same event - but the sense or meaning of these terms is different between Hebrew and English speakers. They can't be said, without more ado, to mean the same. A non-Hebrew-speaking English philosopher cannot think about the Shoah in exactly the ways a Hebrew-speaker can. A barrier of different sense comes down sharply between them.
▻ LINGUISTIC INCOMMENSURABILITY
Each culture is likely to have concepts that are specific to it and in some cases cannot be understood in another language. This isn't the Shoah point repeated. There is a common reference if a different sense between 'Shoah' and 'Holocaust'. But if you take the English term 'moral rights', there is no precise equivalent to this expression in Ancient Greek : there is no common reference. The same, I suspect, is the case with 'duties'. The Ancient Greek moral conceptual scheme did not operate with these notions, not exactly these notions. There are moral reflections an Ancient Greek could not have had because there was no language (either in direct terms or by circumlocution) in which to think about them. The same is true in reverse with Ancient Greek terms such as eudaimonia, diakaiosune, moira. We can approximate these terms in English but only by thinking in Classical Greek can we fully comprehend them : which means that the limits of English set limits to what we can think. Only by stepping into the conceptual scheme of another language can we escape the conceptual constraints of English.


What is the difference between SSH and IPsec?

Question: There seems to be similar encryption going into both.  They both use asymmetric (RSA, elliptic curves, etc) for the initial key exchange, then go to some symmetric (AES, Blowfish, etc) protocol.  I'm wondering if someone here might have a moment to explain the difference between the 2 and how they would appear differently to the user (speed, applications they're present in, etc).
Comment: Also see https://security.stackexchange.com/a/172022/2379
Answer: @graham-hill's answer is correct in general but short and pedantically incorrect, so I'll expand upon it.
SSH is at the Application level - you can think of it as SSH/TCP/IP, or SSH-over-TCP-over-IP.  TCP is the Transport layer in this mix, IP is the Internet layer.  Other "Application" protocols include SMTP, Telnet, FTP, HTTP/HTTPS...
IPSec is implemented using two separate Transports - ESP (Encapsulating Security Payload for encryption) and AH (Authentication Header for authentication and integrity).  So, let's say a Telnet connection is being made over IPSec.  You can generally envision it as Telnet/TCP/ESP/AH/IP.
(Just to make things interesting, IPSec has two modes - Transport (outlined above) and Tunnel.  In tunnel mode, the IP layer is encapsulated within IPSec and then transmitted over (usually) another IP layer.  So you'd end up with Telnet/TCP/IP/ESP/AH/IP!)
So - you ask "explain the difference between the two" -
SSH is an Application and IPSec is a Transport.  So SSH carries "one" type of traffic, and IPSec can carry "any" type of TCP or UDP traffic.*  This has implications described below:
You ask, "how they would appear differently to the user" -
Because IPSec is transport layer, it should be invisible to the user - just as the TCP/IP layer is invisible to the user of a web browser.  In fact, if IPSec is being used, then it is invisible to the writer of the web browser and web server - they don't have to worry about setting up or not setting up IPSec; that's the job of the system administrator.  Contrast this to HTTPS, where the server needs a private key and certificate properly enabled, SSL libraries compiled in, and code written; the client browser has SSL libraries compiled in and code written and either has the appropriate CA public key or screams loudly (intruding upon the application) if it isn't there.
When IPSec is set up, because it acts at the transport layer, it can support multiple applications without any trouble.  Once you've configured IPSec between two systems, it's a really minor difference between working for 1 application or 50 applications.  Likewise, IPSec makes an easy wrapper for protocols that use multiple connections like FTP (control and data may be separate connections) or H.323 (not only multiple connections, but multiple Transports (TCP and UDP)!).
From the administrator's point of view, though, IPSec is heavier-weight than SSH or SSL.  It requires more setup as it works its magic at the system level rather than the application level.  Each relationship (host-to-host, host-to-net, or net-to-net) needs to be set up individually.  On the other side, SSH and SSL are generally opportunistic - using either a PKI (CA) or web of trust model, they make it reasonably easy to trust the other end of the connection without much setup ahead of time.  IPSec can be opportunistic, but it isn't generally used that way, in part because configuring an IPSec connection often requires trial and error.  Interoperability between different vendors (Linux, Checkpoint, Cisco, Juniper, Etc) is not seamless; the configuration used to make Cisco<->Checkpoint is likely to be different than the configuration used to make Cisco<->Juniper.  Contrast that with SSH - the OpenSSH server doesn't really care if you're running Putty, OpenSSH (*nix or Cygwin) SSH, or Tectia SSH.
As far as speed goes - I don't have any good numbers to give you, and part of the answer is "it depends" - if IPSec is involved, then the actual IPSec processing is likely to be offloaded to a firewall or VPN concentrator with dedicated fast hardware, making it faster than SSH which is pretty much always crypting using the same general-purpose CPU that's running the server operating system and applications as well.  So it's not really apples-to-apples; I'll bet you could find use cases where either of the two options is faster.

* This isn't strictly true.  SSH supports a terminal application, file copy/ftp application, and TCP tunneling application.  But effectively it's just a really rich application, not a transport - its tunneling is not an IP Transport replacement.
Comment: Rather well explained, +1 and accepted.  So if I'm understanding you, then SSH (since it's at a different layer, application) could actually be inside some IPsec packet.  So if you're sniffing the network, the TCP packets under SSH would just be nonsense (encrypted).  Whereas the IPsec packets would be fairly obvious that they're IPsec 'type' but beyond that would not have identifying features such as what protocol (HTTP, ICMP) is actually inside.  Actually, you'd have no clue as to the payload at all.
Comment: Yup, you got it.  (In fact, if IPSec is used in Tunnel mode, not just the Application but the actual destination IP can also be hidden by encryption, if the encapsulated IP is different than the outer wrapper IP.  This works when the IPSec tunnel is *-to-net, and the net gateway that decrypts the traffic routes the internal IP layer to a different network or IP than the one the gateway is on.)
Answer: SSH is at the Application Level of TCP/IP while IPSEC is at the Transport level.
Answer: There can be a big difference when running TCP connections over a TCP based tunnel like SSH versus a non guaranteed packet delivery system like IPsec where there is significant packet loss. See http://sites.inka.de/bigred/devel/tcp-tcp.html for further details and a real world example.
Side note - I have run TCP over SSH for a few decades now because it is convenient and I haven't experienced the issue, I will swap to a solution that handles packet loss better but is harder to setup solution only if I need to.


What version of Destiny do I have (Ps4)?

Question: I bought the PS4 White Bundle with destiny (http://www.gamestop.com/ps4/consoles/playstation-4-white-destiny-hardware-bundle/115475)  a month ago. I turned on my new PlayStation 4, and popped in the disc. The disc itself only said "destiny ". No guardian edition, no ghosts edition, none of that. When I played the game today, I noticed down in the "What's New" section, it said that "Younesis32 played Destiny: Digital Guardian edition 3 times." What? Which version of Destiny do I actually have? And if I actually have the Digital Guardian Edition, where is my Expansion pass? Thanks, any help appreciated.
Comment: Also, do you think I got ripped off? In the Destiny game case, I got a vanguard armory code that expired a month before I bought the system.
Comment: As a side note, the vanguard armory code is a novelty - some might say useless. You will have better things than those very quickly.
Answer: You did not get ripped of. You got what you paid for.
From what I could tell, I found that in the bundled items it displays:

PlayStation®4 System (Glacier White)
DualShock®4 Wireless Controller (Glacier White)
Destiny Video Game (Physical Disc)
30-Day PlayStation Plus Trial
HDMI Cable
Power Cable
Wired Mono Headset
USB Charging Cable

I could not find any mention of the "Digital Guardian Edition" so they are not entitled to give you the additional expansion pack.
You may have received the disc for the expansion pack version, and the armory code (although expired), you did not buy the Digital Guardian Edition so your not entitled to anything extra.
Comment: Thanks, but what about the "What's New" section. It said I played the digital guardian edition. smh.
Answer: This is what you get with the Digital Guardian Edition of Destiny: Digital Download of Destiny, Early Access to the Vanguard Armory and Player Emblem pre-order bonus, Exclusive In-Game: Ghost Casing, Player Ship and Player Emblem. This also includes the Expansion Pass.
It seems like you received the standard physical copy of Destiny, plus an already expired Vanguard Armory code. You do not have the Digital Guardian Edition.
Here is the link to the official website - http://www.destinythegame.com/wheretobuy/editions


Till which age, it is the obligation of parents to keep their children?

