What are the oldest copies we have of LXX

Question: What are the oldest copies we have of the Septuagint (LXX)? I know that it is present in Codex Sinaiticus and other codices. Are there any in scrolls and papyri or are there only codices?
Comment: @C.Kelly Re. `The LXX is the Greek OT, Sinaiticus is a NT manuscript, so there is no overlap there.` -- [Sinaiticus includes much of the Greek OT](http://codexsinaiticus.org/en/manuscript.aspx). There are substantial gaps, mostly filled in by the more complete Vaticanus as far as the uncials go, but the OP's sense that there may be more ancient mss available for some texts seems to have merit. Although I agree it's broad, we have several quite broad questions like this that have turned out to be of general interest.  Textual criticism is on topic, as are the ancient versions.
Comment: This, in my opinion, is not off-topic. Historical context (with regards to a particular text) is on topic as are question about  the history of a biblical text. This should have 1 or only a few answers and answers will be fairly obvious, and straightforward.
Comment: @Susan Thanks for the correction, and for all you do here at BH.SE.
Comment: This question is completely outside the rules of this forum - BUT please make an exception and not close it because the answer should be interesting and helpful. I even upvoted it.
Comment: I don't see how this question doesn't meet forum guidelines.  The Septuagint is a Greek translation of the Old Testament.  One of the 5 on-topic areas is "translation of Biblical texts" (see http://hermeneutics.stackexchange.com/help/on-topic).  @C. Kelly am I missing something?
Comment: My opinion is rather moot, now that the question has been here 5 months.  And you're mainly right, though I suppose BlessedGeek disagrees still.  So I deleted the comment.
Answer: As alluded to in the question, the primary (mostly) complete witnesses to the text of the Septuagint are codices bound up with the Christian New Testament. Pride of place goes to Vaticanus in which we have a nearly complete Greek OT. Sinaiticus and Alexandrinus also provide valuable witness. These codices have been dated to the 4th Century CE.
Unfortunately, the Great Uncials are far from perfect witnesses. As is well-known, revisions of the Old Greek translations probably began as soon as the translations had been made and culminated in the work of Origen in the 3rd C. CE. Origen's edition has been called "a monument to misguided industry" in recogntion of the challenge it introduces for modern text critical work.1  Although Origen himself used signs to mark changes in the received Greek text, these were often not preserved. Most subsequent Christian copies of the LXX are a mixture of the original Old Greek text and the editorial work of Origen. Manuscripts that pre-date Origen, then, are gold for LXX textual critics. This is all just to say: the OP's question is an important one!
The available manuscripts (all written on papyrus) fall into two groups. I have included here specific mention of some of the most significant along with brief notes about the nature of their import. They can be considered two groups:

Judean Desert:  The most substantial group of papyri are fragments found among the so-called Dead Sea Scrolls. Although the majority of the DSS are Hebrew and Aramaic documents, a small fraction are in Greek. Among these, most are "documentary" rather than "literary"; that is, they record everyday transactions of a Greek-speaking community. However, a small collection of Greek texts, mostly from Caves 4 and 7, provide a witness to the Old Greek Bible.2
a. 4QLXXLeva:
Lev. 26:2-16, late 2nd C. BCE.  A small number of lexical discrepancies between the Leviticus manuscripts and later available LXX text has led Emanuel Tov to conclude that the 4QLXXLeva represents an earlier version of the Greek text, probably a freer translation of the Hebrew. Notably, John W. Wevers, in producing his Leviticus volume of the Göttingen edition of the Septuagint, did not select any of these readings.3
b. 4QpapLXXLevb:
Lev. 2-5 with lacunae, 1st C. BCE or CE. These fragments are best known for their (to my knowledge) unique representation of the Hebrew Tetragrammaton using the Greek transliteration ΙΑΩ  (iaō) rather than the more common translation (but see 2b., below) κυριος (lord).
c. 4QLXXNum, 4QLXXDeut, etc:
Portions (1-3 vv. each) of other Pentateuchal texts. These are fragmentary and of uncertain text critical value.
d. 8ḤevXII gr:
Portions of the Minor Prophets, 1st C. CE. Arguably the most important of the Greek scrolls from the Judean desert, this scroll includes remnants of 25 columns of the Greek Minor Prophets, together comprising more text than all of the Pentateuchal fragments above. This provides evidence (according to Barthélemy, who published pioneering work on the Scroll) for an intermediate stage of the text (termed the Kaige revision) which dramatically changed our understanding of the history of the Greek text.4
Other Papyri: These are mostly fragmentary bits of Pentateuchal texts, mostly found in Egypt. Among the most substantial:
a. Papyrus Fouad 266:
Portions of Deuteronomy, 100 BCE. Known for its representation of the tetragrammaton using square Hebrew script.5
B. Papyrus Rylands 458:
Approximately 20 scattered verses from Deuteronomy, c. 150 BCE. This is the oldest available manuscript of the Old Greek Bible. In addition to its antiquity, it is known for its blank spaces in place of the tetragrammaton, which have been the subject of much speculation.5

This is by no means a comprehensive compendium of witnesses, and the nature of the evidence is subject to change as further papyrological and textual evidence emerges, but hopefully, this has been enough to answer the OP's question are there any scrolls and papyri or are there only codices? in the affirmative.6
Notes

1. John W. Wevers, as cited in Jobes, below, p. 53.

2. These are mostly not Qumran documents. Naḥal Ḥever and Jericho provide the largest fraction. For this and more: Emanuel Tov. The Greek Biblical Texts from the Judean Desert from Hebrew Bible, Greek Bible, and Qumran (Mohr Siedbeck 2008), Chapter 23.

3. This per Ulrich (The Septuagint Manuscripts from Qumran: a Reappraisal of their Value in Septuagint, Scrolls, and Cognate Writings [Manchester, 1990]), who offers specific analysis of each of the variants, many of which he (with Tov) considers likely original. A more detailed critique of Wever's decisions (and further support for the witness of 4QLXXLeva) can be found in Sarianna Metso and Eugene Ulrich's The Old Greek Translation of Leviticus in The Book of Leviticus: Composition and Reception Ed. Rolf Rendtorff and Robert Kugler (Brill, 2002).

4. Explaining that statement would, unfortunately, require a book -- one which I am not qualified to write, and one which would not fit here anyway. See either of the introductory texts below for an overview.

5. Both the Aramaic letters and the spaces are now widely (?) considered secondary. See also, a BH.SE answer about the representation of the tetragrammaton in the LXX.

6. With regard to the doublet scrolls and papyri: while we have been discussing papyri (the type of material used), these documents were almost certainly also originally in the form of scrolls (the codex being a later invention), but because the manuscripts are mostly fragmentary, they are not patently scrolls.

In addition to Tov's work (n. 2), this answer was largely gleaned from:

Karen Jobes and Moises Silva. Invitation to the Septuagint Baker Academic, 2000. (Link is to the newer edition which I do not currently have access to in full.)

Jennifer M. Dines The Septuagint (T&T Clark, 2004).

Comment: Excellent answer. I was reviewing this now and this question came up in my mind... So which of the ones you've mentioned would definitely be the Septuagint? Since it appears that at least some of these are from a different Greek text.
Answer: Susan has given a typically well-informed answer to this question, and Noah has expanded on it with his link to Hurtado. Allow me to add one small word of caution: There is a wide-spread tendency of New-Testament scholars to date Bible manuscripts solely on the basis of their handwriting to slots of typically 50 years (e.g., "first half of the 2nd century CE"). Scholars working on "profane" Greek manuscripts of the same period do not as a rule accept that it is possible to date manuscripts so precisely merely on the basis of handwriting. There is a fundamental methodological problem here. In my own small area of professional expertise (Syriac and Arabic manuscripts) I would also be very hesitant to date any manuscript to a half-century slot unless it contained a colophon or some other clear-cut dating.
Answer: Larry Hurtado made a table of all known 2nd and 3rd century Christian manuscripts together with what form the manuscript takes.  This includes manuscripts where it is unclear whether it is Christian or Jewish in origin and some definitely Jewish texts for comparison.  I think that means it likely includes all LXX manuscripts.  At any rate it gives a large number of partial LXX manuscripts that predate Vaticanus.
(The main purpose of this table is to provide evidence for Hurtado's assertion that early Christians had an unusual and strong preference for the codex over the book roll.)


For certain locations, why is "au" used in place of "à"?

Question: So for example, why is it "Je vais au travail à pied" instead of "Je vais à travail à pied"? Doesn't "au" mean "to the" in this case? So "au travail" means "to the work" when I thought we were trying to say "to work."
Comment: Je vais [au = à le] travail → *aller à* (je vais à mon travail) —  [à = au moyen du] pied : « Aller à pied » utiliser le pied comme moyen d'action.
Comment: You're right, "*au*" means "*à le*" here, and French for "_work_" is "_**le** travail_". Many words that take no article in English take one in French (*life* is *la vie*, *death* is *la mort*, *love* is *l'amour*, ...)
Comment: @Teleporting goat > It's more that French requires to use the articles more than English does, was it for grammatical reasons or so that you can also get the gender right. "The life is beautiful" is, I guess, correct, while "Life is beautiful" is corect too but also much more idiomatic. French for "work" is "travail", "life" is "vie", etc... but when you actually use these words in a sentence most of the time you indeed use an article, but not always. E.g.: "Garantie à vie","Condamné à mort","Peine de travail",...
Comment: @LaurentS. Of course it's a little more complex than what I said. And words like life, death and love can have articles in English, but in pretty specific conditions. Like "The death of X" or "The love of a mother". It's specific, for one instance of death or love in particular. I'm pretty sure "The life is beautiful" is incorrect, you can't use "the life" for "la vie" as a general concept. You can say "The life you're living is beautiful" though.
Comment: @TeleportingGoat > My bad then, but still this is just a matter of rules on using or not the article in English VS French. I'll look into "The life is beautiful" correctness although I trust you on this as my English is certainly not top-level...
Answer: travail can mean the workplace, not just work in the sense of something one does. Per TLFi:

P. méton.

a) Lieu où s'exerce cette activité. Synon. atelier, bureau, chantier, usine.Aller, être au travail; sortir du travail.

Hence, you should think of it as the French equivalent of the substitution of "work" for "the office," "the factory," etc. (Compare I'm going to work <=> I'm going to the office) Here, work is a location, not an indication of method as is the case in the phrase à pied. (Example for à pied from TLFi: Je remontais à pied les Champs-Élysées.) Consequently, a definite article is necessary, and as the article is le, it is combined with à to form au.
Observe also that this also applies to other expressions that normally would not use a definite article in English. For example:

Je vais à l'école. I am going to school. (But, word-for-word: I am going to the school.)
Pourquoi étudier à l'université? Why study at university? (Refers to the pursuit of higher education, not attendance at a particular university.)

