Why does a Solidity throw consume all gas?

Question: When you throw an exception, all gas is consumed. What is the rationale for this design decision?
Answer: All gas is consumed because the EVM essentially only has 1 exception: Out of Gas.
To see this clearer, take a look at the difference between a "pure" exception, and an error due to bad/buggy/invalid EVM code.  Out of Gas is the former.  Now there are errors such as stack underflow, invalid JUMP, and invalid opcode: they can be called "exceptions" but they are more like errors than an EVM exception, because the developer of the contract has not written valid EVM code according to the rules of the EVM.  For example, an invalid opcode is like writing "class" instead of "function" in Javascript: native Javascript does not have "class", unlike a language like Java.  With stack underflow, it's like the developer telling the EVM to add 2 numbers, but the developer does not provide the 2nd number.
Note also how "exception" in the EVM, is different from other languages.  For example, there is no divide by zero exception in the EVM (though Solidity 0.4+ will generate an exception for this).  Assuming valid EVM code, there are no exceptions except for Out of Gas.
Since there's only an Out of Gas exception, these other errors were made to behave as Out of Gas.  This is relatively consistent with some Ethereum design principles of generalization and minimizing complexity.  In other words, when implementing the behavior of stack underflow, invalid JUMP, and invalid opcode, it's simpler and easier for the protocol to reuse the implementation and behavior of Out of Gas.
Other answers have explained how Solidity compiles throw to an invalid JUMP.  (Serpent instead uses an invalid opcode, and calls it invalid instead of throw.)
EDIT:  The Byzantium release includes EIP 140, a new opcode named REVERT in the EVM that:

provides a way to stop execution and revert
  state changes, without consuming all provided gas and with the ability
  to return a reason.

throw is being deprecated in favor of revert(), require(), assert()
Comment: `class` is a valid keyword in javascript used to define a class.
Comment: @0xcaff Thanks, yes, JavaScript is evolving too and `class` added in ECMAScript 2015 https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes  (still takes time for ECMA standards to become widely known and adopted).
Answer: The Solidity throw instruction gets compiled to an invalid JUMP instruction (i.e. a jump to an invalid location).
The section 9.4.2 of the Yellow Paper explains how exceptional halting works, in chapter 8 it explains that no gas is refunded in this case.
In chapter 8 it states:

Just as with contract creation, if the execution halts in an
  exceptional fashion (i.e. due to an exhausted gas supply, stack
  underflow, invalid jump destination or invalid instruction), then no
  gas is refunded to the caller and the state is reverted to the point
  immediately prior to balance transfer (i.e. σ).

And referring back to chapter 7:

If such an exception does not occur, then the remaining gas is
  refunded to the originator and the now-altered state is allowed to
  persist.

As to why it has been decided this way I have no information about.
Answer: In part I believe this was a security decision to ensure spamming the network with bogus transactions cost the sender gas. For instance, if gas was refunded after a throw, a malicious actor could create a contract that consumed some gas and then would throw. That actor could then send one or many transactions with a high gas price and a lot of gas, making their transactions take priority under default miner terms. Their transactions could take up all the gas in each block so no other transactions could go through, and if the gas was returned to them, they could do this for free indefinitely. With gas being consumed, this would cost them a good deal of ether.
Answer: The throw instruction is a EVM hack that reverts all changes made by current contract execution. It exploits exceptional termination of EVM which also consumes all gas.
Comment: That doesn't explain the rationale for why it is designed that way. Many examples in the Solidity documentation use `throw` as if it's the best way to do things. But it clearly isn't if it costs callers the maximum fee.
Comment: This would require adding a system to track all state changes, including those in calls to other contracts, which is not realistically possible
Comment: By your explanation, it should cost _at most_ as much gas as has already been consumed in the transaction up until the point that the exception occurred (a naive implementation might "undo" each operation one at at time).