Question: As a general rule, it is the obligation of parents to keep and train their children till they become able doing their affairs. I was wondering if Islam has mentioned any specific age for it. For instance till the age Tamayyoz (التمییز) or puberty (البلوغ)  or even more than it?
Answer: 1- The Messenger (PBUH) said "Teach your kids to pray when they are 7 years and enforce it on them when they are 10 years."
2- In another Hadith (Prophetic statement) he said "Play with your son for 7 years, discipline him for 7 years and take him as a friend for 7 years. Means between 14 and 21 according to the conditions. Some youth become independent before others. It is not the age in years but the employment and financial ability.
3- The female children are different and you take care of your daughter until she gets married. Concerning their marriage the messenger said. "If comes to you one who you accept his faith and his Morals (ethics, and Akhlaq), then accept him for marriage. If you do not do, then it will be severe tribulation and corruption in the Earth. Means Wealth, and family status, etc. should not be considered a barrier in accepting men for marriage.
Comment: References please.


Alternate proof that a sequence is Cauchy

Question: Is it sufficient to show that for any $\epsilon > 0$ that there exists $N$ such that if $n$ is greater than or equal to $N$, then $d(s_n, s_{n+1})$ is less than $\epsilon$ to prove that a sequence ${s_n}$ is Cauchy?
Comment: FYI, this is actually true in the p-adic world. ;)
Comment: http://math.stackexchange.com/questions/75840/cauchy-sequence-of-real-numbers/79531#79531
Comment: also of interest: http://math.stackexchange.com/questions/55735/pseudo-cauchy-sequence
Answer: No. For example, take $s_n=\sum_{i=1}^n \frac{1}{i}$.
Answer: In general, no, common counterexamples being $s_n = \sqrt{n}$ or $s_n = \log n$ or $s_n = \sum_1^n \frac{1}{i}$ as mentioned by Chris.
But it's not completely unfeasible that there exist metric spaces $(X,d)$ in which this is true. For example if $d$ is the discrete metric on $X$ then it is certainly true; I wonder if there are others.
Comment: It's true, for example, in any ultrametric space. This includes the $p$-adics, as Mark notes in the comments.
Answer: Intuition and or process when looking for such a series: Firstly, it's going to be easier to look for an unbounded solution - it will require ingenuity to find a bounded one (though simple sequences can be found). Secondly, we don't want it to grow very quickly - we can tell if something grows quickly by taking it's derivitve. Canonical functions with such things are log(x).
Answer: No, this is false in general. A standard counter-example is the harmonic series: $H_n = \sum \limits_{i=1}^n \frac{1}{i}$. It is well-known that $H_n$ diverges while $H_{n} - H_{n-1} = \frac{1}{n} \to 0$.

This situation is the sequence-analogue of the "$n$-th term test" for series:

If $\sum a_n$ converges, then $a_n \to 0$. Equivalently, if $a_n$ does not converge to $0$, then $\sum a_n$ diverges.

You might know that the $n$-term test is a divergence test and cannot be used to assert convergence of a series. Similarly, we cannot conclude that a sequence $s_n$ converges given only that $s_{n} - s_{n-1} \to 0$.
In fact, this connection is not a coincidence. Let $\sum a_n$ be a series, and define $s_n$ be its $n$-th partial sum. Then $s_{n} - s_{n-1} = a_n$, so asking if $s_n - s_{n-1} \to 0$ is equivalent to asking whether the $n$-th term of the series goes to $0$.

Finally, let me also touch upon Mark Schwarzmann's comment. In any ultrametric space (with an addition operation satisfying the usual properties), a sequence $s_n$ converges if and only if $s_{n+1} - s_n \to 0$. By the preceding discussion, it follows that any series $\sum a_n$ in such a space converges if and only if it passes the $n$-term test!
Examples of such spaces include
(i) any discrete metric, (ii) the $p$-adic numbers, and (iii) the ring of formal power series $\mathbf C[[X]]$ (with the metric $d(a, b) = 2^{-i}$ where $i$ is the smallest index where $a$ and $b$ differ).


Why snort uses "alert" rule only?

Question: Among community rules and registered rules, all are "alert" type rules only. Since there are more rule types like log,pass,activate,dynamic,drop,sdrop available , snort official rule sets use only alert type. Why other type of rules are not included in the snort official rule sets ?
Comment: Is there any specific reason behind it to use only "alert" rule in all snort official rule sets?
Answer: "Alert" type is the default one that can be used if you want to generate that kind of event when the signature matches.
Based on your need, and to customize the use of the application, you might want to not be alerted all the time, for example.
You might want to set "log" type for some rules that might be too noisy, so that you can analyze them once a day, with some analytic methodology.
Answer:
Is there any specific reason behind it to use only "alert" rule in all
  snort official rule sets?

It's a safety precaution.  Defining all matches to alerts allows snort to be initially installed without causing problems for your users.
Imagine you're on the network team that's set up snort for the first time in your organization.  Suddenly your app team is getting calls that the website stopped working!  Nobody would know why, and it would be a big emergency.  Eventually, we can hope that someone would be clever enough to break out Wireshark, and discover these strange RST packets coming from the new snort box.  When you went to investigate why, you discovered that there was a snort rule that matched one of your existing packets and it was configured to reject, which killed the legitimate connection.  People in your organization would be very unhappy with you.
By setting them to alerts first, when you install snort your organization will not be impacted.  Of course you can (and should) investigate all the alerts you see.  You'll see that some of them will likely be false positives - had any of those been configured to reset the connections or drop the packets, snort would have been the cause of a catastrophe.
The converse is also true.  By not setting them to log only as a default setting, they also are being cautious about not allowing actual malicious traffic to be ignored.
The idea is that when you turn on snort, you'll suddenly get a bunch of alerts as part of your normal traffic, and you will have to evaluate them all.  Most will have safe explanations, and you can choose to pass on those; some will be kind of weird and you may choose to log them.  Some may be legitimate alerts of data that you want to drop.  And anything else you discover, well, that might be really bad traffic you don't want on your network, so you set those to reject.
Answer: Well, ALERT is the default rule actions. It is more a best practice issue to use ALERT as the rule actions for the community rules and registered rules. No technical reason behind it up to my knowledge.
Here is why? The other rule actions options are really user-specific. You will only use them to achieve a specific purpose or reach a goal specific to your network. The community rules and registered rules are also general purpose rules most likely you will end up customizing them.
In addition using the other rule actions without really understanding how they work could be harmful especially for people who are new to snort. For instance, Why would you have a rule with a log or ignore actions in the default rule set? This could be the desired behavior in your SNORT deployment but not for others.

Why snort uses "alert" rule only?


Why does HIV belong to a group of retroviruses called lentiviruses?

Question: HIV is an enveloped retrovirus. Each virus particle contains two copies of an RNA genome. The virus also has a number of enzymes: reverse transcriptase, integrase and viral protease. But, once I also find that HIV also belongs to lentiviruses.
I am not very familiar with the relationship between retrovirus and lentiviruses. And does here have somebody know why HIV is a kind of lentivirus?
Answer: In Latin, lente means slow, so lentiviruses are retroviruses that are characterized by long periods of latency.

lentivirus A group of retroviruses that include human immunodeficiency virus, HIV-1. They cause disease after a long incubation period.
latency A state in which a virus infects a cell but does not replicate.

-Janeway's Immunobiolgy, 8th Edition, Kenneth M. Murphy
HIV-1 is characterized by long incubation periods. It can often take months before the virus is detectable on HIV tests.
It is also known that HIV reservoirs will exist in a patient, such that even if they have undetectable viral loads when on Antiretroviral Therapy, once they come off of the treatment, they will begin to produce virons again. The likely reason for that is that the immune cells that they infect can be very long lived, and as the viral genome has been incorporated into the host cells genomic DNA, it is incredibly stable. It will stay dormant for years until it is able to replicate again.
Answer: There're 6 genuses under the family"orthoretrovirinae": alpharetrovirus, betaretrovirus, gammaretrovirus, deltaretrovirus, epsilonretrovirus and lentivirus. So the question came down to the differences between lentiviruses and other retroviruses. Here are some major differences:
Lentiviruses have a cone shape capsid while other retroviruses have a icosahedral capsid.
Retroviruses share 3 common genes: gag, pol and env. While lentiviruses share 2 additional genes: tat and rev.
Lentiviruses have an overlap between gag and pol, and a gap between pol and env. While retroviruses have no overlap between gag and pol and an overlap between pol and env.
Retroviruses can only infect mitosis cells while lentiviruses are capable of infecting non dividing cells.
Pathogenesis of retroviruses and lentiviruses are also very different.