In these cases, we are arguably referring to work, school, or university as abstract general concepts. French typically requires articles for nouns unless they are modified by determiners or possessive pronouns, and for general concepts, the definite article is used.

Mais les femmes sont toutes belles. But women are all beautiful. (Refers to women in general, not a specific group of women.)
L'amour est un oiseau rebelle. Love is a rebellious bird.
Comment: "*work*" is "*le travail*" not just by metonymy for "*the office*". Ex: "*Allez, au travail !*"
Comment: @TeleportingGoat Good point, I was focused on "le travail" as location because of the example and hadn't thought to that.
Answer: (note that I would have commented directly under the post if StackExchange would let me do it...)
Although Maroon's answer is correct, Maroon also misunderstood the question. I think the OP did understand that "travail" here was a location, the question is why is it "au" and not "à" for certain locations. Plus, in English "work" is also

a place where someone goes to do their job

as per what Macmillan Dictionary says.
So basically yes, you would need an article (le) in French, and à + le is au. Articles are nearly always required in French.


Move from HTTP to HTTPS and crawling requests

Question: I have moved my web site from http to https, http is fully redirected to https and they are identical in content. I have added both properties to Google Search Console, when I ask Google for crawl (Fetch as Google option in Search console) do I do it for https or for http or for both?
Comment: You don't need to ask.  Googlebot is always crawling.
Comment: With "ask" I have meant - to use "Fetch as Google" option in search console
Answer: From here on, you should only do these kind of checks (fetch-render, sitemap, crawling, mobile usability etc..) in the HTTPS version of the GSC property.
Side node: You should only monitor the HTTP property in GSC to see if traffic decreases in time. And at some point, you should have no traffic data in the HTTP property (assuming HTTP site is correctly redirected to HTTPS)
Answer: If redirecting properly, Googlebot can't crawl the http site. If you have a fresh sitemap of the new https site, I would upload it under the https property, and check back in a few days. Always keep check for errors and suggestions that Google Search Console gives you.


Am I allowed to study and read Ibn Qayyim's books without an Ustadh?

Question: Am I allowed to study and read Ibn Qayyim's books without an Ustadh? What about reading ahadith and the Quran (translation)? What about tafsir?
Comment: These are many questions in one post (take a look at [why we should avoid multi-question questions?](https://islam.meta.stackexchange.com/questions/2965/why-we-should-avoid-asking-multi-question-questions)). One of these questions is discussed here [read quran with tafseer alone](https://islam.stackexchange.com/questions/33343/read-quran-with-tafseer-alone). The overall answer might be the same, but the details might end up in different answers.
Comment: What is your purpose of studying Islam? The answer will depend on whether your goal is to know your religion, or to know about someone else's religion, or to become a scholar in the Hanbali school.
Comment: My purpose of gaining knowledge is to increase my iman and become a Muhsin
Answer: Gaining knowledge has many venues, including learning through a teacher (sheikh, Arabic: شيخ) or from books without a teacher. The common point of any method is to ensure that the source (let that be a teacher or a book) is a valid one as Ibn Sirīn said:

إِنَّ هَذَا الْعِلْمَ دِينٌ فَانْظُرُوا عَمَّنْ تَأْخُذُونَ دِينَكُمْ
Indeed this knowledge is faith, so carefully consider from whom you take your faith.
— Sahih Muslim (Introduction)

There is no doubt that Ibn Qayyim al-Jawziyya is one of the greatest scholars of Islam in jurisprudence, tafsīr, and hadith. Suffice it to say that he was the student of Ibn Taymiyyah, and his students included Fairuzabadi, Ibn Kathir, and Ibn Rajab. Studying the works of Ibn al-Qayyim will actually cover all the areas you inquired about (including tafsir and hadith).
One has to keep in mind, though, that reading books may result in some faults in knowledge either through misunderstanding of the written text, overlooking the context, or a multitude of other reasons. One should always seek clarifications when in doubt, and one should keep in mind that even without ambiguity, one's understanding may still be incorrect.
Imam Abdur-Rahmān al-Awzā'i used to say that the knowledge about Islam used to be an honor that people learned from each other. When books came into the picture, those who are not qualified started to have an opinion.
The point of Imam Al-Awzā'i is that without a teacher, there could be potential errors that do not get corrected. The most dangerous ones are not when the person is not clear on the meaning of the text (they may eventually seek clarification); rather, when the person thinks they fully understand the text when in reality they do not. It is rather difficult to guess what the reaction of Imam Al-Awzā'i would have been to the Internet and Google.
Comment: AS Imam a-Shafi'i said: The great Imam ash-Shafi R.A', he went to his teacher Waki` *
Complaining about the weakness of his memory. **
He told him, 'abandon rebellion, for knowledge is a light *
And the light of Allah is not bestowed upon a rebel.'  شكوت الى وكيع سوء حفظي *
فارشدني عن ترك المعاصي **
واخبرني بان العلم نور *
ونور الله لا يعطى العاص


Grocery Shopping by bike, expanding my cargo options cheaply

Question: Can I take a shopping cart like say this one and safely attach it to a bike? I'd like to also take it into the store with me as well.
Comment: I took a child trailer and renovated it into a little wagon that I attach to my bike. I took off the canvas, cut the top rails down and it works really well.
Comment: Not something I would pull on a bike.
Comment: How much groceries are you carrying? More than fit in 2 panniers and/or a milk crate bolted onto a bike rack? What kind of bike do you have?
Comment: I have broken such a cart by hitting a crack while pushing laundry down the sidewalk. I don't think it would handle a pothole at 15mph (or 7mph, for that matter) very well.
Comment: With those little wheels, hitting even a small bump would upset the cart and might make it fall over sideways.  If you have really smooth roads and road/driveway transitions, then it might work.
Comment: Make more trips. Eat less. If you're shopping for others, get them to get out on their bikes and help you haul.
Answer: I think you would be much better off converting an old kids trailer.
They can be found pretty cheap on Craigslist.
Answer: There are trailers in a similar form factor, like the Burley Travoy (no recommendation, just the first I found), that are made for the purpose. These will be a lot more stable than any home made conversion of something that was never designed to go faster than slow walking pace.
Comment: I've seen a sack truck modified to something rather like a travoy, which looked great for large items (it had a dismantled small garden shed on it) but wouldn't be up to much for groceries. Does the travoy have any clever way of stacking small bags. Otherwise a kids' trailer modified or otherwise would be better (as well as having a lower centre of gravity).
Answer: For my daily commute and for moderate sized weekly grocery trips I use two waste baskets, permanently mounted, each with 6 gallon capacity.
They serve as waterproof hard shells into which I can insert any bags, backpacks, clothes etc, that I want.
Answer: I go grocery shopping with my bike all the time, I put groceries in touring panniers mounted on a rear rack.  They are a little cumbersome to load but I can fit a lot of stuff.  You can't fit a week's worth of groceries for a family of 5, though, so it depends on your situation.
For a solution more elegant for shopping, you can buy square-shaped shopping panniers which will more easily allow you to carry canvas shopping bags full of groceries.  These are very popular in Holland where bikes are a primary mode of transit. (http://dutchbikeshop.ie/wp-content/uploads/wp-checkout/images/fastrider-pannier-cargo-1423825156.jpg)  One word of caution for most bikes in the US-- you'll have to watch out for heel clearance with a square bag like this.
If you're serious about hauling a lot of stuff then you should get a cargo bike, like the Surly Big Dummy or something with a frame-mounted front rack (Workcycles).
Answer: There are several manufacturers who offer rod supported quick connect pannier bags which you can easily detach from your bikes rear rack, shop and load them, then rehook to the bike and ride home.  They work great and are inexpensive.  One such company is Performance Bicycle.
Answer: I agree with Blams comment, I don't think you would want to pull that cart behind a bike. Since you still want to take it into the store, maybe use that cart & mount it on a flatbed trailer with bungee cords. Something like this maybe: http://www.burley.com/page_453/flatbedsuptmsup
Answer: Many grocery stores do deliveries. There's also taxis. Living without a car and having to get groceries for my family has taught me that bicycles aren't always the best solution. About once a month plan a big trip and get a list of all the heavy, non-perishables items you will need. This will include canned goods, frozen meats and vegetables, dried goods like pasta or rice, and other non food items like laundry soap. The other stuff you need to bet fresh doesn't weigh that much. Exceptions are things like milk, but sometimes I make a special trip just for that. It's an excuse to ride my bike more, and I just go to the corner store because it's only a short trip, and it's the same price everywhere. Also, learn to shop light. Don't buy liquids like pop or juice. Water is healthier anyway and comes right out of a pipe fed into your house. If you want juice, try frozen concentrate. You'll spend money on deliveries, about $8 a shot here, but you won't have to buy a trailer, and surely your bike will appreciate not having to tow heavy loads.


Will my Windows OS be kept if I install Ubuntu

Question: I use Windows 8 Consumer Preview, and since it is going to expire, I wondered if I can keep Windows 8 when I install Ubuntu or not, because I want to still use Windows 8 as well as Ubuntu.
THANK YOU!
Answer: You can choose to run both Windows and Ubuntu together, on the same machine. This is called Dual-booting.
The easiest and safest way is to create an empty drive in windows, delete it to get free space, and install Ubuntu using Live CD or Live USB, choosing the option 'Install alongside Windows'. The setup will automatically install into the free space created by you.
Even if you directly boot from cd without creating free space, The installer will attempt to resize your windows partition and will install alongside. But there is one in a thousand chance that this may cause data loss, since NTFS file system is not effectively manipulated from Ubuntu (yet), I suggest making free space from windows before proceeding.
Answer: yes you can . after inserting the LIVE CD , moving with regular steps , stop at this .
so there 1st and 3rd options are good for you to keep windows and install Ubuntu .Remember dont touch the WINDOWS partition while doing the installation process  and remaining all are same as a normal installation process .
All the best .
But as our one of the answer saying its not possible with win-8 , i too heard about win-8 is going to behave like as he said but give a try my friend ,we loss nothing with a try .
Comment: it works perfectly, atleast for now. we need not worry until win8 is officially released. and there is still a chance that it might never be implemented. UEFI is much debated thing these days.