Gaussian distribution in Astronomy

Question:
What does it mean for a data set to have gaussian distributed noise?
What does an error bar on a data point really imply?
What does a 5-sigma result mean? How often is a 5-sigma result wrong?
What does a 3-sigma upper limit mean?
Answer:
One believes that the measurement differs from the true value by a random variable chosen from a Gaussian distribution.  Many things can contribute to the errors of measurement.  The Gaussian distribution is convenient, but often wrong out on the tails-the tails are too small.
The author should tell you.  It will usually be some number of standard deviations in the Gaussian approximation.
A 5 sigma result is a measurement that is that far from some background level.  There may be a broad source of light that you can put a smooth curve through and a bright spot in the middle of it.  The null hypothesis is that there is nothing there and the bright spot is a fluctuation in the noise.  A 5 sigma result says the null hypotheses is ruled out to that level.  If you believe the Gaussian model, it would only be wrong about one time in $10^{12}$.  Experience says otherwise, however.
A 3 sigma upper limit is given when your measurement is consistent with zero.  It is the value that would have to fluctuate down by 3 sigma to match your data.  You are trying to convince the world that the true value is less than this.

Gaussian distribution in Astronomy


Why kernel source is not installed

Question: I want to install kernel source in Ubuntu 12.04 which is not installed. I have checked the same using the following command :
dpkg -s kernel

Output is Kernel is not installed no information available
Hence I have followed the following steps to install the same:
1.Installed the Dependencies by the following command:
sudo apt-get install gcc libncurses5-dev git-core kernel-package fakeroot build-essential
sudo apt-get update && sudo apt-get upgrade

Download kernel source by the following command:
wget http://www.kernel.org/pub/linux/kernel/v3.x/linux-3.5.tar.bz2
tar -xvf linux-3.5.tar.bz2
cd linux-3.5/

3.Compiled the source code to generate the .deb packages by the following command
make-kpkg clean
fakeroot make-kpkg --initrd --append-to-version=-spica kernel_image kernel_headers

Install the .deb packages (two .deb packages generated, one is to install the kernel headers, another one is to install the kernel image) by the following command
sudo dpkg -i linux-*.deb

But after reboot it seems kernel is not installed (checked by dpkg -s kernel). Please tell where I am going wrong.
Also, In step 3, I guess I am installing a new kernel (named spica kernel_image) but during the boot up this new kernel is not showing as option.
Please help me
Comment: The fact that you can boot indicates that the kernel is installed :)
Answer: The kernel package isn't called "kernel", it's called "linux-image-<version>-<type>".

Why kernel source is not installed


what is the difference between a host adapter and network card?

Question: what is the difference between a host adapter and network card? I know that a netword card is the electrical circuits in the computer where you plug your ethernet cable into in order to gain internet access. But how is the host adapter different from this?
Comment: The latter is a specialization of the former. http://dictionary.com
Comment: Why is this question getting marked down? Seems like a fine question to me.  +1
Answer: A host adapter is required for a network card, but the host adapter controls many different port. It connects network AND storage devices.
"In computer hardware, a host controller, host adapter, or host bus adapter (HBA) connects a host system (the computer) to other network and storage devices. The terms are primarily used to refer to devices for connecting SCSI, Fibre Channel and eSATA devices, but devices for connecting to IDE, Ethernet, FireWire, USB and other systems may also be called host adapters. Recently, the advent of iSCSI and Fibre Channel over Ethernet has brought about Ethernet HBAs, which are different from Ethernet NICs in that they include TCP Offload Engines. There are also converged HBAs that support both Ethernet and Fibre Channel called Converged Network Adapters (CNAs)." (Wikipedia)
As you can see, a network card is what you said, but the host adapter, controls things like ethernet, firewire, iSCI, SATA, and other kinds of storage devices. You can think of it as the device that controls external ports and network devices. The network card (Or network adapter), simply allows you to connect to ethernet.
Other devices are needed here as well, including the Host Controller Interface:
"A host controller interface (HCI) is a register-level interface that enables a host controller for USB or FireWire hardware to communicate with a host controller driver in software. The driver software is typically provided with an operating system of a personal computer, but may also be implemented by application-specific devices such as a microcontroller."
The HCI requires a Host Controller Driver, or HCD. This is because the software side of things require it.
Answer: A Host Adapter "connects a host system (the computer) to other network and storage devices"
An Ethernet Host Adapter is a host adapter for Ethernet networking and is also called by a variety of other names such as: Ethernet Adapter, Network Adapter, LAN adapter, Ethernet Card, Network Card, Network Interface Card (usually abbeviated NIC) and so on.
Ethernet is just one family of networking technologies, 802.11 or WiFi or Wireless is another family of neworking technologies, sometimes computers don't have this built-in and so rely on a plug-in card of some sort or on a USB-connected device.
The term card is often used because early computers (especially personal computers) did not include built-in networking circuitry, this capability was provided on plug-in circuit-boards (often called "expansion-cards" or just "cards") which were plugged into expansion-sockets inside the computer. Nowadays most server, desktop and laptop/notebook computers have built-in networking but the term persists.