Make a complete new installation, or upgrade continuously to new releases?

Question: I have an Ubuntu system, which has been upgraded to the actual release since 9.04. Now I am wondering if upgrading during such a long time has any disadvantages. Can I relay on the upgrading process to remove really all those old components, which are not used anymore, as HAL for example? Or do I have to do some manual work on that? For example I found out that a package named has is still installed, although in a brand new Ubuntu 11.04 installation it isn't?
So is it recommended to make a complete new installation from time to time, rather than to upgrade continuously?
Answer: I have upgraded continually since about 8.10 or 8.04, and I'm currently running 11.10. So, it's proof you can upgrade, upgrade, upgrade, and still have a working system.
However, it requires a bit more work to keep doing this. Usually I upgrade manually (using Synaptic), but if you keep your system relatively free of useless applications, you can upgrade using the upgrade-manager and it will remove most of the old stuff (that is, things that are no longer able to work with the system, or that need to be replaced with something new).
However, there is a catch, in that a lot of things can end up remaining installed, but marked as auto-removeable, or as local/obsolete, but they will only go away after you specifically remove them yourself.

Doing a fresh install is advisable if you want a completely clean system each time, and many people do it that way. However, if you want to keep your old applications, then I would not suggest doing that.
Answer: As a good recommendation I would say save your needed files and you ~/user folder in another pc or media and re-install.
A lot has changed since 9.04!
Answer: I concur with this advice.  My practice is to keep all of my data files on a separate disk partition.  This (a) makes the data very easy to back up regularly, and (b) makes it simple to do a new install of any new version of Ubuntu.
Comment: You don't need to keep a separate partition to do a new installation of ubuntu, the installer keeps your data by default during a reinstall.


Bacula over s3fs

Question: We are constantly running out of space on our backup server so I want to move the backups to S3. Does it make sense to run Bacula on top of s3fs? Does anybody have experience with that? Is it reliable? What about bandwidth use? Can I expect s3fs to work nicely on Debian 5.0?
Answer: theres no reason why s3fs won't work reliably under debian 5 that I'm aware of.
Re bandwidth, you need to understand that your backups may run a lot slower, they can only run as fast as the bandwidth you have to amazon because everything is pushed over the internet to S3. On this basis you need to carefully consider your data volume being backed up, the bandwidth you have available and consider whether this is going to exceed the window you have available to conduct your backups in.
Answer: I personally prefer S3QL over S3FS. Here's a great feature comparison table (made by S3QL, so it is biased).
If you are at all concerned about bandwith, S3QL's compression and de-duplication are certainly advantages.
Answer: s3fs developer here :)

Can I expect s3fs to work nicely on Debian 5.0

Yes, I primarily develop under Debian (now squeeze)
Bandwidth use will be slightly higher than something such as FTP due to the HTTP overhead. As ccame mentioned, it's limited to your server's upstream bandwidth. I've benchmarked s3fs from EC2 instances at rates over 100mb/s

Bacula over s3fs


Travel insurance for flight cancellation

Question: I know this is a stupid question, I am travelling to Colorado this Friday to see my Uncle, but he has to go to other place due to some work issues. My flight ticket is non refundable and it costs 400$. Is there any insurance policy that would cover the ticket cost if I cancel my flight for no reason, or if I missed the flight (I could make up that I missed the flight on Friday ;).). Please let me know if there is any kind of policies available, I can even buy it for 100$.Thanks.
Comment: Airlines sell insurance for this sort of thing, but it's generally quite expensive - it's called a flexible ticket!
Comment: Also, it sounds like you're wanting to buy a policy when you already know you're going to cancel the flight.  Technically, that's usually against most insurance companies' policies, and they may make you prove you have a valid reason - generally just "deciding not to go" is not enough :/
Comment: Where are you from? USA? Which state?
Answer: Non-refundable means they can't refund you, but you might be able to cancel the flight and get credit towards a future flight. If this works, there may be a fee involved, but it could be minimal or non-existant.
Most travel insurance companies require valid proof. You can look at World Nomad's (or some other company's) policies online to see if any of the inclusions for trip cancellation could apply to you. No one here will try to condone lying, and I would simply try to get the flight cancelled, or sell it off.
Answer: There are a number of companies that offer "Cancel for any reason" travel insurance. Googling for that phrase will find you countless hits that cover the various options available.
However, all of these plans have very specific requirements about when the policy must be purchased (near to the time the ticket is purchased), and when the trip must be cancelled (generally more than 48 hours before flight). They are also generally very expensive.
So in short, whilst what you're asking for does exist, it's not going to work for your specific circumstances.
However, you've said that your ticket is "non-refundable".  Non-refundable means that you can't get the value of the ticket back as cash, but with most airlines you can get the value back - minus a fee - for use in purchasing a future ticket.  Presuming it's a domestic flight, the fee will normally be in the $150-200 range, so you'll be able to keep at least half the value of your ticket.  In order to get this value you normally need to cancel before the time of the flight.
Given that your plans are changing due to "work issues" you may also be able to ask your employer to cover the change fee. Obviously that will depend on exactly what the "issues" are...
Comment: It's his uncle that's away for work though, not him :/
Answer: I was in a situation where I bought a non-refundable ticket without any insurance that I could not use. Of course I wasn't allowed a refund but I was allowed to change the ticket to future trip for a change fee. Just be sure to call the airline before the departure date and they should be able to do it.
When purchasing tickets I almost always see an offer for travel insurance but it's not something I've ever taken advantage of. I assume it works though.


Is adding IDs to everything standard practice when using Selenium?

Question: My team is using Selenium WebDriver and the QA team is having the developers add IDs to almost everything. Is that normal or is Selenium flexible enough that tests can be written with the markup that is provided without having to go back and add IDs? For example, if you have a list of similar items you would usually just use the same class for each item and not put a unique ID on each item in the list.
Comment: One alternative I've used with teams in the past is to create CSS classes specifically for automation use: `qa-accept-button`, `qa-firstname`, and the like.  Developers may find this less obtrusive than an ID, and they're probably less likely to change them as long as they know why they're there.
Answer: It's not unusual.  IDs tend to aid greatly in creating more stable UI tests, since class name changes or other refactoring is less likely to break tests.  Also, dynamically populated lists tend to be high-maintenance in UI automation, and having predictable IDs can help with that.  Using XPath is not only slow, as squeemish pointed out (and slowness itself can break tests if there are any timeouts, which leads those tests being fragile), but XPath also tends to be more vulnerable to UI changes, again increasing test maintenance.
Basically, your test engineers are probably finding that their test UI automation is fragile.  This is a change from developers that can dramatically reduce test maintenance costs and, in turn, reduce the turn-around time on testing new builds.  Fragility in UI tests is hard to manage, so other issues might also need to be resolved before you see the benefits of adding IDs.
Answer: We've found that developers have a tendency to change IDs willy-nilly. We can control it happening in our team, but when we merge with other teams' code, stuff breaks, xpaths change , and css breaks.
So now we've implemented our own css attribute - [data-automation] with its own ID. As long as this gets moved around with any elements, we can always find it and it's made our testing far more robust.
What works for one, may not work for another person tho ..
Answer: I try and use ID's as much as possible.
I don't like using link text as this prevents you from running test in other languages, and link text can also be changed due to settings, e.g. different customers may have different terminology.
ID's are less likely to change than the CSS, so I find that ID's on as many items as possible is all-ways best. This requires the developers to make sure that everything has ID's so you will need buy in from you whole team. (But it good practice for everything that can be to have ID's anyway.)
Some items however will not be ID-able, if there are many instances of that item on the page. For these I have found that the best solution is to have a custom tag and have HTML5 custom attributes that can be targeted by X-Path. This way they very unlikely to be affected if the page layout changes.
Answer: +1 to support an answer by Ethel Evans. It is reliability of UI automation in the long term that unique IDs can help with.
One other point to add is that sometimes you may have semantic CSS classes with elements instead of IDs. For example, on this site, question element is enclosed into

...

while all the answers are within

...