Fragmentation of phenyl radical cations in mass spectrometry

Question: What are the main fragmentation pathways and products of a phenyl radical cation in mass spectrometry ? I notice sometimes that there's a m/z = 51 peak after the m/z = 77 for phenyl radical cation; I assume due to loss of acetylene.
Comment: Can you give more details?  What type of ionization?  What is the source of the phenyl radicals?  What types of collision cells etc. are used in the mass spectrometry you're thinking of?
Comment: Phenyl radical source: benzylic acid; I'm not familiar with the experimental method.
Answer: What about $\ce{phenyl^{*+} -> Dewar\ benzene^{*+} -> cyclobutadiene^{+*} + C2H2}$?
Comment: That's definitely a plausible pathway, thanks !
Answer: Without more experimental details, it isn't 100% clear that the small peak is exclusively derived from the phenyl radical cation.  It also isn't clear that observed the "phenyl" radical cation is actually cyclic!  It could be another isomer of $\ce{C6H5^{+.}}$.  With benzilic acid as the ion source, it is possible that the small peak contains an oxygen atom, although searching for possible formulas with appropriate "exact" masses makes me think this isn't true (see below)
The fragmentation spectrum of benzoic acid shows similar peaks.  The "exact" mass of the observed ion 51.0237 Daltons, within a milliDalton of the mass of $\ce{C4H3+}$.   The negative mode fragmentation spectrum of benzoic acid shows a corresponding ion at 49.0084, corresponding to $\ce{C4H1-}$ within a mDa.
I think the existence of something like $\ce{C4H1-}$ in negative mode supports Klaus's answer of cyclic structure because it's hard to see how else such an "unsaturated" cluster of carbon atoms can form.  Perhaps a tetrahedrane-type structure is even possible?
The easiest way to get more information on this ion's structure would be to fragment a variety of partially deuterated benzene rings.
Update: I found a paper talking about fragmentation of fluorobenzene, and they argue the "phenyl" cation fragment derived from that molecule may not be cyclic after all:

In combining these approaches, several mechanistic variants can
  be excluded, while a new, hitherto not considered pathway
  involving the formation of acyclic $\ce{C6H5+}$ is proposed.

I don't know if the same thing happens during the fragmentation of benzilic acid, but if so, then I wonder if the observed peaks could be protonated butadiyne (positive mode) or deprotonated butadiyne (negative mode).


MDADM not able to connect to EBS drives on Ubuntu 11.10

Question: I have been using a raid.sh script [1] for over a year now and has never gave me trouble. I recently started moving from Ubuntu 10.04 machines on AWS to Ubuntu 11.10. I updated the raid.sh script from the old sdF style drive names to xvdF. Since doing that, while attaching my 4 EBS drives, mdadm is getting a "device is busy" error on some of my drives (say 1 of 4) when using brand new drives, and on all 4 of 4 drives when trying using volumes created from a snapshot. I have attached the errors below as well [2].
After trying this, and the device being shown as "busy", when I properly make sure everything is unmounted and stopped via mdadm and detach the volume using AWS web console, the drives will be stuck on detaching and sometimes even say "busy" forever.
Any light on the situation would be greatly appreciated, thanks!
[1]
<code>#mdadm == Raiding software
#xfsprogs == XFS filesystem creator/manager

sudo apt-get install -y mdadm xfsprogs;

sudo mdadm --create /dev/md0 --level 0 --chunk=256 --metadata=1.1 --raid-devices=4 /dev/xvdf /dev/xvdg /dev/xvdh /dev/xvdi;
echo DEVICE /dev/xvdf /dev/xvdg /dev/xvdh /dev/xvdi  | sudo tee /etc/mdadm/mdadm.conf;
sudo mdadm --detail --scan | sudo tee -a /etc/mdadm/mdadm.conf;

sudo mkfs.xfs /dev/md0;
echo "/dev/md0 /raiddrive xfs noatime 0 0" | sudo tee -a /etc/fstab;

sudo mkdir /raiddrive;
sudo mount /raiddrive;

sudo blockdev --setra 65536 /dev/md0;

df -h /raiddrive;

sudo chown ubuntu:ubuntu /raiddrive;
</code>
[2]
<code>mdadm: super1.x cannot open /dev/xvdf: Device or resource busy
mdadm: /dev/xvdf is not suitable for this array.
mdadm: super1.x cannot open /dev/xvdg: Device or resource busy
mdadm: /dev/xvdg is not suitable for this array.
mdadm: super1.x cannot open /dev/xvdh: Device or resource busy
mdadm: /dev/xvdh is not suitable for this array.
mdadm: super1.x cannot open /dev/xvdi: Device or resource busy
mdadm: /dev/xvdi is not suitable for this array.
mdadm: create aborted
DEVICE /dev/xvdf /dev/xvdg /dev/xvdh /dev/xvdi
ARRAY /dev/md/ip-10-4-237-174:0 metadata=1.1 name=ip-10-4-237-174:0 UUID=151438b6:98297ef2:e40fb2e3:a1ee73f3
</code>
Comment: There is some bug in the Networked Drive -> EBS abstraction where latency delays cause mdmadm to report the drive as busy.  @rpkelly is correct, just retry several times until it works.  Actual drive performance seems unaffected.

I would set a retry limit of 5, it usually takes 1-3 times to succeed.
Comment: I've been having this same problem. It's been driving me crazy. Currently we just start over and keep trying until it succeeds... eventually.
Answer: This link answers it:
http://dev.bizo.com/2012/07/mdadm-device-or-resource-busy.html
Comment: Welcome to Server Fault! Whilst this may theoretically answer the question, [it would be preferable](http://meta.stackexchange.com/q/8259) to include the essential parts of the answer here, and provide the link for reference.


What is a good measure of self-regulation?

Question: Is there self-regulation or self monitoring test. I am interested in measuring why people are not commited to their goals (quit smoking, dieting etc) so you can suggest another measure instead of self-regulation failure.
Comment: Both, and only the former applies. [Self-monitoring](http://en.wikipedia.org/wiki/Self-monitoring) is a different construct. As for self-regulation, have you considered checking any of the references on [Wikipedia](http://en.wikipedia.org/wiki/Self-control)?
Answer: You can also use a stop signal task that measure response inhibition. It has been associated with dependencies like smoking http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3077266/ and http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0018898 for a go/nogo task, though the stop signal task is preferred. There is also some studies linking inhibtion to self regulation and self control, although I dont remember the sources at the moment. Sorry for poor formatting, new here at s.e.
Comment: I agree with this approach insofar as the effects of such behavioral tasks typically exhibit less variability than questionnaires.  I would also suggest stroop and Simon tasks, although my first bet would be Go/NoGo as well.
Comment: Go/NoGo is good for neuroscientific research which measure moments, but I am looking for something whicha are measuring past in much wider sense.
Comment: All for its uses :) Though if you want to implement some sort of training paradigm or look at small group differences, adding a stop signal task might give more sensitivity. And to the go/nogo users out there, stop signal task is regarded as the more "pure"/valid option, regarding inhibition. See the later articles by logan/verbruggen.
Answer: You could look at the Self-Regulation Questionnaire. See here for a summary of the instrument in terms of what it purports measure, items, response scales, psychometric properties such as reliability and validity.
References
Brown, J. M., Miller, W. R., & Lawendowski, L. A. (1999). The Self-Regulation Questionnaire. In L. VandeCreek & T. L. Jackson (Eds.), Innovations in clinical practice: A source book (Vol. 17, pp. 281-289). Sarasota, FL: Professional Resource Press.
Comment: Looks pretty solid to me! I guess I'd prefer seven Likert rating options over the five they use, but otherwise, looks promising. Wish I had a nice big dataset to play around with, since they don't seem very clear on how to score it...A little MIRT analysis could fix that though! Saved it to my archive of measures for someday when the world is my respondent recruitment oyster...Thanks!
Comment: This questionnaire is exactly I was looking for.
Answer: Also consider more fundamental personality differences such as Conscientiousness, "...the personality trait of being careful, or vigilant. Conscientiousness implies a desire to do a task well, and to take obligations to others seriously. Conscientious people tend to be efficient and organized as opposed to easy-going and disorderly. They exhibit a tendency to show self-discipline, act dutifully, and aim for achievement; they display planned rather than spontaneous behavior; and they are generally dependable." https://en.wikipedia.org/wiki/Conscientiousness
It can be measured along with other major personality factors in the Big Five Inventory (BFI): https://www.ocf.berkeley.edu/~johnlab/bfi.htm


Fight or go for a inhibitor when the enemy team is doing Baron

Question: The enemy team is doing Baron. Both teams are equally strong, but you see a opportunity to get a inhibitor in exchange for a Baron. What is the best course of action?
Answer: To be quite honest, there are WAY too many variables at work here to be able to give you a straight answer.
The simple/quick answer? It depends.
A few variables for example: Gold lead, Positioning, Lane push, Character lineup, among tens of others of things.
If there was always an easy answer, there would be far less "comebacks" than you see already. In some cases, if you feel like your team can take an inhib safely AND defend/turtle during the duration of baron, it could work.
PERSONALLY, I feel like giving baron is far too much. Even in exchange for an inhib, Baron is game ending. If a team is any good, they'll maximize their advantage of Baron and either force a teamfight (which if they win will result in you losing mutliple inhibs) or just split push you and take turrets/inhibs that way. Again this is all hypothetical, if you could give me a more specific scenario, I could help you out :)
Answer: Inhibitor will usually be worth more than a Baron. But if you wait to initiate while they are fighting baron there is a chance for a baron steal + killing stragglers. If your team comp helps to get a jump on the enemy team it might be beneficial to fight at baron.. but most of the time an inhibitor will be worth more as you can still defend your base afterward and wait out the baron.
Answer: I'm going to have to say it depends on time of the current game. Early/mid game it is normally worth it to take out an inhibitor due to it forces the team back to defend that lane and they don't have the pushing power to retaliate on your base inhibitors while baron is still in effect (assuming teams are near equal). However, late game letting the enemy team get baron can often prove fatal due to the pushing power and finished builds can easily cause the lose of the game as turrets can be tanked and they will have a large team fight advantage.
Answer: Fight them because if they get the baron buff they can probably win a team fight if they are smart on positioning baron buff is one the things u need to have control of in order to win so in the long run baron>inhibitor if u are certain u can win a baron fight or steal a baron do it u can always get an inhibitor after the fight


Does Non Repudiation offer proof of receipt ?