A "Network Card", one type of "Host Adapter"


using "touch" to create directories?

Question: 1) in the "A" directory:

find . -type f > a.txt

2) in the "B" directory:
cat a.txt | while read FILENAMES; do touch "$FILENAMES"; done

3)
Result: the 2) "creates the files" [i mean only with the same filename, but with 0 Byte size] ok. But if there are subdirs in the "A" directory, then the 2) can't create the files in the subdir, because there are no directories in it.

Question: is there a way, that "touch" can create directories?
Comment: Cross-posted on ServerFault: http://serverfault.com/questions/223267/using-touch-to-create-directories
Answer: What is your desired end result?
If you are trying to duplicate a directory structure there are easier ways. Syncing solutions come to mind first.
e.g.
rsync -a /source/dir/ /dest/dir/ --include \*/ --exclude \*
This will recreate the entire directory structure. You can then run your touch command to finish the rest.
As previously mentioned we may be able to find a more elegant solution if we know the full problem :)
Answer: You could use this script. You need to use chmod +x scriptname
#!/usr/bin/env ruby
# touchMakingDirs: a Ruby script to be able to touch a file and optionally create the directories necessary
# by Dan Rosenstark 2013-03-18
if __FILE__ == $0
  if ARGV.length == 0
    puts "Sorry, include one argument for filename"
    exit
  end

  filename = ARGV[0]
  puts "Will touch file #{filename}"
  path = File.dirname(filename)
  #filename = File.basename(filename)
  `mkdir -p \"#{path}\"`
  `touch \"#{filename}\"`
end

It's worked in a few small tests. Let me know if there is a test case I've missed. I use it like: touchMakingDirs blah/blah/whatever/who.txt where I've named the script touchMakingDirs and put it in my path.

using "touch" to create directories?


Is there a penalty in link building?

Question: If I have a satellite site with content constantly renewed, and within each new content, there is a link to a target URL, for passing link juice. Will I get a penalty from Google for that?
Answer: No, you cannot get any penalties from links to your site (otherwise it would be easy for competitors to harm you). However, if Google decides the satellite site is of little value or the content is spam, the links may only give you little or no value. They may be classed in the same vein as "footer links" which tend to have low value.
Edit: An update by Google a little while back has confirmed that links from very spammy sites could harm your ranking. This is still pretty rare and not something you really need to worry about.
Comment: Yes, you can penalties from links to your sites, see the answer I am about to create (or http://tshikatshikaaa.blogspot.com/2012/05/how-to-build-backlinks-efficiently.html for more details).
Comment: just for the sake of clarity: at the time of the original answer this was not possible, but with the latest Google updates it now became possible.
Answer: The accepted answer to this question may have been correct at the time, but it is clearly now wrong.  On April 24, 2012 Google released a new algorithm which they called "penguin" which targeted web spam.  Part of the algorithm was focused on bad inbound links to a site.
It is now the case that inbound links to your site that are spammy can hurt your site.
To lessen the impact of "negative SEO" where a competitor creates spammy links pointing at your site, Google has released a disavow links tool that lets you report to Google any links that are poor quality but which you are not responsible for creating.
Answer: This is not normally a problem and it's something that's commonly done. For example, one client I did some work for had a charitable .org website where every post linked back to the main corporate website or to a sponsor. You can even see this in action on stackexchange websites in the footer and sidebar links. The value of these links may not be as strong as other links from unrelated sites but they won't hurt you.
There can be certain situations where certain shady and deceptive practices might lead to an algorithmic and/or manual penalty but these are quite difficult to trigger by accident. One example is what I call the "NY Times Penalty" where the SEO practices of a big name company get exposed in the press and Google manually takes action against them.