Relying on semantic CSS class names may be a smart option as well if IDs are not available
Answer: Is adding IDs to everything standard practice when using Selenium?
From what I've observed in various companies over several years, this is a great approach, often championed by QA, but rarely done in practice.
In most companies that do web development I usually see markup decisions like this being made primarily by two groups - application and ux developers.  They also have needs for ID's and classes for layout, ajax, styling.  The most common practice I observe is that QA has to work using those identifiers placed there by the other two groups.
I have tried myself to push for unique ID's on all fields but developers and ux folks have their own needs to address first and they take priority.  I've also tried the 'use data-attributes just for QA' but then the feedback is that we don't want that test markup in our production code being served to users.  Most places are going to require you to work as a team on these decisions.
So, in my experience, the standard practice is that generally you have to work within the bounds given to you by developers and ux markup folks.
This means you need to learn a little bit more about good ways to identify fields and learning not to do things like
//tr/td/div/div/div/div/span
# XPath Too specific and thus too fragile, breaks when page layout changes

td.header
# CSS path Too general, applies to several elements.

I call it the goldilocks approach.  Not too general, not too specific, short but unique is "just right".
Comment: hi, another question is placed here if you can answer, thanks https://sqa.stackexchange.com/questions/46093/how-do-top-companies-handle-writing-automation-tags-for-selenium
Answer: Selenium2/WebDriver can find elements with ID, Name, by CSS, by XPath, by Link Text, by partial Link Text, or by Tag Name.  You need to specify what element you want to interact with by identifying it with a findElementBy*
Some elements I work with don't have IDs, so I use the XPath when interacting.  Caveat...XPath is the slowest identifier to use.
Answer: The only benefit I can see from having the developers manually inject unique id's into all significant web elements is:

That it will make Selenium tests that you write much more human readable
as opposed to using Css selectors.
Using IDs, you create an environment very friendly to using recording and
playback ,such as with Selenium Builder because Ids are what Builder
recordings "go for" by default.  Therefore, builder won't fallback to creating
whacky xpath or class name identifiers.

When it comes down to it, if you aren't looking for what I describe in the previous 2 items, then using css Selectors, etc. will work just fine for most people, especially if you use browser plugins, such as FirePath, to auto-create selectors for you when you need them.


Why can't I use Electrum seed to recover my wallet on Trezor and Blockchain.info?

Question: I wanted to use my BCH after the hard fork so I was going to import my wallet on Trezor or Blockchain.info since they support BCH (Electrum doesn't support BCH AFAIK) but it seems both Trezor and Blockchain.info can't recover seed generated from Electrum. Am I missing something ?
Answer: Electrum uses a custom mnemonic format so yes Electrum wallets cannot be restored in any other wallet using the seed mnemonic. Instead I suggest installing and running electron cash to get access to your BCH straight away. It's a fork of electrum for bitcoin cash and by default it imports all your electrum wallets automatically. The official site is https://electroncash.org/


If $A$ is dense in $\Bbb Q$, then it must be dense in $\Bbb R$.

Question: I have $A$ is a subset of $\mathbb{R}$. If $A$ is dense in $\mathbb{Q}$, then it must be dense in $\mathbb{R}$. I am confused because $A$ is dense in $\mathbb{Q}$. Does that imply that between any two rational numbers, there exists a real number? I understand for anything to be dense in $\Bbb R$, there must exist something that lies between any two real numbers. However, how does knowing something is dense in $\mathbb{Q}$ prove that it must be dense in the reals? Any help is appreciated.
Comment: Not a proof, but it may help you to understand what's going on. For any pair of distinct real numbers, and any $a$ in-between, there is always a pair of rational numbers that also straddles $a$ and is also in-between the pair of real numbers. In fact there are infinitely many.
Comment: And yes, there is always an irrational real number between any pair of rational numbers. In fact there is an uncountable infinity of them.
Answer: $A$ is dense in $\mathbb{Q}$ if for any two rationals $q_1 < q_2$ there is some $a\in A \cap \mathbb{Q}$ such that $q_1<a<q_2$. The dyadic rationals would be an example. Here is the way to think about the puzzle of nested dense sets. If you give me two reals $r_1$ and $r_2$ can I find a $q_1$ in between them? Yes. Why? because $\mathbb{Q}$ is dense is $\mathbb{R}$. Can I find two values? $q_1$ and $q_2$ in between $r_1$ and $r_2$? Because if I could find two... then I could exploit the density of $\mathbb{Q}$ to finish the job.
We are given two reals and then we find $q_1,q_2$ inbetween the reals and then we find some $a\in A$ inbetween these rationals. All told we have the following inequality: $$r_1<q_1<a<q_2<r_2$$
Comment: Isn't it 'for any two $q_1, q_2 \in \mathbb Q$ there exists $a\in A\color{red}{\cap\mathbb Q}$ such that $q_1 < a < q_2$' ...? (Plus, of course, an assumption of $q_1<q_2.$)
Comment: @CiaPan. I think you are right. This was written hastily.
Comment: :) I'm not familiar with the definition, but I thought about irrationals – they are certainly dense in reals, but I would hesitate to accept they are dense in rationals, which would fit your former formulation. Hence the red part.
Comment: I don't think that it would have fit because I defined it as a subset. So it would read "the irrational reals are dense in the rationals if (1) they are a subset of the rationals and (2) there is an irrational between any two rationals." So it wouldn't meet the (1)st criteria. So we might be dancing around equivalent or similar variations on the same concept.
Answer: Since $A$ is dense in $\Bbb Q$ so $\overline A \cap \Bbb Q = \Bbb Q \subseteq \overline A.$ So $\Bbb R = \overline {\Bbb Q} \subseteq \overline A \subseteq \Bbb R.$ Therefore $\overline A = \Bbb R.$ This shows that $A$ is dense in $\Bbb R.$
Comment: Looks good to me. I would add the line: Where $A$ overbar refers to the closure of a set. It's unclear to me whether the OP is familiar with this expression so maybe a link to? http://mathworld.wolfram.com/SetClosure.html. +1
Comment: This proof is true for an arbitrary topological space no?
Comment: This is true for any arbitrary metric space @Mason. Let $(X,d)$ be a metric space. Let $Y$ be metric subspace of $X.$ Let $A \subseteq X.$ Then the closure of $A$ in $Y$ say $\overline {A}^Y = \overline A \cap Y,$ where $\overline A$ is the closure of $A$ in $X.$
Comment: Yes @N00ber I also think so.
Answer: There is a more general statement of this theorem. Let $X$ be a topological space and suppose $Z \subset Y \subset X$. If $Y$ is dense in $X$ and $Z$ is dense in $Y$ (w.r.t. the subset topology) then $Z$ is dense in $X$.
To prove this, suppose $Z$ is not dense in $X$. Then there exists $A$ open and non-empty in $X$ such that $Z \cap A = \emptyset$. Then we have two cases depending on if $A' = Y \cap A$ is non-empty. If $A'$ is empty then $A \cap Y = \emptyset$ therefore $Y$ is not dense in $X$. Else $A'$ is non-empty and open in $Y$ w.r.t. the subset topology, and $A' \cap Z = \emptyset$. Hence $Z$ is not dense in $Y$. Either way we have a contradiction.
Answer:
Does that imply that between any two rational numbers, there exists a real number?

Well, if $a$ and $b$ are distinct rationals, then $(a+b)/2$ is a real number that's between them. It's also rational. And, if you want an irrational number that's between them, take something like $a+(b-a)/\sqrt{2}$.
Comment: @RingØ Or $a=b=\mathrm{anything}$. Edited to add the necessary "distinct". Thanks!
Answer: Another proof. $A$ being dense in $\mathbb{Q}$ means that for any $q\in\mathbb{Q}$ there is a sequence in $A$ converging to $q$.
Let $r\in\mathbb{R}$. Since $\mathbb{Q}$ is dense in $\mathbb{R}$, there is a sequence $\{q_n\}_{n=1}^\infty$ converging to $r$. For each $n$, pick a sequence $\{a_{n,i}\}_{i=1}^\infty$ in $A$ converging to $q_n$. For each $n$, choose $k_n$ such that for all $i\ge k_n$ we have
$$|a_{n,i}-q_n|<2^{-n}.$$

Claim: The sequence $\{a_{n,k_n}\}_{n=1}^\infty$ converges to $r$.