Question: I understand that non repudiation intends to provide a mechanism that reliably proves that the sender of a message cannot deny sending the message. But does it also provide similar mechanisms to ensure the recipient of the message cannot deny receiving it.
Comment: It can work both ways. Sender A signs a message and send to receiver B. If a proof of receipt is need then we need another extra step - Receiver B signs the return message and send it to A, indicating that he has received it.
Answer: Non-repudiation binds the sender, not the recipient.  I could, for example, digitally sign a message intended for you.  I wouldn't be able to deny composing the message, but suppose I never sent it?  You'd never get it, even though it was digitally signed.
I can't think of any mechanism not requiring the cooperation of the recipient that proves receipt of a message.  (There's the email return receipt, but a recipient can disable that.)
Comment: I agree. You can bind the recipient (proof of receipt). It gives you non-repudiation of receipt. Often this is just a signed message that says "i received the following message: ...". It always has to involve the recipient, as he is the one to sign the answer.
Answer: Non-repudiation doesn't just involve sending messages. It can also mean proving a user performed an action. Let's say a recipient receives bitcoins from a sender, but moves the bitcoins to a remote location. He then says that he did not receive them and asks the sender to send them again. Non-repudiation will be able to prove that he moved the bitcoins (via technologies like logging). Hence non-repudiation, in a way, prevents the recipient from denying he received the message.
Comment: Bitcoins aren't sent and received like an email, they are transferred atomically by transactions on a public, distributed, (nearly) tamperproof log called the block-chain. If your claimed payment to me is "confirmed" on that log you did pay (even if I deny it), if not you didn't.


British War Medal

Question: Could you tell me where I might apply for my grandfather's British War Medal his name is John Percy Blackburn, he joined the NREF in Murmansk 1918 Middlesex Regiment and was a 2nd lieutenant?
Answer: As Antony states, WW1 Campaign Medals can no longer be issued on behalf of those who never received them. According to the MoD site describing  eligibility for WW1 Medals

All World War 1 medals were issued to the recipient or next of kin after the war ended. You can only apply if the medal was returned, and you can provide a medal card or roll which includes this information.

Note also

If the veteran has died, you must be the official next of kin. The general rules are:
the person's spouse or civil partner has first claim to the medal, and then the eldest child
if there's no spouse or child, the eldest grandchild is entitled to apply

In general terms, it might have been that he never applied for them in the first place. Officers in WW1 had to apply for their campaign medals - unlike enlisted men, who got their's sent automatically. You need to check the Medal Index Cards (Ancestry or TNA) to see if he applied or not. For instance, if I recall correctly, JRR Tolkien, a lieutenant in WW1 who saw action, must not have applied as he isn't in the Medal Index Cards.
The other thing to bear in mind is that eligibility for the Russian campaigns is complex and he might not have been entitled. I'm still not totally sure, for instance, why my G-uncle, in the Caucasus post 1918, wasn't entitled.
In this instance (thanks to @AndyW for the prompt) we can see from the Medal Index Card that

2/Lt JP Blackburn applies for BW Medal 14-9-19

At this point he was at 9 Cann St, C-on-M, Mancester. However, the medal appears to have been returned 1-8-20 (or that might be the date of the annotation). Why it was returned, I don't know.
Comment: There are likely medal cards for John Percy Blackburn on Ancestry that suggest he did apply for the British War Medal in 1919. The rest is hard to parse, but I think it says that they were returned. So there's a chance of eligibility, but the records will have to be checked by someone who understands them, i.e. not me :)
Comment: Thanks @AndyW - I have editted my reply to match. I also agree that I cannot assess eligibility either.
Comment: I don't understand the Medal Roll (not Index Card, the Roll itself) for him on Ancestry, which describes eligibility for *both* BWM & VM with entry to North Russia theatre 30/5/18. So why is the reference to just applying for the BWM? This is where my knowledge falls short but the MoD Medal Office needs to be involved.
Comment: TNA's guide is here: http://www.nationalarchives.gov.uk/help-with-your-research/research-guides/british-military-campaign-and-service-medals/ "British War Medal  For service abroad (including India) 5 August 1914 – 11 November 1918, or 1919-1920 in Russia." so I couldn't say why your relative wasn't entitled.
Comment: FYI Paul Nixon has decided not to add new posts to his British Army Medals blog but will leave the old content up: http://britisharmymedals.blogspot.com/
Answer: Are you sure he was never issued with it? Check the medal cards on Ancestry to check.
I applied for my Father in Law's military record from WW2 and when the papers came back they said he had never applied for his medals and included a form for my wife to sign if she wished to claim them. They arrived in the post a couple of weeks later.
You can probably do the same thing for WW1 but bear in mind that not all WW1 military records survived, again those that did are on Ancestry.
Answer: According to the MoD website, you can only apply for WW1 medals if the original was returned:
https://www.gov.uk/apply-medal-or-veterans-badge/apply-for-a-medal

British War Medal


Is it generally a bad idea to have other types of virtual appliances installed along side a firewall appliance?

Question: I want to run my Firewall/NAT software (pfsense) and an internal NAS (looking at freenas right now) for my SOHO on one machine. Right now I have them separated on two different machines, but I'd like to consolidate them. Is this generally a bad idea? I see the security concern where if the firewall or host OS is compromised, then your data is essentially screwed. But is it really a concern for me?
Answer: How valuable is your data?  Machines are cheap.  Your time isn't.
I would tend to use separate machines to keep configuration and maintenance as simple as possible.  Plus getting to your data requires two break-ins, not one.
Answer: For a small office use, as long as you're not pumping a large amount of traffic through it, I don't think it's a bad idea necessarily from a security standpoint so much as it would be a bad idea from a redundancy/reliability standpoint. Make sure you have a good backup routine in place!
Personally I like separate machines just because if one is fried, it doesn't take everything down. VM servers save energy, noise, and management headaches, but do create a single point of failure, so it's something you need to consider when budgeting; if you were to completely lose your computer tomorrow, how much is it worth to you to have that data back? That should be how much you're willing to spend on backup hardware.
For security, while there could be some concern argued, practically speaking you can set it up with networking in such a way that it will be partitioned off well enough to prevent most attack problems you're likely to encounter as long as you keep things updated and monitor the logs (and make decent backups, of course). The only other security issue I'd see is that generally you can't trust a machine to monitor itself; something gets compromised then it can report what it wants for logs and data. An external system doing IP monitoring/logging/etc. is more reliable at monitoring what is happening inside your network (unless the firewall system is what's compromised, but many appliances can be run from a read-only image with just a data partition to save logs/cache to).
Again, from what you described, I don't think you'd have much to lose in consolidating, and probably will gain more in the long run, as long as you have a plan in place for backups and spare equipment if necessary for getting your VM server online after hardware failure. You don't mention the VM server you're considering but I had assumed, perhaps incorrectly, you're thinking of something like VMWare ESXi or a Linux "bare metal" hypervisor? It's generally less of an attack surface to run a bare-metal hypervisor instead of running it on top of a regular server or workstation OS...
Comment: I was looking at ESXi and Xen. I wanted to have about 3 or so appliances installed, so I'm probably going to end up leaving the firewall machine alone, and virtualizing the fileserver to handle a few more internal tasks.
Answer: That is up for you to determine.  Anything that is directly Internet connected, even if a firewall, is in most cases going to be one of the first points of compromise.  The same could be said for any internal network server that is NAT'd, which would essentially make that segment a type of DMZ.
In SOHO environments, I often like to have the main border router/firewall and file server as separate servers.  Even so, depending on resources available, I might do otherwise.  SOHO and enterprise are vastly different service levels, even for my home office.  I will make compromises personally that I professionally wouldn't encourage clients to make.


Investing in a bench-top power supply

Question: For my projects I have always used hacky power supplies: wall plug + 7805, a USB port or an ATX PSU. I am looking at getting into 3.3 V circuits and I am having trouble getting hold of the necessary regulators in Australia. Can anyone recommend a good lab/bench-top power supply or alternatively a cheap way of getting 3.3 V?
Comment: so, where are you located?
Answer: Decent bench power supplies are quite expensive.  However, you've said that you have used an ATX power supply in the past.  This is what I've done - I've modified an ATX supply to have a switch, an LED and some 4mm sockets on it with +12V, -12V, +5V, -5V and 3.3V.  It's a cheap and very effective way of having a range of voltages available for development.
The ATX supply pin-out is here.
Alternatively, there are a wide range of 3.3V regulators available, including switching ones like the LM2574 or linear ones like the 78L33.  Farnell or RS will ship to most places (and they stock both of these parts); alternatively some bits are on ebay (search for 78L33 and you get a few links from "international sellers") and you can probably get them shipped pretty much anywhere.  Where are you located?
Apologies for the lack of links for Farnell, RS or ebay: it'll only let me add one hyperlink as I'm a new chiphacker.com user!
Comment: This is what I've done and it works quite well.  You need to be careful with your ATX power supply choice, however, as not all of them have the voltages you reference. Mine's missing −5V, which is an optional (now obsolete?) part of the spec.
Comment: I didn't know that ATX had 3.3v in it. Thanks!
Answer: You could use an LM317 and set that to 3.3V.
The standard part for 3.3V would be the 1117 at 3.3V I suppose. It is indeed somewhat hard to get, even here in Germany (for example, Reichelt only has the SOT 223 variant of the LT1117).
As a lab supply I have a cheap chinese 15V 3A unit. That should also be ok, though I prefer the regulator approach.
Answer: For quick prototyping designs with dual rails supply voltage (i.e. 3.3V and 5.0V), I have recently been using a breadboard with an adjustable breadboard power supply. With this, I can use it with a USB plug or with an ac-dc power adapter and a JST-DC adapter, or for more portability, I use a 9V battery with the JST connector. Search "JST" at seeedstudio for the adapters.
Answer: LittleBirdElectronics in australia sells a simple, build it yourself kit capable of taking in DC from a standard 2.1mm center pin positive barrel and outputting selectable 5v or 3.3v. I myself have one and find it extremely reliable.
Answer: I recently picked up a kenwood bench psu for a very low price on ebay as new old stock, it's only 1A 60V but that's quite enough for 99% of things I will ever do and it cost about 10% of it's list price. So ebay might be worth a look.
Answer: I use one of these:
PSU 140
It's very good. The only shortcoming is the sockets, I'd have preferred terminals with sockets.
Answer: The surplus company Marlin P. Jones (http://www.mpja.com) sells a variety
of supplies made by MasTech. I don't know the quality of this brand but
if they meet the specs and are reliable they would be a good value.
The Extech power supplies look good. Same caveats.
Answer: Electronics Express has some power supplies.  http://elexp.com/tst-pwr.htm
They also sell some kits such as this one.
http://www.elexp.com/tst_3010.htm

Investing in a bench-top power supply


Best browser solution for secondary window

Question: I'm writing a web app for accountants managing employee salaries in organizations.
The accountants request is while editing an employee salary form parameters to preview it's
Paycheck report.
The only solution I have is to open the paycheck report in a separate browser window or tab.
But the drawback is that for every employee then it will open a new browser window which is heavy in terms of user experience and OS memory.
Is there a better UX solution here?
Is there some js plugin that can help?
Answer: There is no reason to open it somewhere else. Your window is an endless canvas, open it there. This also probably solves the users' problem much better: they want the information on paycheck to be present while editing salary, but having to switch to a new window, commit the needed information to working memory and come back to the original editing window is terribly expensive in cognitive terms. Showing the needed information side-by-side is a much better solution.
The right hand space would be a standard placement for such a preview. Make your editing form narrow(-ish). Then put the paycheck report to the right. See the Windows Explorer preview feature for MS Office and PDF files as an example of what I mean. There is no Excel instance running in the screenshot, it is Windows Explorer which renders a preview.