Is there a penalty in link building?


How to sustain with Fizz

Question: I just started playing with Fizz and as an Ahri Player I really wonder how you sustain with Fizz on the lane (or at all)? He hasnt any Build-In Spellvamp and also no AP Build foresees someting like Hextech Revolver for him. Don't get me wrong - I am not like "oh my god I can only buy according to my build" but there must be a reason you don't build something like spellvamp on him. I mean I really have a hard time to hold the lane when I get harrassed...So my question is - How to sustain with Fizz? Maybe to give some kind of half answer - does it all depend on evading harrass with my E ?
Comment: If you're looking to experiment with items on Fizz, hextech gunblade gives good sustain with an active that fits very well with the rest of his kit.  (disclaimer:  I don't play Fizz and this may be a very poor suggestion).
Answer: The key to midding fizz is, as someone already mentioned, managing trades to get a pressure lead early on.
I like to think of it in these terms. In starcraft 2, it is commonly said that to win with zerg, as a general rule, you should always have one more expansion than your opponent, and you do this by expanding early.
With fizz, begin initiating trades as early as level 2, and maintain a health advantage by constantly diving in for trades as your counterpart approaches the wave to last hit.
For the trades, as they approach the wave, dash in with Q. As you land, activate W before your first attack lands. Auto attack them twice, maybe three times if you can get away with it, then E to get out.
Usually once you Q to them, they either start to panic and don't cast anything, or show absolutely no sign of worry and start throwing spells out. Just juke a bit between auto attacks if they don't instantly back off, which will put them in an even worse position, and tilts the trade even more in your favor.
As you E away, occasionally they will try to even out the trade a bit and throw a spell out, if that is the case, you can elect to use the second half of your E to go back to them and apply more pressure, as they just wasted a cooldown.
After 6 it doesn't get much more complicated, just add the ulti into the rotation wherever you see fit.
Of course, the usual laws of midding in general apply, after you do it multiple times, you can get predictable. Be aware that while your E does make escape pretty easy, if baited into a situation just that little bit too risky, it can be fatal. Remain aware of how your lane counterpart changes in behavior from level one onward.
Other than that, make sure to get your farm in whenever you aren't harassing.
You could say that it isn't really "sustaining", but ensuring that your counterpart is even less sustainable than you are. If done correctly, they aren't even "trades" really, because often times you will take no damage other than some auto attacks.
Hope this helps, and sorry for the large response.
Comment: +1 Buying spellvamp doesn't help you trade as well as focusing on damage. Keep in mind spellvamp scales off of your damage as well, so rushing spellvamp will likely end up with you being behind. That being said, you could try AD Fizz with Blade of the Ruined King, but make sure you have a replacement AP carry and expect to get yelled at if you do poorly since it's unconventional.
Answer:
does it all depend on evading harrass with my E ?

Honestly? Yes.
Fizz, as a Champion, is incredibly different from most other APs that you would take mid. His skill set is the definition of high risk, high reward. If you can master Playful/Trickster, landing Chum The Waters, and using Urchin Strike not only to get in, but to get out as well, then you will become a very good Fizz player.
While buying Spell Vamp might seem worth it on Fizz, from my own experiences, it really doesn't help much.
Good luck!
Answer: You can rush a catalyst for sustain later. However, fizz isn't really that great early against lanes with sustain.
I see fizz having a lot of strength in his ability to initiate and control trades. If someone like an oriana tries to harass you with a q, initiate a trade because she just blew some of her damage and if you dodge it, you'll win the trade for sure. I personally think fizz is situational for non sustain lanes where his initiation/trading is highly in his favor.
Answer: Fizz is really easy to use if you get the flask and some health potions.  I easily out-sustain even a Twisted Fate with full health pots and a flask.  That being said, somethings to watch out for are pushing for that tower dive kill, while backup arrives. It's very easy to trade with Fizz and one low health champ can easily become two.  Jungles thrive on baiting the mid laner to their own turret and charging them.  A good Hecarim can totally mess up your mid lane even if you outpace your enemy.  It's a very good idea to ward when you go mid.
Answer: the main reason why you don't go spell vamp on fizz is because he is a burst class assassin not a constant damage dealer like katarina (kat has very less cool down on her skills so its worth buying spell vamp on her)but with fizz you are going to focus one or two people and flank them from the sides
early you feel like you dont have sustenance but once you max out your playfull/trickster
you are a monster you have an inbuilt zhoniya's hourglass in your package with a added flash why you need sustenance
your playful/trickster is a very strong disengage if used properly i would suggest to buy cool down reduction items than others.
laning against certain champions will be more difficult like leblanc, mordikeiser,yasuo,vladmir, etc
the key is take advantage of their cool down time to trade damage also try to cs without using skill in the lane and force the opponent to use skills to cs which will give you good results