Proof: Let $\epsilon>0$. Choose $N$ such that for each $n\ge N$ we have
$$|r-q_n|<\frac{\epsilon}{2}\qquad\text{and}\qquad2^{-N}<\frac{\epsilon}{2}\ .$$
Then for each $n\ge N$ we have
$$|r-a_{n,k_n}|\le|r-q_n|+|q_n-a_{n,k_n}|<\epsilon\ ,$$
consluding the proof.
Answer: Another definition of "dense" is that every open neighborhood of $\mathbb Q$ has a member of $A$. And using that definition, we want to prove that every open neighborhood of $\mathbb R$ has a member of $A$. So suppose we take a neighborhood $N_1$ of $r$ in $\mathbb R$. Since $\mathbb Q$ is dense in $\mathbb R$, there is rational $q$ in $N_1$. We can take a neighborhood $N_2$ of $q$ that is a subset of $N_1$, and there will be $a$ in that neighborhood, and thus $a$ will be in $N_1$ as well.
Basically, $A$ being dense in $\mathbb Q$ means that for every $q$, there is $a$ "close" to $q$, and $\mathbb Q$ being dense in $\mathbb R$ means that for every $r$, there is $q$ "close" to $r$. So given any $r$, we take $q$ "close" to $r$, then we take $a$ "close" to $q$, and $a$ is "close" to $r$.
It's analogous to "If everyone lives close to a school, and every school is close to library, then everyone lives close to a library.", although there's some additional rigor regarding the term "close" that has to be introduced.


What are Ripple's weaknesses?

Question: Bitcoin's vulnerabilities are well documented. Is there are similar document for Ripple?
Comment: > fast (ten seconds) Is it average time to get consensus in the network? Will it grow?
Answer: These are Ripple's largest known weaknesses (not including weaknesses common to Ripple and Bitcoin):

Because the Ripple network can't move actual fiat currencies, balances in Ripple act like bank balances. If the bank somehow defaults on its obligation, people won't actually have the money they thought they had. This is not a big problem if you're just using Ripple to make payments, but it's a weakness if you're using Ripple as a store of value for fiat currencies. Your money is only as safe as the "bank" that holds it for you.
If some unified group were able to get control over a significant number of validators that other validators trusted, they could temporarily disable the Ripple network (by refusing to reach a consensus) until manual intervention by other validators removed them.
Ripple's internal mechanics are still largely untested. As happened with Bitcoin, problems may be discovered as people begin to heavily use the system. Ripple's scalability has only been tested in simulation.
While the hope is that gateways will be all close enough to equal that people will be able to just sum their balances to get a sensible total, it's possible that different redemption policies or perceived trustworthiness will result in some balances being "devalued". So you might deposit $10 at a gateway, get a "$10" balance in Ripple, but find that because your gateway is not considered reliable, nobody actually values that balance at $10.
People might get tricked into extending trust to entities that are untrustworthy. We are already seeing "you trust me and I'll trust you" type offers. People may not realize that the system will give them what they ask for, giving them balances they have no way to collect on. Essentially, Ripple simply records balances. The system can't force people to settle them outside the system.
The stability and possible future changes in the value of XRP is unknown. If XRP is usable as a bridging currency, that should make cross-currency payments cheaper. But there's no assurance that it will be.
Until the network is decentralized, OpenCoin is a single point of control and possible failure.

In exchange, Ripple provides a way to perform fast (ten seconds), cheap (less than a penny) cross-currency payments with no way to lose money unless someone you chose to trust defaults. Ripple provides decentralized exchanges between any number of currency pairs. Ripple provides a way to seamlessly federate payment systems so people who use one payment system can pay people who use another, even across currencies. Ripple's design requires no central authorities and OpenCoin is committed to making it an open source, decentralized payment network.
Comment: @Manish: The need to rely on gateways, at least for the near term, is probably the biggest weakness. By "bridge currency" I mean one that you can use to exchange between unpopular currencies. If I hold XYZ and you want ABC, and these are both unpopular currencies, it may be hard to find a payment path. But if there are lots of offers of XRP for XYZ and ABC for XRP, then XRP can "bridge" the two currencies. You can also combine those two order books to get an "ABC/XYZ" synthetic order book to trade on.
Comment: @Manish: The problem with using USD as a bridge currency is that someone has to hold that USD. Say XYZ is big in Poland where USD is handled largely by a Polish bank while ABC is used in Slovenia where some other USD gateway is popular. Now you need intermediaries between those USD exchanges. There's only one XRP. There's no technical reason inside the Ripple system why XRP wouldn't be used, it's a matter of whether it's suitable for that purpose which would require things like price stability and low risk.
Comment: "Your money is only as safe as the 'bank' that holds it for you." This is funny, because I don't know if my money is safe with any of the "real" banks (example: Cyprus depositor tax). Thanks.
Comment: "There's no assurance that it will be." Is this because the design isn't complete, or that the implementation isn't complete, or that it hasn't been tested in the real world? XRP is supposed to work as a bridge currency (transaction fee) as per the design, right? Or am I missing what "bridge currency" means here?
Comment: If I understand correctly, USD could just as well be used as a bridge currency over the Ripple network (owing to USD's popularity), but XRP would be even more efficient because it's native to Ripple. "If XRP is usable as a bridging currency...." Why wouldn't it be usable? Any technical reason, or just a matter of people not accepting it?
Answer: In 2019 several exchanges suffered an attack called 'ripple partial payment exploit'.
It uses an integral part of the ripple network (partial payments) in conjunction with sloppy implementation by exchanges to defraud large amounts of money(in crypto) from those exchanges.
The largest known victim of this exploit is the beaxy exchange that got affected by some $570000 USD worth of cryptocurrency drained out of their wallets.
https://finance.yahoo.com/news/beaxy-exchange-likely-lost-570-142053125.html

What are Ripple's weaknesses?


Are connected doubled (4 pawns on 2 files) a weak structure?

Question: It seems to me to be a very solid structure that can play very good in the endgame.
Example: pawns on c2, c3, d2, d3.
Comment: Depends on what else is on the board.
Answer: It is a solid structure indeed, avoiding one of the most important weaknesses of doubled pawns, which is that they're generally hard to protect. Also, they're less easy to block than just a c- and d-pawn, since the backward pawns can attack enemy pieces on the blocking squares.
However, this structure only influences the b-, c-, d- and e-files, while a 'regular' connected pawn structure influences 6 files. Such a structure is also more likely to be able to force breakthroughs. Everything depends on context, as user1583209 notes, but in general it's better not to have doubled pawns.
Answer: The structure itself isn't weak, since as you mentioned all of the pawns are solid and protecting each other. The problem is that you can't do much offensively with these pawns. Take the following example:
Black has his c-, d-, e-, f- pawns.
White has the pawn structure you mentioned.
Notice how White is unable to block Black's f-pawn. Also, how is White supposed to do anything in the center? He can only really use his front 2 pawns in an attack, while the back 2 have to sit and wait.
It depends on the position though. Maybe you don't want to move any of your pawns, and controlling the squares around that 4-pawn block is more important.
Answer: The structure lacks mobility and is therefore inherently weak.  The c2 and d2 pawns can not advance if attacked.


Can Chang'e 3 take photos of the stars from the moon

Question: Chang'e 3 now on the Lunar surface has imaged the Earth. Is it possible for that same camera to take a time exposure and show us the stars?
Comment: It is equipped with a UV telescope, [LUT](https://en.wikipedia.org/wiki/Chang%27e_3#Lunar-based_ultraviolet_telescope_.28LUT.29)  to observe stars among other objects.
Answer: Looking through this image archive, they haven't taken photos where stars are the main subject. All photos in that archive were done in daylight.

I've removed the processed photo - I found 2 bright pixels in the shadowed region of the Earth, so those can't possibly be stars.
Chang'e 3 hibernates during the lunar night. It's not clear if the probe contains an RTG that can provide electrical power, or if it has radionucleide heaters only.

The majority of power provided by the RTG is used for the heaters and a small fraction is used to sustain the lander's Lunar Night Sleep Mode in which all systems except for crucial control and housekeeping systems are shut down.

I tried to find out a bit more about the camera:

Three panoramic cameras are installed on the lander, facing different
  directions to allow the lander to acquire images of the lunar terrain
  surrounding the landing site and take photos of the rover as it
  departs the landing site. The cameras can take still images as well as
  video. Exact technical details for the cameras were not released. (emphasis mine)
Camera technology tested on previous missions that is employed on
  Chang'e 3 include auto-exposure, focus adjustments, high-speed
  compression of color imagery and static gray image, and sub-sampling
  methods.

I also found a paper (PDF) that analyzes the performance of the cameras on Yutu in more detail. It doesn't mention exposure times specifically. It does say the camera has both manual and automatic exposure modes. The PDF contains a few graphs that show camera performance, these have an X-axis marked 0-1200 with a label of "0.5 ms". I think that means the X-axis shows an exposure time of 0-0.6 seconds, but I'm not entirely sure. If this is true, the camera has been designed to use exposures of up to 0.6 s, which sounds long enough to see a few stars.