Modern programming languages will support this well, so it is easy to achieve programmatically. For example, ASP MVC has partial views which can be rendered in different divs.
If you haven't been doing responsive design, this is an additional reason to start now. Your horizontal space will become much narrower for both the entry form and the paycheck display. The mobile size will possibly need a way to slide from the one view to the other, because you can't possibly fit both on a phone display. On the other hand, if your accountants have to change salary data on a phone, the company has a problem which has to be addressed outside of your application.
Answer: Agreed with Rumi - use your current window.
As far as I see it there are two basic solutions you could use: live preview or preview on request.
Live preview
With this, you have a permanent preview pane in your window, preferably above the fold or at least in the same area as the edit pane. Whenever the user makes a change in the fom you live-update the preview to reflect this. Pros are that it's always there and the user can just look over at it; cons that it takes up screen space and may squash your edit form too much.
Preview on request
This option has you include a Preview button in your form, that when clicked will bring up a preview of the report. This can be done in several ways:

Bring up a modal dialog box containing the preview
Use a lightbox effect and overlay the preview
Open a separate window

As you say, I wouldn't recommed opening a new window. But the other two idea have equal weight and either would work: it is appropriate to open a modal dialog as the user has requested something, and lightboxes are similar. Either way, you would need to recalculate the report as you haven't already got it.
Answer: Pimcore CMS has an example how to solve this. Instead of opening the preview of a page into a new browser tab or window, it has it's own tab (it's build on ExtJs) where the page is loaded into an iFrame (see the image). I doubt users use this very often though, in edit mode the page is already shown as a "real page" but with inputs and drag and drop stuff. I recommend doing it this way so users don't have to switch back and forth, but use the preview button only when they are unsure what it will look like.


How to test Line-of-Sight radio at 5GHz at range 25Km or less?

Question: I'm thinking of buying PowerBeam ac. It's a wireless device to connect two computer networks that are about 25 kilometers apart. But it depended on line-of-sight they say. So is there a cheap way to test for line-of-sight at that distance and frequency? Like a homemade thing or something?
Comment: Binoculars and a signal mirror work pretty well.
Comment: @PhilFrost-W8II if you look at this image from 20km you would see that you want more than a mirror. https://commons.wikimedia.org/wiki/File%3AHorizon%2C_Valencia_(Spain).JPG
Comment: @RowanHawkins If you look at this video from 43 miles (70 km) you can see the mirror works great. And that's without the binoculars... https://www.youtube.com/watch?v=GwCbgQGmID4
Answer: 5 GHz and 25 km range is hard.
Let's do a quick free space loss calculation:
$$\begin{align}
a &= \left(\frac{4\pi d}{\lambda}\right)^2\\
&= \left(\frac{4\pi dc}{f}\right)^2\\
&= \left(\frac{4\pi \cdot 2.5\cdot 10^4 \text{ m} \cdot 5\cdot 10^9\text{ Hz}}{3\cdot 10^8\frac{\text m}{\text s}}\right)^2\\
&= \left(\frac{5\pi}{3}10^6\right)^2\\
&\approx {5.2}^2\cdot10^{12}\\
&\approx 130\text{ dB}
\end{align}$$
130 dB is a lot. Remember, in the ISM band you're normally not allowed to send more than 20 dBm, which leaves you with but -110 dBm at the receiver – not a good thing, considering that thermal noise in a 20 MHz WiFi channel is already $-174 \frac{\text{dBm}}{\text{Hz}}\cdot 20\text{ MHz}= -101\text{ dBm}$, i.e., you'll get ten times the power of the signal in thermal noise.
Hence, your antenna gains must be very high on both receiver and transmitter side, and hence, your equipment must be very well-aligned.
Furthermore, adding to the thermal noise mentioned above, you got the receiver's Noise Figure  (NF) – building a low-NF receiver at 5 GHz isn't easy, either.
So, physics and legislation are stacked against you here: If non-LOS communication over microwave over 25 km would easily work, everyone and their mom would run to use it. But then, the spectrum would be crowded – because each station would "spoil" the spectrum for a 25 km radius.
The fact that the higher the frequency, the higher the path loss, is the only reason we actually have these large ISM bands in the 2.4 and 5 GHz region.
Directional links in the Gigahertzes are thoroughly common; look for the horizontally pointing dishes (typically encased in white plastic) on larger cell phone towers. However, these are usually run with an explicit license for stationary earth-earth directional links, and hence legally do much more power than your AC router. Now, you could possibly build a similar link yourself with off-the-shelf satellite dishes, but I've got no experience nor appropriate license knowledge on doing so on ham bands.
So, coming back to your original question:

so is there a cheap way to test for line-of-sight at that distance and frequency?

well, line-of-sight can be tested by sight! So, good binoculars are your weapon of choice.
Now, I feel like I should also point you to Fresnel zones: it's recommendable to not only have clean LOS, but also a clean first Fresnel zone, to avoid diffracting/reflecting your microwave beam. In other words: this is something to be mounted high if you want to go the full distance!
Aligning those dishes won't be easy, but maybe they come with some straight piece that points exactly parallel to the beam direction, on which you can mount a laser pointer, and tell a friend over the phone to turn the dish until you can see the laser pointer pointing at you.

Another thing to consider: WiFi was never meant to be used over huge distances. It's a carrier-sense system, ie. a station must listen for a predefined time, before it considers the channel free and may device send. That way, the chance of packets colliding in the air is reduced significantly enough to make such systems work without a central instance to coordinate spectrum access in fixed time slots.
This is achieved by first sensing, then sending a short "Ready To Send" packet containing the coming packet's length, which the intended receiver detects, and then again waits for a fixed period, to make sure no one missed the original RTS, before replying with a "Clear To Send". The idea is that all other stations hear at least either the CTS or RTS and know that they need to stay silent for the specified amount of time.
However, if the CTS->RTS mechanism, being completely superfluous for a fixed length, takes 2 times the sensing period in addition to 160 µs of signal flight, the consequences for throughput are dire, because the channel will be used nearly only for waiting and sensing – a full IEEE802.11n symbol is about 4µs the most, and there's about 30 of those in a frame; though a couple of frames can be aggregated under 802.11n, the relation of flight time to active data transmission is unnecessarily bad:
If your wave has to travel 25 km, it'll be in flight for $\frac{d}{c}= \frac{2.5\cdot 10^4\text{ m}}{3\cdot 10^8\frac{\text m}{\text s}} \approx 80\, \mu\text{s}$.
In this scenario, having a system with separate up- and downlink frequencies (FDD), halving the available bandwidth, is significantly more efficient.
Comment: Good points, I added an config example for RTS/CTS control which may be useful
Comment: I think that particular device, when running in point-to-point mode, alters the standard 802.11ac protocol to be more amenable to high latency.
Answer: One way to estimate line-of-sight before you even step outside is to calculate the radio horizon. For the horizon distance in miles and antenna height in feet, the formula is
$$ \text{horizon} = 1.23 \sqrt{\text{height}} $$
For kilometers and meters, it is:
$$ \text{horizon} = 3.57 \sqrt{\text{height}} $$
The horizons of each station add. If you need a link distance of 25km, then you might aim for a horizon of 12.5km on each side. That would require an antenna at height:
$$ \left(12.5 \over 3.57\right)^2 = 12.26\:\mathrm m $$
This is an approximation based on a smooth Earth with no terrain. If a station is on a hill, add that to the height. If there's a hill between the two stations, that may be a problem.
If you want a quick and easy real-world check, a signal mirror and binoculars work well. A true optical line of sight isn't strictly necessary for a working RF link, but it's nice to have if the objective is a high quality, reliable path.
There are other complicating factors which must ultimately be considered, such as atmospheric effects, Fresnel zones, interference, and other practical issues other answers have mentioned. However, if this estimation shows the radio horizon is grossly insufficient, there's little reason to continue your endeavor unless you can secure taller towers or better sites.
Answer: To add to the already given answer.
Most countries have a 100mW / 20dBm ERP maximum for license free usage of those bands. (You will have to check if this is your case, but chances are)
As this is usually expressed in "ERP" which is "Effective Radiated Power" it will take antenna gains into account. Example by using an antenna with 3dBi gain, you will have to compensate your TX-output power to 50mW, to ensure you comply with legislation. This is probably the single most limiting aspect of what you are asking.
However (!!)
If you can design your system, where you stay within your 100mW ERP for TX, but you somehow can improve your RX by means of an antenna with extreme directional gain, you may have a chance.
For this to be successful, you need to use devices with separate TX antenna and RX antenna connections, or use of a "Circular Coaxial Coupler" (for the bands you intend to use)
Lets use a calculation from another answer already given:

So we know that path-loss is about 130 dB.
We have seen the noise figure at -101 dBm.
And we take a maximum of 20 dBm.