How to sustain with Fizz


Dropbox can't find live wallpapers on android phone

Question: I would like to add live wallpapers to my dropbox from my android phone but I can't find them. I can find my apps from the backup folder, but how do I find the live wallpapers? I have tried going to the android folder, but it doesn't find anything. At all.
Answer: The stock live wallpapers are actually apps found in the /system/apps folder. User installed ones are in /data/app. You will need to be rooted and have a root explorer to access these folders.


How can some politicians become cabinet ministers without winning elections in India?

Question: Smriti Irani and Arun Jaitley both lost the elections in the recent Lok Sabha elections in India, so how can they become cabinet ministers if they didn't even win the election? Mustn't one be a member of Parliament first to become a cabinet minister? Is there any special provision for becoming a cabinet minister without winning the election?
Comment: This is also allowed in Australia.
Answer: Under the Constitution of India, Part V.—The Union.—Arts. 75—76., the President on the advice of the Prime Minister appoints cabinet Ministers.

(1) The Prime Minister shall be appointed by the President and the other Ministers shall be appointed by the President on the advice of the Prime Minister.

Yes, you must normally be a member of Parliment to continue to serve as a cabinet minister, but that doesn't prevent you from being appointed in the first place.

(5) A Minister who for any period of six consecutive months is not a member of either House of Parliament shall at the expiration of that period cease to be a Minister.
Answer: India has two houses in parliament - the Lower House (Lok Sabha, for which the elections were held) and the Upper house(Rajya Sabha, where people are nominated from their respective parties).
The Constitution of India dictates the following regarding a Minister:
     " A minister should be
          (a) a Member of Parliament
          (b) should become an MP in 6 months"
So, going by the definition and its loophole, a Minister need not be an MP directly elected to the Lok Sabha. (Looking at you, Mr Manmohan Singh!) Rather, he/she may come from the Rajya Sabha too. So that's how Smriti Irani, Ms Heptullah are going to be Ministers.
By the point (b) in the definition above, a Minister should strive hard to become an MP in 6 months. Scenario: When Modi resigns from one of his seats, Arun Jaitley can contest there if he wishes and become an MP from there (he's already an MP in RS though, so that probably won't be necessary)
Answer: The President may appoint any chief member of the Council of Ministers, these being the prime minister and the cabinet members, if they are not members of Parliament for a duration of six months. After that time, the minister must secure his position through election. During this time, the cabinet members will not be able to vote in Parliament.
It is uncommon in India to do this, but may be done (by custom) if no party has secure control over the Parliament and the politicians appointed to the positions of cabinet are believed to be excellent candidates that may have only recently fallen out of favor with the electorate for a short time.
http://www.desikanoon.co.in/2014/05/how-is-prime-minister-appointed-elected-india.html
Answer: As per constitution any one can be appointed as a minister (even Prime minister) by the president. He/she enjoy all benefits of being a minister.
But, within 6 months from the date of appointment they need to be elected as member of parliament. They can either contest election and  become a member of Lok Sabha or they can be nominated as a member of Rajya Sabha.
Ex-prime minister Manmohan Singh is a nominated member of Rajya Sabha from Assam constituency and not an elected member of Lok Sabha.
This provision also applies for state governments of India.
Answer: You may lose the Lok Sabha elections - like Arun jaitley and Simriti Irani - but they can nominate you to Rajya Sabha - and then you can enjoy the cabinet rank or whatever until your Rajya membership expires - after which they can nominate agian if needed and the cycle goes on.
Answer: The last terms cabinet ministers as in , the previous five years cabinet ministers continue their position because such ministers stay as long as the president likes....they do not have any term of office. Remember their term of office is not 5 years just as is an MLAs .
So even if the next batch of lok sabha have come in, the cabinet ministers wont change even if they lose elections .
If they lose elections, in such a case, the president has to approve these ministers into the lok sabha within 6 months.