Edit:
More information on the Chang'e 3 image archive can be found in these blog posts:
one
two
The archive itself is here but is available in Chinese only.
Comment: The bright spots in the image are most likely hot pixels of the image sensor and no real stars. Easy to spot: Most are fully saturated colors of single pixels. A star should show up as a slightly blurry spot of several pixels due to the finite resolution of the optics.
Comment: Good point, I've removed the photo.
Comment: Looking at the camera specs, do you think  it is capable of imaging the stars? Daylight should make no difference to the camera as long as it is not looking at the Sun, stars should show up with a few seconds exposure time.
Comment: It seems the Chinese space agency hasn't published details on the camera specs.
Comment: Is that image archive still available? I can't seem to get a connection from where I am.
Comment: The archive is still available, but can take a while to come up, connections from overseas can be unreliable. I've also added some new links at the end of the post.


Important question about svchost virus was closed

Question: I asked this question and It was closed as a Exact duplicate of a question which does not resemble it at all. First of all, nowhere in this "dupilcate question's" answers does it mention the stop error my question was based on. Also, there is no mention on the specific virus I was dealing with that I asked the question about. The answers were irrelevant to my situation. How do I report the five guys that closed this, and how can I reopen it?
Comment: Could you please not post your answer in the question, but use the "Answer your question" button to add the answer there?
Comment: Never remove the duplicate question reference from a post. I rolled the question back to when it was inserted. Feel free to bring back your changes from the [revisions page](http://superuser.com/posts/524418/revisions) now.
Comment: Can I still change the title?
Comment: Sure, go ahead. Just the note at the top is important *while* the question is closed. Once and if the question is reopened, it will be removed automatically.
Comment: Ok thanks. I didnt realize it was placed automatically, I thought someone had edited it into there.
Comment: Ok, when I have some time I will.
Answer: In this case, I think closing it as a duplicate was slightly overzealous. The question yours was closed as a duplicate of has a lot of good information, but is meant to be a general reference, not the end-all be-all for every virus question ever.
Generally speaking, when one is asking a question about a virus, they should try the main concepts listed in the referenced how to deal with viruses post. If that doesn't work, or if there ends up being a very particular way to remove the infection, then creating a new question for it is appropriate. Just make sure to reference the canonical virus removal question and say something to the effect of "I tried <all this> and it didn't work" so that it doesn't get closed as a dupe.
For the reasons above, I reopened your question.
Comment: Thanks, I'll work on the question to add the stuff it needs.


What are the positions on drop bars called?

Question: I've heard the different places to put your hands on drop bars given different names, like "hoods" which I think is the flat part on top or maybe the brake levers, or "drops", referring to the lowest part of the bars.  Can anyone tell me what the actual names of places to put your hands are?
Comment: See also: http://bicycles.stackexchange.com/questions/5251/how-to-use-drop-handlebars-properly/
Answer: "Middle of the bar", "On the tops", "Middle", "Top", "Tops", "Center"
Hands on the part of the bar closest to the stem. The middle has some width and you can slide your hands around there.

"On the corners", "Corners"
Hands at the outside end of the section where the bar turns. You can grip the corner different ways. There's a sort of position in between corner and hood ("ramp"?), too.

"On the hoods", "Hoods"
Hands on the brake hoods. The brake hood is that rubbery thing that the brake lever comes out of and on a modern road bike likely also contains the shifter. There's different ways to put your hands on the hoods, I'm showing two I use.

"On the drops", "In the drops", "Drops"
In the lower curved part of the bar. With the drops you can be leaned way down with your hands far forward/up in the hoods, you can be about where I'm showing in the picture, or you can be on the lowest part of the bar (where you can't reach the brake lever).

Note for all of these:
There are various different hand positions in each of those main positions, but not necessarily names for them. Some of the differences can be subtle and don't really look much different, but change the pressure points on your hands.
Most road-bike riders primarily use the hoods, and that's generally what's currently encouraged. It's the most comfortable position that you can brake and shift from and gives you good leverage. The drops are your fastest position and also give you good leverage. Middle is most comfortable for your hands, but gives you the least control.
Comment: Unintuitively, there is [some evidence](http://road.cc/content/news/133598-want-ride-faster-hunker-down-hoods) that the drops aren't the fastest position.
Comment: There's also the variation of in the drops which is a bit farther back were the controls aren't accessible that many sprinters use which might be like "on the ends" or some such
Comment: I think the corner position is more commonly called the ramp. One example here: http://lovelybike.blogspot.com/2011/10/must-haves-dropbars-with-flat-ramps.html
Answer: I hate to simply copy+paste Sheldon, but...

Drop bars give you (normally) 4 hand positions: Middle of the bar...On the corners...On the hoods...On the drops.

I find that the middle is good when you need to breath more, the drops are good for aerodynamics, the hoods are for steep hills and I spend most time on the corners.
Comment: Yeah, those are the basic positions, but with each position there are generally 2-3 options.  On the drops, eg, your hands can be on the horizontal (most rearward) parts of the drops, or they can be up onto the bottom of the curve.  (A position does not have to have a name to use it.)
Comment: Do you mean palms facing up? I suppose that'd relieve downward pressure over a long distance
Comment: No, I mean towards the front, where the bar begins to curve.


Should this be a tag synonym? [big-o] and [big-omega]

Question: I noticed big-omega has 6 uses and a stub description that is a synonym for big-o which has a full fleshed out description.  I don't have any privileges regarding tags yet, so I thought I'd mention it here.
Comment: I think this is a much better synonymization: http://meta.stackoverflow.com/questions/251548/proposing-tag-synonym-big-o-and-friends-%e2%86%92-complexity complexity <-- big-*
Answer: Revising my position.
No, big-omega should not be a synonym. Big-Omega is a different notation; see the Family of Bachmann–Landau notations on Wikipedia. There is another related tag here: big-theta.
The terms are related, but not synonymous. The wiki perhaps needs to be updated to reflect the relationship.
I've erroneously proposed a synonym for this; I'd like people to vote against this request.
Comment: Will CS people read the tag description and use the right precise tag, or use the wrong one or a commonly-used one?  My experience is that questioners are not always up on formal nomenclature, to the point of that being part of the problem.
Answer: Alternatively, big-omega and big-theta should have better tag wiki content.1
And note that one would expect their usage to be significantly less: only used with the most rigorous analysis of algorithmic complexity.

1 My CS knowledge is not really up to a useful expansion.
Answer: The current big-omega wiki is not just short, it is incorrect. The various complexity notations describe the growth of some function in terms of the growth of another function as their argument tends to infinity. The function whose growth is being described may itself be worst case time, best case time etc.
The issue is complicated by the fact of alternative definitions, but for SO the Knuth definition is the more relevant. The Wikipedia page http://en.wikipedia.org/wiki/Big_O_notation covers all this well. Maybe copy some material from there, with attribution?
Comment: Yes, if the tag description made the difference very clear, it would be different.  But as stated it's imprecise.  I think people would not make such a fine distinction when tagging, and a single tag for complexity of algorithms is proper.  Or, would people interested in the subject know to subscribe to every form separately or end up missing the rare but specific ones?


demand impact on shares price

Question: Knowing that share prices are determined by the forces of supply and demand, other factors can also play a role in this but lets consider the fact that prices are only based on supply and demand.In this case is it possible for someone to buy a big amount of shares thus making the price rise and then sell them to make a profit ? If so how can the impact on the price be calculated depending on the amount of shares bought ?
Comment: It might be your while worth thinking through what it would mean if things worked as you describe.
Comment: So share prices do not depend on tge current demand/supply ? I am asking this question because I need explanations
Comment: @EnergyNumbers and when I am talking about buying big quantities I mean quantities that can actually have an effect on the price...
Answer: What you're failing to consider here is that in buying a large amount of shares, the buyer also makes the price high for himself. And if he sells a large amount of shares at the same time, he's likely to lower the price of the actions while selling them. Poor boy has no chance.

demand impact on shares price


Why do Japanese speakers have difficulty pronouncing "L"?