But now we take the RX antenna at 40 dBi (huge parabolic dish).
20(dBm) - 130(dB) + 40(dBi) = -70dBm for RX with a -101 dBm noise figure.
Theoretically this should be possible, but you will have to precisely line up the parabolic dishes, you need to have the correct equipment, and-so-on-so-forth.
It would be a nice experiment if you have the equipment laying around... If you have to buy it all from scratch, then my practical advice is: 25 km is just a little to far for bridging networks with "consumer" equipment and staying within the confines of the law.
[EDIT] based on the answer already given: regards "Timing"
Some equipment may have settings to further control the RTS/CTS and other parameters of the protocol, to enable longer distances.
Here a typical screenshot of an EnGenius device:

There are various settings which would help longer distances:
"Data Rate" - the lower the data rate the longer the distance, experimentation needed
"Transmit Power" - the higher the Transmit Power the longer the distance, this should be as high as (legally) allowed (taking antenna gain, coax/feeder loss, into consideration)
"RTS/CTS Threshold" - the lower the threshold the longer the distance, at a cost of bandwidth, experimentation needed: set as high as possible without collisions. This might require monitoring of re-transmits
"Distance" - this should be set as accuarately as possible, as according to this document it affects the ACK timeout on longer links.
"Short GI" - should be disabled, this affects the timing between "characters" on the link, and should be set to standard/disabled (timing 800ns) which prevents/avoids collisions within the same character due to reflections. The longer the distance the more risk of reflected signals you have
"Aggregation" - this should probably be disabled, but experimentation may be needed.
Obligatory disclaimers:

other brands may call settings differently
EnGenius has different models, not all of them available to consumers
I have no affiliation with EnGenius, this is only an example
and... ofcourse... YMMV
Comment: good points! Another aspect is timing... added that to my answer.
Answer: If you want to calculate the height required for a 25 km radio link. Check out this link.
http://www.everythingrf.com/rf-calculators/line-of-sight-calculator
This link gives not visual line of sight but also the radio horizon which is slightly longer. At 5 Ghz with sufficient gain antenna to direct the signal as close to the horizon as possible and height above ground you should be able to establish a good strong link. This should be at both sites. At 5 Ghz the radio noise and interference would be quite low which is a bonus.
40 meters above ground should do it. If you have an unobstructed point to point link you could probably do it on relatively low power.
Answer: 25KM is a really long distance. You need to be up fairly high to beat the curve of the earth. If you are at sea level on a beach, and your eyes are 1.7m above the ground, the horizon would be at 4.7km. Source wikipedia:Horizon
In actuality both ends would need to be elevated so that the beam path passes high enough over terrain between the 2 points.
A cheap way to test line of sight is binoculars and a really large white or Black sheet depending on the color of the surrounding terrain.
A signal mirror would need to be aimed and it would need to be huge to be really visible. If you want to use a light, do it at night, and get one of the aircraft approved Xenon strobes.
You wouldn't want to use a laser with binoculars for 2 reasons. 1) coliniated light can damage your retina when intensified by a lens. 2) the divergence of a non-commercial laser point over 25 Km probably wont be visible anyway.
A bunch of things which other answers have hit on are various type approval issues with using this for WIFI. If anyone had looked at the link, they would see that the PowerBeamAC is not working under the 802.11ac specification.
It has a separate 802.11 antenna for configuration, but the 25km, link is advertised using a supposedly proprietary TDMA signalling for point to point. and is advertising 450Mbit/s bandwidth. It is separately type approved by the CE, FCC and IC. All of this is in the DATASHEET


Why do older computer cases turn yellow?

Question: Why do plastic cases of old computers turn yellow? Is this yellowing a problem that has been solved in modern plastics?
Comment: See also [How to get rid of yellowing in plastics?](http://retrocomputing.stackexchange.com/q/379)
Answer: In this article, it states:

In nearly every case it's flame retardants. Although plastics can yellow in the absence of flame retardant chemicals, the chemicals accelerate the process when exposed to heat, oxygen, and ultraviolet rays.

The process of the plastic reacting with the heat, oxygen, and UV rays makes the plastic yellow. Flame retardants (very common in old computers) will only speed up the yellowing process. It could be any UV light, not just the ones from the Sun. Heat made from the device itself plus some poor choices can also make your plastic a nice yellow:

The plastics used in early Super Nintendo units, however, had a terrible flame retardant mixture that yellowed as a result of the oxidation caused by the heat of the machine itself (let alone any heat in the environment).

Using new formulas, the process can be slowed down, but not necessarily stopped completely. You can always try to remove the stains though, chemically or physically:

Some people use ultra fine sandpaper or melamine sponges to sand away the very surface of the plastic and remove the yellowing. This works for very light yellowing and plastic types where the yellowing hasn't penetrated very deeply into the plastic. Unfortunately it only works by removing the actual yellowed plastic...
The other method, chemical removal, relies on using bleaches to attack the yellowed molecules in the plastic and restore the color. This method is best for very old and very yellowed plastic as it gets right into the structure of the plastic and does a very thorough job.
Comment: This explains the problem well enough.  One way this excellent explanation could be improved would be a little on the history of retr0brite in particular and some pointers to techniques and pitfalls for doing it.  Ideally that should be a separate question and answers here, or external links to discussions elsewhere, though here might be better for the long-term.
Comment: Bleach is itself an oxidiser, which could worsen discoloration and/or embrittlement...
Answer: Assuming the yellowing isn't caused by nicotine, if you're a smoker, usually it's because of UV light interacting with flame retardants (usually bromine) in the plastic of the device's case. UV exposure can be from sun or fluorescent lights - the yellowing plastic isn't picky about the UV source.
I'm not aware of it being solved in modern plastics.
Answer: As has been mentioned already, the most common reason is flame retardants.
I've found that the easiest way to restore the original color is hydrogen peroxyde + UV light (either from the Sun or from an UV lamp).
Some time ago I restored an MSX turbo R and documented the process, including whitening the case and keyboard. Here's the link.
Comment: Link doesn't seem to work, or is slow, or the page contains flash... for whatever reason, it doesn't seem to load. It might be a good idea to include the details in your answer here, rather than relying on an external link.
Answer:
Degradation may be initiated or accelerated by numerous factors
  including ultra-violet light, visible light, ozone, pollutants,
  manufacturing additives, oxygen, heat, or by carton storage without
  protective liners.

As mentioned in other answers, one of those "manufacturing additives" is flame retardants.
Also, make sure the storage room is well ventilated, especially if it's in a basement.


What is the wattage of my iPhone?

Question: I am trying to make a transportable charger for iPhones, but before I can start this I need to know how much power my iPhone uses when I make an emergency call.
If nobody knows this, I will be glad to have any information related to this.
Answer: Anandtech has some nice charts detailing the wattage of the iPhone 4S running various apps, which could be a good starting point for you.
You may also want to check out existing kits/projects, such as MintyBoost - they seem to work reasonably well with iPhones. There's a specific MintyBoost page detailing Apple device charging that may also yield some useful info.
Answer: The power use is going to vary somewhat depending on external factors on the RF side. Network mode in use (GSM/3G) and the power required to establish a stable connection to the cell site in use together with any other usage on the device (speaker/bluetooth/GPS)
Best guess is that your baseline should be the specified 1A required to charge the device and work up/down from there with a meter connected

What is the wattage of my iPhone?


Why don't we calculate swaps and other steps except comparison for finding time complexity of a sorting algorithm?

Question: I was learning some basic sorting techniques with their complexity. However I cannot understand why only the number of comparisons are taken into account while calculating time complexity and operations such as swap are ignored. Link to selection sort analysis. Please help me understand.
Comment: This is only a simplifcation for beginners and lazy people. There are plenty of analyses of other operations, too. It just so happens that comparisons are often the dominant operation in sorting (for *comparison* sorts!) so for the sake of "Big-Oh" analysis, it's quite enough to investigate them.
Comment: Maybe look at this answer http://cs.stackexchange.com/questions/1354/quicksort-vs-insertion-sort-on-linked-list-performance/1386#1386
Answer: There are 2 major reasons:

For most algorithms the number of other operations can be bounded by a multiple of the number of comparisons (i.e. runtime in $\mathcal{O}(\#comparisons)$). This is because e.g. a swap does not occur without a prior comparison requiring this swap (depends on algorithm).
Using comparisons you can prove an $\mathcal{O}(n \log n)$ lower bound for any comparison based sorting algorithm on $n$ elements.
Comment: i am sorry but i didn't understand the second reason's relevance to question
Comment: Well, we like to compare the algorithm to other algorithms and the comparisons provide a notion of optimality.
Answer: <code>...while calculating time complexity and operations such as swap are ignored
</code>
You are wrong. All the operations are taken into account when calculating time complexity. But as loops are dominant compared to other operations, we ignore other operations and only consider dominant operations(Because for large input value, cost of all other operations are much smaller than the dominant operations) .
As an example with selection sort: When you consider all statement costs into account then you get a function $f(n)=an^2+bn+c$  ($a$,$b$ and $c$ are constants and depend on machine architecture). Here dominant term is $an^2$.So we can say Time complexity of selection sort $O(an^2)$.We also ignore leading terms coefficient $a$ ,as $a$ does not change the rate of growth.
I suggest you to study some standard algorithm textbook to understand it properly.


How to create QtCurve theme KDE

Question: I want to make my gtk2 theme for KDE.
Is there a good instructions/tutorials/tools for creating a QtCurve theme.
Can Images be used as elements (buttons, badgrounds, handles, scrollbars...)
Answer: Your question is a bit strange, since QtCurve is a Qt/KDE theme already.
If you want to build your own theme based on that, the easiest thing to do is to download its source from kde-look.org, start by building it without modifications to make sure you've got that part right, then start changing things progressively.
If you want to write a Qt style yourself (those are written in C++), you should take a look at the Qt Style Plugin examples. (This isn't trivial. Also have a look at Qt Stylesheets while you're at it, and Qt Styles and Style Aware Widgets.)
An alternative to this is to use QuantumStyle which you can use to create SVG-based styles without requiring C++ code.
For Plasma themes, you should head over to TechBase and read the Creating a Plasma Theme in 7 Easy Steps guide.
For custom KWin decorations, you should have a look at KWin decorators part of the KWin Hacking page, which links to a few options (deKorator, AuroreaDesigner and Smaragd). The other option is again C++, and a HOWTO is also linked there (but it's out of date, the other options are most likely better these days anyway).
Answer: QtCurve is a theme engine, not a theme in and of itself, although it does come with a default set of themes.  The themes are just text files that define how you want each component to look, e.g., the scroll bar, etc.  If you go to the KDE settings page for Application Appearance -> Style, you can see if QtCurve is installed by seeing if it is listed in the drop-down box near the top of the window, labeled "widget style".  If not, install it via apt-get or whatever. Once it is installed and appears in the Style drop down box, select it, then click the "configure" button to the right of the drop down.  In the window that appears, you can either choose a theme to start with or just begin modifying the current theme (named "current").  On the left side of the window, there is a large array of choices to choose from, where you can change everything about your selected theme.  It is a bit cumbersome to see your changes, as you need to keep clicking the top-most item "preview" to inspect your changes and see if they are too your liking.  I sure wish there was an instant preview box that keeps being updated as you change things, but not yet.  In any case, when you are satisfied, select "Save" just below the "presets" drop down box near the top of the window, and select a new name for your masterpiece. :)  QTcurve is surprisingly configurable.  The text files end up getting deposited in: ~.kde/share/apps/QtCurve, but you can use the "export" button to save off your work for backup.  Importing is similar, using the included "import" button.

How to create QtCurve theme KDE


Isolating a voice

Question: I have a recording of someone talking while driving. There is a lot of road noise that I cannot get around. I want to isolate the voice from the road noise. I have tried using Audacity but the frequencies of the voice and road noise are too close. I haven't had any luck. Any suggestions to help me isolate a voice from a noisy car ride.
Comment: [Izotope RX](https://www.izotope.com/en/products/audio-repair/rx/) might be able to get rid of a fair bit of it, but it's not cheap, really not cheap.
Comment: If you have the option of re-recording this and getting the mic closer to the source that might be an easier option in the end.
Answer: Selective eq-ing is the option for this kind of restauration.
Using an spectral imaging software/plugin can be useful to visually see loudest sounds and be able to separate that from ambient noise. Software that would enable spectral editing like above mentioned Izotope's RX would make it easier.

Restoration requires patience and even so - if applicable - recording another take with properly set microphones will generally yield better results

Isolating a voice


How can one parallelize a multigrid method for solving a linear system of equations?

Question: As I understand it, the multigrid method solves a linear system by solving a coarser version of the same problem (there by eliminating low frequency error) then projecting back to the fine grid to smoothen out the high frequency errors.  For large systems, i can see how an iterative method can be implemented in parallel at each gridlevel.  Does this approach scale well in parallel?  Is there any other source of concurrency in the algorithm that one can exploit in parallel?
Answer: Parallel geometric multigrid is straightforward to implement on structured grids. Algebraic and unstructured multigrid are more technical, see this answer for links to implementations.
In a multiplicative method (e.g. $V$-cycles), only one level can be computed on at a time. Since the number of levels is $\log_{c} N$ where $N$ is the number of degrees of freedom and $c$ is the coarsening factor (usually about $2^d$ or $3^d$ in $d$ dimensions), this logarithmic term is not removable. Additive methods sacrifice some constant factors, but can compute all levels concurrently, thus reducing the logarithmic factor to $\log_2 \log_c N$. I have yet to see an demonstration on real hardware in which the increased concurrency justifies the poorer constants and reduced robustness of additive methods.
Gauss-Seidel is a very popular smoother which is multiplicative and would seem not to parallelize efficiently. For simple discretizations on structured grids, and when memory bandwidth is not a concern, the classical solution of red-black Gauss-Seidel is reasonable. For more complex problems and on modern hardware, Adams (2001) shows a much more efficient algorithm. For many problems, the simple approach of using independent Gauss-Seidel on each subdomain is entirely satisfactory. An alternative to Gauss-Seidel is to use damped Jacobi or polynomial smoothers, see Adams, Brezina, Hu, and Tuminaro (2003) for a comparison. The performance model for these smoothers is similar to any other stencil computation, and thus have optimal weak scalability, $\mathcal O (N/P)$ for subdomains large enough to cover latency.
In practice, coarse grids quickly reach the strong scalability limit (beyond which adding more processes increases run-time), so they should reside on ever smaller MPI communicators. This adds some mild complexity to the implementation. For problems in which the coarse levels have too much structure to continue coarsening, the coarse level solve can become a bottleneck.
For testing various parallel multigrid methods, I recommend using a library like PETSc which allow you to run many different algorithms with very little user code.
Comment: The Adams (2001) link no longer works.  I believe the article you meant is this one: http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=1592790&tag=1.  "A Distributed Memory Unstructured Gauss-Seidel Algorithm for Multigrid Smoothers"

Let me know if I'm wrong.


Root Samsung Galaxy Core i8262 manually

Question: I tried almost every readymade app like Framaroot, Root Genius, etc. to root my Samsung Galaxy Core i8262 , but I didn't succeed.
I want to root it manually but did not have Linux laptop. I have a Windows 8 laptop. How to root it with that?
Comment: While I cannot answer your question, I can give a quick hint: you might wish to check with our [rooting tag-wiki](http://android.stackexchange.com/tags/rooting/info) and [rooting index](http://android.stackexchange.com/q/1184/16575) for available generic methods. Good luck!
Comment: I read about " how to root android manually" but it has a portion where it need Linux to root it ... is there any way to root it with windows?
Comment: I already used many applications to root my phone but it didn't work.
Comment: I want to root it manually.
Answer: Before rooting your Galaxy Core GT-i8262, you need to know that CWM have bug to get caught in bootloop. But don't worry about that if your phone get caught in this situation. Here is solution  for all problems during rooting of your phone. So, here I prefer TWRP (Team Win Recovery Project) over CWM , which has no such problems.
Here, you need to flash a custom recovery before because, after rooting if you you reboot then your root access vanishes. It happens as stock recovery sets them back. It was happening with me. Such a waste of time in rooting your phone again. Then, I got this process for permanent root access.

Before you continue to the tutorial, you please prepare the following files:

Make sure your computer has installed SAMSUNG USB Driver and Odin307. If not, please download.
Download file TWRP Recovery, and save it on your computer.
Download flashable SuperSu zip file.

Steps to follow:
Step 1: Download both the above files in a specific folder on your computer. The Odin that you've just downloaded is an essential tool & is must for installing TWRP recovery on any Android device. We remind our users that this tool is an illegal way & is not officially released by Samsung. Even though it will not cause any damage to your device unless you follow the instructions properly.
Step 2: Enable mode USB Debugging on your android by going to the settings, click on the phone, click on the version 7x formation, then you will receive Developer options.
Step 3: Power off your Galaxy core completely. You need to boot your mobile into downloading mode now.
Step 4: For Galaxy core, downloading mode can be entered by pressing & holding Home button + Volume Down key + Power button simultaneously until you see a warning message.
Step 5: Leave all the three buttons as soon as it appears. And just press the Volume up button to land into downloading mode.
Step 6: Now you need to flash the TWRP recovery on your device. For that keeping the device in download mode, connect it to computer using the USB cable.
Step 7: Launch the Odin Multi Downloader v3.07 that you've downloaded earlier. If your device is connected properly, then you'll see that an Added message has been appeared on the Odin window. Along with that you'll see the ID:COM section has turned blue with a random com port number below it. If this doesn't happen, then ensure that proper device drivers are installed on your computer. You can install them from the link mentioned above.

Step 8: Make sure that the Auto-Reboot and F.Reset Time options present on the Odin window are checked. Do not tick or untick anything else.

Step 9: On the Odin window, click on the PDA tab and browse to the twrp2.7_arubaslim.ODIN.tar recovery file which you have downloaded earlier & select it.

Step 10: After completing everything as mentioned above, simply press the Start button located on the Odin window to execute the TWRP flashing process. Your phone will automatically reboot itself while going through the process. It is recommended not to touch or remove the USB cable until the process completes.

Step 11: Be patient until the TWRP flashing process completes. It will hardly take 2 minutes to complete. As soon as it completes, you'll see the ID:COM section on the Odin window is turned green with the message Pass below it. Now you can safely remove the USB cable.

Note: Unfortunately due to some reason phone if the process fails or if Odin gets stuck or in case your device gets boot loops, then don't panic. Just close the Odin window, remove and re-insert the battery from your device. Again go into downloading mode and repeat the whole procedure as given above.
Step 12: As all the flashing processes are completed now, let's see whether the TWRP recovery is properly installed or not. For that, firstly switch off your Galaxy Core.
Step 13: Now boot into recovery mode. For Galaxy Core, recovery mode can be entered by pressing & holding Volume Up key + Home button (Middle button) + Power button simultaneously until you land into recovery mode. You'll notice that TWRP Custom recovery is properly installed on your device.
Step 13: Now select 'Wipe data/factory reset' option followed by 'Wipe Cache partition' option for wiping out the data & caches present onto your phone memory. You can now reboot your phone by selecting 'reboot system now' option from the recovery mode itself.
Step 14: Now, copy SuperSu zip to your sdcard and put it in your phone.
Step 15: Open recovery mode again as stated above. Click Install, then choose your SuperSu zip file and flash it.
Step 16: Reboot to system and search for SuperSu app. If you have it, then you have full root access.

Don't forget to enjoy it by utilising ROOT access.

Disclaimer: To build this answer I used my experience ( Yup! I have same device ) and two main sources as follows: first and second.

Root Samsung Galaxy Core i8262 manually


Why is it that the least reactive metals are the best electrical conductors?

Question: Silver, Gold and Platinum are amongst the best conductors of electricity, but also the amongst the most unreactive. Since electrical conductivity depends on the number of delocalized electrons (along with resistivity), if an element gives up its electrons readily (making it highly reactive), then shouldn't that mean it will be a great electrical conductor? Why isn't Cesium the best conductor?
Comment: In the trio of metals you quote the most reactive (silver) is also the best conductor. And copper is a better conductor than gold despite being a lot more reactive. Aluminium would be very reactive were it not for the passivating oxide layer on its surface and it is also a very good conductor. are you sure there is a real trend or correlation?
Comment: I realize there may be no definite trend with regards to these two properties, but the fact that less reactive materials tend to be better conductors seemed spurious from my reasoning. Am I missing some sort of logic here?
Answer: One of the reasons why non reactive metals are good conductors is that they are good at staying as metals. Most metals react with the atmosphere to form oxides. And the majority of oxides are insulators or semiconductors. Of course there are few exceptions to this rule.
Also, just a note: calcium and iron have better conductivities than platinum.
Answer: First, why are they "non-reactive"?  They're really not "non-reactive", gold and silver certainly appear in a variety of complexes, but they are highly resistant to corrosion and oxidation, which is what makes them far less reactive with typical reagents than most metals.  They have this property because they have a high electronegativity (EN), so its harder for oxygen or acids/bases to remove their electrons and force them to react.
The requirements for good conductor is partially filled valence bands (alot of electrons) and strong delocalization (electrons can move freely between metal atoms).  Ag, Au, Cu have alot of electrons per atom, and due to the distance from the nucleus and shielding, valence electrons aren't so tightly bound to an individual nucleaus that they can't be shared equally.
The equal sharing is key and makes this bonding is highly energetically favorable because it fills each atoms valence, but leave no Au, Ag, or Cu atom deficient of electrons.  If these metals bonded covalently to a higher EN atom, they would lose some of their electron density, which is unfavorable.  You'll notice however, that gold dissolves perfectly into mercury (slightly lower EN, many free electrons) because of this strong metallic bonding.
The delocalization of electrons is mainly s and p electrons (probably not what you would expect), so heavier atoms that have strong relativistic effects causing the contraction of s orbitals don't act as well as conductors because the sea of s and p electrons isn't as free.  Au, Ag, Cu hit the perfect spot on the periodic table where they have many electrons, don't have a full valence (so want to form covalent bonds), but also have a higher EN than most metals so its preferable for them to bond with themselves.
An atom like Cs, has relatively few electrons to move around its metallic bonding complex, so doesn't fulfill both the requirements for a conductors even though the electrons are highly delocalized.
If you're really interested you should read about the tight binding model and the quantum mechanical approach to metallic bonding.
Comment: You should precise with which scale of electronegativity you think because there's a big difference between Pauling and Alfred-Rochow for example.
Answer: The simplest answer I can give is that silver (e.g) is a good conductor of electricity because of its sea of delocalised electrons between the layers of metal ions. However it is not highly reactive because you would need a lot of energy to break the metallic bonds within each of those layers.
Comment: Is there a more precise explanation you could provide (perhaps I suspect involving periodic properties such as electronegativity/ionization energies or thermodynamic properties such as enthalpies of atomization etc.)? That would be very useful.
Comment: I cant give much of an answer at the minute, but to truly understand electrical conductivity of metals, in depth, I would read up on [band theory](https://en.wikipedia.org/wiki/Electronic_band_structure). I'll try to reply when I get chance.
Answer: The electrical conductivity of elements does not really depend on its reactivity. Then silver would be one of the best insulators. It actualy depends on the number of free electrons in the atoms. So, diamond has no free electrons thus it is a good insulator whereas silver having a lot of free electrons is the best conductor.