Question: Why is it that native speakers of Japanese have a hard time pronouncing "l"?  Whenever a western word contains "L" I see that they pronounce it "ru", "ra", "ro", "ri", or "re".
Comment: @AndrewGrimm: It is possible for someone to be born in Japan to Japanese parents, but then move to, say, England shortly thereafter and consequently become more fluent in English than Japanese. As for Chinese and Korean, the root cause is essentially the same: there is no phonemic distinction between rhotics and laterals in those languages. (Mandarin does have a phoneme that is often transcribed as /r/ in addition to a genuine lateral /l/, but /r/ closer to a fricative than an approximant, really...)
Comment: Why do westerners so often sound so awful when speaking Japanese?

Because when learning another language, so many people try to learn that language as if it is a direct translation of their own. Sure the words are different and the grammar might be rearranged, but they think it's a direct equivalent.

Japanese has fewer sounds than English, yet I've met almost fluent Japanese learners who do not bother imitating the sounds and just speak Japanese with English vowels.

Can you imagine how hard it must be for a Japanese person to learn English sounds! There are sooo many more than in Japanese.
Answer: If you ask a Japanese person to say a word like renraku fast, and then gradually ask them to say it more and more slowly, you will notice that what initially sounded like an r becomes an l as they slow down (usually earlier on for women). So the claim that l and r don't exist is simply wrong -- they both do, but as variants (allophones) of the same sound (phoneme).
If you tell them that they first said an r and then an l, you will surprise the heck out of them as they usually have no idea about this and will probably be unable to tell the difference. The reason is fairly simple: it's the same sound in Japanese. The only difference between the Japanese l and the Japanese r -- not the English r, which is quite different -- is mostly the duration of the contact between the tongue and the roof of the mouth and while this feature allows the two sounds to be distinguished in European languages, it is NOT a distinguishing feature in Japanese.
Similarly, English speakers fail to distinguish many sounds. Some languages have nasal vowels (French, Portuguese, Polish, etc.): when the soft palate is lowered to allow air to pass through the nose as a vowel is produced, it changes a vowel to a nasal vowel and this is a distinguishing feature -- but not in English. So in a word like bank, the a is nasal because it is following by a "ng" sound, and if you ask English speakers what vowel is it, they'll say it's an a, while for a speaker of Portuguese, it's a nasal vowel, not an "a". Another example is how English t, p and k sounds are aspirated (there is a puff of air) at the beginning of words or between vowels, but not after an s, for instance. English speakers usually have no idea about this, but in other languages, such as Korean, aspirated and unaspirated t's are two very different consonants. So if you want some insight into how it feels to not tell l and r apart, ask yourself why you, as an English speaker (if you are), can't distinguish nasality or aspiration.
So the first problem is that Japanese people have a hard time hearing whether an l or an r is being produced because they are two possible realizations of the single phoneme to them, and consequently, they have a very difficult time remembering which is which. You'll find the same thing with z/dz. Ask a Japanese if you should say zettai (with a z) and they'll say yes. Ask again if it's dzettai (with dz) and they'll also say yes. Ask which they prefer and they'll tell you they only heard one pronunciation, even though you clearly said z in one case and dz in the other.
To make matters worse, they start out learning English using the wrong kind of flapped r (as in Spanish or Italian), while the English r is not flapped, but is rather a retroflex r where the tongue is curled back and doesn't touch any part of the mouth. To make matters even worse, the way English words are borrowed into Japanese interferes with their perception of English words, ie. they are spelled in a way that doesn't distinguish l and r (love=rabu), final r's are turned into a's (computer=kompyuutaa), etc. They really need to stop using katakana transcriptions. Note also that the English r involves lip-rounding, but since this isn't used in Japanese -- and they start learning to pronounce English without ever using lip-rounding -- they fail to see this as an important feature of English r.
The second problem is that even if they remember which is which, production is very difficult because the brain is used to treating both sounds as one, and they usually spent a fair bit of their lives as learners of English in a context where you simply said l and r the same and that's how everyone did it. It's a bit like relearning to walk after an injury.
So, while it's true that some Japanese people have a difficult time saying the English r, most can usually make l and r in isolation once they've understood how the sounds work. The biggest problem is that they have a hard time telling l and r apart when hearing it in speech, remembering whether a given word actually has an l or an r (and they often misspell words containing these sounds), and then producing either sound with regularity is ultimately the most difficult exercise.
One final note: to make it easier to explain, I left out some details. For instance, the English l is sometimes curled and English l and r are sometimes devoiced (compare ray and pray). These are small details that also make the sounds harder to produce accurately.
Comment: "To make matters even worse, the way English words are borrowed ... They really need to stop using katakana transcriptions." Borrowed words are *Japanese* words, so it's completely natural to write them with the Japanese script. Or do you expect people to write "kimono" with kanji in English text? Using katakana to transcribe pronunciation of *English* is a different matter, though, and here I tend to agree with you. But the question would be what transcription to use instead. IPA would be quite a steep learning curve. And English spelling is just as misleading as katakana if you ask me.
Comment: [l] and [r] are free variation allophones in Japanese -- allophones needn't be in complementary distribution. The fact that /r/ can surface as [l] in slower speech (and more so in certain positions) IS proof that it's an allophone; [r] is undefined for laterality. As for my comment about using katakana, it was poorly worded: I meant that using katakana for words of English origin makes learning them as English words more difficult, and use of katakana when learning English does learners a disservice. English spelling can indeed be misleading, but not when it comes to l and r.
Comment: You are right, l is sometimes written but not pronounced, however l and r and never spelled one way and pronounced the other. In that sense, I meant that they aren't misleading.
Comment: "If you ask a Japanese person to say a word like renraku fast, and then gradually ask them to say it more and more slowly," - any chance you have video of this? :D I kinda don't have Japanese person to ask to do that
Comment: Sorry, but I fail to see how the fact that you can get speakers to pronounce [l] by forcing them to speak slowly proves that [l] is an allophone of /r/. In order to prove that they are allophones, you would need to show that the two sounds are in complementary distribution, which is *not* the case. /r/ is always pronounced as a flap in normal spoken satndard Japanese.
Comment: I guess it comes down to what we define as "part of Japanese". I acknowledge that some - small kids/anime characters/pop singers while singing/people speaking at an unnatural speed - might pronounce /r/ as [l], but I don't consider this "standard". Yes, I do realize I had my definition of allophone wrong, I didn't realize sounds in free variation were also called allophones. "English spelling can indeed be misleading, but not when it comes to l and r" How do you pronounce "salmon"?
Comment: Well, to me that doesn't seem like a strong enough argument to use English spelling for transcribing English pronunciation :)
Comment: @dainichi, at least if English spelling is used for transcribing English pronunciation, then the mistakes made are going to resemble native-speaker mistakes, since that's the system we English speakers learn from.

English speakers have a number of spell-it-out pronunciation systems for explaining how to pronounce difficult words, and those are not a terrible idea either. To your example, you might write "salmon (SAM-in)" to explain to a child how to pronounce it. I concur that katakana is not a great system for teaching English.
Comment: I feel like this doesn't explain why Japanese speakers struggle to distinguish l and r in English. The English r is a completely different sound from the Japanese r, so even if l and r are allophones in Japanese it doesn't make sense that that would make l and a completely different sound hard to distinguish. The learning environment not making a distinction probably doesn't help, but that doesn't explain why Japanese people struggle to hear a distinction in audio from native English speakers.
Answer: The problem with L and R is that niether of them exist in Japanese. The Japanese sound is more of a cross between the English R and L, so it's very difficult to distinguish the two, hence Engrish. A proper hard R is actually just as difficult to pronounce as an L for Japanese speakers, and the hardest words to pronounce are those with both sounds (for example, parallel).
It might be hard to understand from an English speaker's viewpoint, because the difference is as clear as day, but go ahead and try to distinguish the Chinese X and SH
Comment: l and r both exist in Japanese, but they are allophones of a single phoneme. And I'm not sure what a cross between English R and L would sound like, but certainly not what Japanese would pronounce in a word like Engrish. I would downvote, but I think your last comparison with x and sh is valid.
Comment: @alexandrec: I disagree completely. らりるれろ is neither "la, li, lu, le, lo" nor "ra, ri, ru, re, ro". When trying to pronounce words in English, of course the Japanese try to pronounce R/L not る/り, the same way English speakers try to say かえる not "kaeru". (I guess this applies for serious language learners.)
Comment: Japanese /r/ can surface as either [l] or [r], but these are not English r or l, if that's what you mean. There are several kinds of l's and r's across languages.
Comment: @alexandrec, oh...I guess what I mean is "in the context of learning Enlgish", not "in the context of phonetics" heheh :) In that I am way over my head!
Answer: Same reason English native speakers have problems with the Spanish /r/ (both versions)  They don't exist in the language.  The Japanese ら、れ、etc is a combination of /r/ and /l/ (it's actually quite similar to the soft Spanish /r/)  It is possible for Japanese people who have been studying the language for a long time and have traveled abroad to pronounce them correctly, but for a beginner, or somebody who just studied the basics (in a very deficient way) at high school, it's probably difficult.  Same happens with English natives learning Spanish, and vice versa.  And obviously, it's also difficult for us to pronounce correctly the Japanese ら、れ、etc (among others)
Answer: There is a very good article in the Scientific American Mind magazine: http://www.scientificamerican.com/magazine/mind/2014/07-01/ (pages 24-25).
The authors also talk about an experiment that was developed at Carnegie Melon University, which teaches native Japanese speakers to learn the difference between L and R sounds with reinforcement learning.
There is also a free app which utilses the same reinforcement learning technique discovered by the scientist at Carnegie Melon University at: http://www.lirril.co
Answer: The answer here summarises my feelings on the matter as someone who's grown up with both languages:
http://japanese.about.com/blqow13.htm
I'd strenuate the use of the English "L" noise when attempting Japanese words, I've never really understood why learning systems adopted "R's" when "L's" seem closer in sound.
Comment: Thanks silvermaple. Perhaps I'm putting unnecessary emphasis on my own view of how "ra" etc. is pronounced.