not sure if my wudu is valid

Question: Keeping wudu is a struggle for me because I have a lot of gas which is on and off. I can guess sometimes when it might go away although I am not really 100% sure and if it does go away then i end up reading the prayers really late, for example, i read Dhur when it is almost Asr time. I read that if you cannot control it and it happens a lot then you do one wudu for each prayer. But on another website it says if it goes away then you have to read it during that time. The thing is this is causing me a lot of stress and anxiety because it is a random thing. I am not sure what to do. Should I read the prayers on time even if I am having these problems or delay the prayers to see if the issue goes away and read the prayers then?
Comment: Have you ever wondered why you get a lot of gas?
Answer: The answer is simple and clear:

Do one wudu for each prayer.
Read the prayers on time.

حَافِظُوا عَلَى الصَّلَوَاتِ وَالصَّلَاةِ الْوُسْطَىٰ وَقُومُوا
  لِلَّهِ قَانِتِينَ
"Maintain with care the [obligatory] prayers and [in particular] the
  middle prayer and stand before Allah , devoutly obedient."
Al-Baqarah (2:238)

.

إِنَّ الصَّلَاةَ كَانَتْ عَلَى الْمُؤْمِنِينَ كِتَابًا مَّوْقُوتًا
Indeed, prayer has been decreed upon the believers a decree of
  specified times.

not sure if my wudu is valid


Torsion subgroup quotient

Question: Let G be an abelian group, $T$ the torsion subgroup of $G$. If $G/T$ is torsion-free, then $T$ and $G/T$ must be disjoint. $G=T \bigoplus G/T$ implies this as well. I don't understand why they are disjoint in the first place.
Comment: This is a mess: $T$ and $G/T$ don't even live in the same group. And aren't you assuming that $G/T$ is finitely generated or something?
Answer: What you write does not make sense: $T$ "lives" in side $G$, but $G/T$ is a quotient of $G$, not a subgroup. It does not really make any sense to talk about the intersection of $T$ and $G/T$.
What is true is that if $G/T$ is finitely generated, then since $G/T$ is torsion free then it is isomorphic to $\mathbb{Z}^r$. Pick an isomorphism. Then for each of the elements $e_i \in \mathbb{Z}^r$ ($e_i$ has a $1$ in the $i$th coordinate and zeros elsewhere) we can pick an element $g_i\in G$ such that $g_i+T$ corresponds to $e_i$ in $G/T$ under the chosen isomorphism.
Then the $g_i$ generate a subgroup isomorphic to $\mathbb{Z}^r$: if $a_1g_1+\cdots+a_rg_r=0$, then projecting onto $G/T$ we get $a_1e_1+\cdots+a_re_r=0$, hence $a_i=0$ for all $i$; so $\langle g_1,\ldots,g_r\rangle = \langle g_1\rangle\oplus\langle g_2\rangle\oplus\cdots\oplus \langle g_r\rangle$.
And now we have the important point: $\langle g_1,\ldots,g_r\rangle\cap T = \{0\}$. For if $a_1g_1+\cdots+a_rg_r\in T$, then there exists $n\gt 0$ such that $n(a_1g_1+\cdots+a_rg_r) = 0$. This gives
$$na_1g_1+\cdots+na_rg_r=0,$$
hence $na_i=0$ for all $i$, so $a_i=0$ for all $i$. That is, the intersection is contained in $\{0\}$.
Since $T+\langle g_1,\ldots,g_r\rangle = G$ (e.g., by the isomorphism theorems), and the intersection is trivial, we get
$$G \cong T\oplus \langle g_1,\ldots,g_r\rangle \cong T\oplus \mathbb{Z}^r\cong T\oplus (G/T).$$
Answer: What is not true. If $G=\prod_{p}\mathbb{F}_p=\mathbb{F}_2\times\mathbb{F}_3\times\cdots$, then $G/T$ can not inject to $G$. In particular, $G$ is not isomorphic to $T\oplus G/T$.
Proof It is clear that $\bigcap_ppG=0$. It suffices to show $\cap_pp(G/T)\neq 0$. The image of  $x=(1,1,\ldots)$ is in $p(G/T)$ for every prime $p$.

Torsion subgroup quotient


What is the difference between a neutron star and a white dwarf?

Question: What is the difference between a neutron star and a white dwarf? I know that both are very dense even if they go through different phases.
Answer: In a neutron star, the force of gravity is strong enough to press the protons and electrons together to form neutrons {1}, White dwarfs are only very compact. With even more mass, you get a black hole.
All three types are outcomes of star death, when the failing fusion in the middle of a star is no longer able to counteract gravity. What a star become when it collapses is depending on its mass.
Comment: @LorenPechtel The same can be said about the plasma in the Sun.
Comment: True, but the sun is held up by the pressure from it's heat.  A white dwarf isn't.  Note that the inside of a star can become basically a white dwarf.
Comment: White dwarfs are dense enough to force the electrons out of their shells--you have electrons and bare nuclei, not complete atoms.
Answer: A white dwarf is less than 1.44 solar masses, and is held up by electron degeneracy pressure, the rule in quantum mechanics that says electrons strongly resist being squeezed together. They're made of highly compressed but still more or less normal matter, mainly carbon and oxygen. Dispite their mass they're only about as big as Earth, meaning one teaspoon of their material would weigh several tons.
After 1.44 solar masses, electron degeneracy is not strong enough to overcome gravity. At this point, it collapses even further, electrons and protons merge to neutrons, forming a neutron star, only the size of a city, and composed almost entirely of neutrons. One teaspoon of neutron star matter would weigh a billion tons. Neutron stars are held up by neutron degeneracy pressure, which is far, far stronger than for electrons.
Comment: Neutron degeneracy pressure is actually weaker than electron degeneracy pressure because neutrons weight lot more than electrons. Neutron degeneracy pressure alone can only support a star up to 0.7 solar masses. It is the strong nuclear force that is the main reason behind a neutron stars stability up to 2.74 solar masses.
Answer: Additionally, others haven't said it, is that a neutron star is also much smaller than a white dwarf:
White dwarfs are Venus- and Earth-sized (e.g. about 7000 mi / 11000 km in diameter) while neutron stars (and stellar black holes) have the size of the Martian satellites Phobos and Deimos (e.g. about 10 mi / 16 km in diameter).
The neutron star (or maybe a quark star) PSR B0943+10 is the smallest known star at all, at 5.2 km (3.2 mi) diameter. It is also one of the least massive known stars. Anyway, it is orbited by two known gas giants.
Answer: Please see this article that describes the Chandrasekhar limit. Any star remnant more massive than the Chandrasekhar limit, 1.4x sun mass, will become neutron star or black hole. Otherwise it will become white dwarf.
This mass limit is the threshold at which gravitational will overcome electron degeneration pressure, causing the electrons to join the nucleus. The protons in the nucleus combine with the electrons to become neutrons. Thus the term neutron star.
When the mass of the neutron star surpasses the level at which the escape velocity for the neutron star exceeds the speed of light, then even light cannot escape its gravitation, thus becoming a black hole.
Comment: The Chandrasekhar limit is relevant, but white dwarfs that exceed the limit are too small to become neutron stars or black holes.   They do go supernova, but they don't become neutron stars or black holes.   That requires a larger mass and a different process.
Comment: My bad , I corrected it to say star remnant.  The initial star mass pre-collapse is much larger.
Comment: It's still incorrect in more than one way.    You need a LOT more than 1.4 solar masses to create a neutron star.  The "remnant" is a bit confusing, but if you look at the Neutron star as the remnant, then the remnant can be less than 1.4 solar masses and be a neutron star.  See here:    https://physics.stackexchange.com/questions/143166/what-is-the-theoretical-lower-mass-limit-for-a-gravitationally-stable-neutron-st  Your red giant statement is also strange.   Most stars above a certain mass go red giant (not 100% sure about the largest ones).
Comment: Ok, I corrected so we are referring to star remnants; ie what is left of the star core after red giant and planetary nebula or supernova explosion.


Wifi Issues with Elementary OS

Question: Whenever I connect to my network, it connects fine for about 10-15 minutes until it still shows up as connected to the network in the upper-right corner but if I try to do anything that requires internet, it just doesn't work and says I'm not connected. Then after about 15 seconds, it pops up a notification saying I'm disconnected from the network. Please help me, I really just want to use my computer but I can barely do anything.
Answer: this sounds like a driver issue, do you know which wireless chip you have
I'll give you and update on what to do based on that
lspci
lspci -v
lspci -v | less
Try doing that and post a screenshot with what pops up
I'll know how to help you after that

Wifi Issues with Elementary OS


Do I need a DI to put a Guitar through my Interface?

Question: I recently picked up an MBox 2 Mini so that we can send a guitar into our DAW and put effects onto the guitar in live time. I know that this will work with the configuration of software and interface hardware I will be using, however because of the nature of this recording work, I do not know how to feed the guitar into the interface. The interface has 2 inputs- A Line input (1/4" jack) and an XLR input for microphones, or if I had a DI, that too. What I want to know is if I could plug a guitar right into the line input, no amp or DI, or if I should be using a DI and using the XLR input on my interface. I realize that using a microphone with an amplifier is most times ideal, however because we want the sound to be as clean as possible when it goes into the computer, that would not work for us. Any ideas?
Answer: No DI needed. Just your 1/4" guitar cable straight to the "line in" on the MBox. The output of the guitar is an unbalanced line level signal. The MBox is designed to take that input on either input 1 or 2.
Page 34
MBox 2 Mini Manual
Answer: 1/4 inch jack lead should do it. Just plug your guitar straight in. On my interface, it has 2 x Jack/XLR combo sockets(1616M).
Comment: I got the wording wrong. I read from the manual, but I was reading another question too, and mixed them up. Edited it now.