In brief, if I were to explain to an English friend (with little Japanese knowledge)  how to pronounce テーブル, I'd most certainly tell her to say ル as lu, rather than ru, not least because it flows better for an English speaker, sounds more similar to "table," and in my opinion sounds more like the Japanese pronunciation.

Interestingly though, when I'm intoxicated, I do mix my r's and l's when speaking English which perhaps signifies that I'm less aware of the phonetics then I think!
Comment: I don't know why this was downvoted without explanation, but if you could explain a little more that would be helpful :)
Comment: "I've never really understood why learning systems adopted "R's" when "L's" seem closer in sound" -- I'm guessing here you are refering to romaji using r instead of l. Maybe you are thinking about the English r, but generally, the Japanese r sounds the same as r in most other languages, so it does make sense to use r instead of l in romanization systems.
Answer: Because [l] simply isn't in the native phonetic inventory of Japanese. The lack of phones in your native phonetic inventory is the reason for most pronunciation difficulties.
Just to add an observation, though:
When trying to explain the [l] sound to native Japanese speakers, I usually tell them that it's like an [n] sound, except you should point your tongue a bit to allow air to escape on both sides of it. But it seems that this can be harder than it sounds. And thinking about it, no Japanese sound requires you to point your tongue. Doing things with your tongue you're not used to requires muscle training, and native Japanese speakers would probably lack this training.
Comment: @alexandrec, there might be spakers who pronounce r in ringo as an [l], but I would definitely consider that a small minority. The Japanese /r/ is usually a postaveolar flap, whereas [l] would be an alveolar lateral. You are right that tongue placement is similar for the consonants you list, since they're all alveolar. I just prefer to use [n] as comparison, since both [n] and [l] are sonorants.
Comment: l does exist in Japanese, but not as a distinct phoneme (ask a Japanese speaker to say ringo slowly, it should sound like l, especially for women). Furthermore, if you are going to use n as an analogy, it would be to indicate where the tongue is placed, and that place is the same for l, n, d, t, s, z, ts and dz -- all of which are Japanese sounds.


Displaying OpenOffice Base data in Excel

Question: Is this possible?  I assume I need an ODBC driver or other recognised from Excel/Query to access Base.
Answer: I don't think it is possible, at least not easily. OO Base is a front end that actually uses HSQLDB as its database engine. There is this thread in the OO community forum where a workaround is proposed to unzip your .odb file and use JDBC to connect to the HSQLDB database. (I haven't tried this). The OO forum thread is a bit old, so it might be a good idea to try to find alternatives.


Gas In dip tube is missing in my corny-keg

Question: I bought two used cornies. They're pin locks, and neither of them came with Gas tubes. Will everything still work? Or do I need to buy replacements?
Answer: Are you sure they're not there? I ask because I have six pin lock kegs, and two of them have tiny gas dip tubes that are maybe a quarter of an inch long. (Just long enough to hold the O-ring.) They usually stick inside the gas post when I remove it. I can attest that these short dip tubes work just fine.
To answer your question, though, you will need dip tubes because the O-ring that creates the seal slides onto the dip tube. Without the dip tube, there's no O-ring. Without the O-ring, there's no seal.
Also, I'm not sure the poppet valve inside the post will function properly if the dip tube is missing. The gap left between the bottom of the poppet valve and the top of the nipple probably would allow the poppet valve to slide down out of position, leaving it incapable of creating a seal at the top of the post.
If you really don't have the dip tubes and just want to see what happens, you can try putting thread tape on the nipple, screwing the post on, pushing some gas in, and checking for leaks.
Comment: The o-ring needed replacing anyway, so I used a knife to pry up, using the o-ring for leverage, rather than the tube. Thanks! For anyone in the future, it looks like the gas dip tubes are *so* short you can't feel them from the inside...Thus my mis-understanding.
Comment: I'm pretty sure. I've checked both the hole and the post. I'll re-check, because they guy I bought the kegs from said he'd used one of them...
Comment: It looks like you're right. I think it's stuck *in* the keg. I looked closer and the whole for the gas peg looks like it has a rubber gasket on it. I think it's the o-ring. I"ll have to try prying it up tonight.
Comment: If it's not too stuck, you should be able to grasp the top edge of the dip tube with some needle-nose pliers and lift straight up on it. Good luck! Be careful when prying :)


MINI Cooper Oil Leak

Question: I bought a 2007 MINI Cooper 8/27/14. On 9/9, as I drove to work, I noticed smoke coming from the back & front of the car. When I drove home late that night, the car was not smoking. After that time the car would smoke if I drove it longer distances. I have been told that oil is leaking from the vacuum pump & full flow oil filter seals onto the exhaust pipe. Why would the car only smoke on slightly longer drives rather than constantly if the oil is leaking onto the exhaust pipe? Wouldn't it leak whenever I drive and so smoke whenever I drive? Thanks!
Comment: Is this a turbo car (the Cooper S)?
Answer: The Mini has an unusual set up on its oil filter element. If you look at the filter you will see one end has a rubber sealing on it, whilst the other end does not. When you fit the filter element, the rubber ended end must be on the outside, away from the engine and into the cap you take off to replace the element. If you dont know it, it is easy to think the rubber end goes towards the engine but it does not. Fitted the wrong way round it causes havoc with the oil pressures.
Answer: Perhaps because your exhaust pipe is hotter when it runs longer, and is cold enough when not running much to burn the oil. Anyways you could buy a special paste to fill-block the leak.

MINI Cooper Oil Leak


Adding external video source to RPi Cam Web Interface

Question: I am using RPi Cam Web Interface to stream raspberry pi camera over browser. But I need to integrate a Wi-Fi camera as well, which produces rtsp or http output. I want to know if the RPi Cam Web Interface has any inbuilt support for external video coming from a WiFi camera, or if there is any hack available for this?
Comment: Welcome to Stack Exchange, Samrat! Can you be a bit more specific - which WiFi Camera are you looking at and which part of connecting it up are you curious about?
Comment: Thanks, I wanted to integrate wifi based ip cameras, what I found is, most of the wifi camera are giving output as http or rtsp. So i just wanted to know if RPi Cam Web interface, supports external video from a http or rtsp stream.
Comment: I have edited your question to make this more clear
Comment: What "RPi Cam Web interface" are you referring to?
Comment: http://elinux.org/RPi-Cam-Web-Interface this one
Answer: Rpi Cam Web Interface cannot do that. UV4L can do what you want. Not only it can live stream the native or an USB camera to any browser via WebRTC thanks to the Rpi camera or UVC drivers, but can also create a virtual device from an external source like IP camera thanks to the MJPEGStream driver. You can also have both streaming at the same time as the Streaming Server can be per-device. Audio is also supported. No configuration is required.
Comment: what i found, raspimjpeg is used for creating stream of pics for RPi Cam Web Interface, if we can do to alter the input of raspimjpeg, providing output of ffmpeg (it will be listening to some stream) through some pipe, and utilize it. is there any similar way to do it? also in RPi Cam Web Interface wiki, i had seen about some ispy connect, which they had written for achieving the same