Usually , the Prime Minister c , through the President , changes the cabinet ministers.
Answer: This was the question that struck my mind when last year Lok sabha election results came out.Arun jaitely , head of finance and smriti irani  head of education in india plays vital role in shaping future of India.Now the question is if already people have rejected both of them in Lok sabha elections yet these two are making laws for the people.Isn't this wrong.
Well I thought about it and found out tha logically its correct.In political system there are so many hidden facts , people see what is at surface.I would say that when smriti irani was fighting election for representation , it was not that she didn't got any votes.She got votes less than what rahul gandhi got.She was still supported by people but with lesser votes.The fact that she is able to make laws for education system whereas rahul gandhi cannot is due to the political system that our country has adopted.We are governed by majority party.Now  members of majority  party decides whom they want to elect as thier leader, which will be Prime minister.Then PM will advice president what what all ministers he wants to be in cabinet.PM and cabinet ministers need not be elected members of lok sabha.Now the question remains that if we the people have not elected them how can they make laws for us.This is where the fault lies.when a rajya sabha member becomes a cabinet minister, he may not be elected by people directly but indirectly by people themselves.We vote in sate assembly election.When we do so we have faith in our representatives.They then elect rajya sabha members.Hence indirectly we have elected cabinet ministers.
One important aspect is that each law (not money bill) are made by voting of members of both houses.Cabinet ministers head each departement.They frame laws.In discussion if MP find that there needs changes ,draft bill is reconsidered.cabinet ministers are just initiators ,bill wont get passed unless it gets majority in voting.also, cabinets ministers are executive head of thiers departement .hence thier role also lies in implementing of new laws.Hence I find that it is more important that who gets elected in state assembly and loksabha, they are the one whom we can choose directly.In turn they ten choose PM,President,vice president,governers and so on.So people vote and vote for better future of our country.
In India we are limited in terms of involvement in politics.We cry when prices rises.Gets angry when corruption comes in news.I believe thats because we are aware whats happening but cannot correct it.Only chance we have got to correct is in elections.But truly that time also we just vote first by seeing what party we are voting for which is based on caste and religion.Then again as last year election people took revenge on congress because of so much corruption involved.Voting should be based on records of the person whom you elect as representative, at state , village, city or national level.
Comment: Your answer is long and rambling.  It looks like there might be an actual answer somewhere in there, but all of your other commentary makes that answer hard to find.  You need to remove all the parts of your answer that are not crucial for answering the question.


T-duality approaches

Question: The textbook approach to explaining T-dualities is to show that a type of T-duality transformation "inverts the radius of the circle, that is, it maps $R\rightarrow\tilde{R} = \alpha'/R$ and it leaves the mass formula for the string invariant provided that the string winding number is exchanged with the Kaluza-Klein excitation number." 1
However, the broader classification of T-duality appears imply that theories are T-daul if there is any type of transformation that changes on theory into the other.  So the question is whether there are other approaches to showing T-duality, or is there a "canonical" approach for T-duality.
String Theory and M-Theory: A Modern Introduction [Hardcover] Katrin Becker (Author), Melanie Becker (Author), John H. Schwarz (Author)
Answer: "Any transformation that changes one theory into another" (or the same) theory is not called T-duality. It is just a "duality". A condition is that the two theories seemingly look different - otherwise the equivalence would be vacuous - but it must be true that their spectrum and the strength of interactions between their states must be totally isomorphic: physics has to be indistinguishable. A duality is therefore just a very fancy redefinition of coordinates that can't be quite done in the classical limit so you can't really write any "explicit" redefinition. But the impact is the same - the two theories behave in the same way. For each state/object on one side, you find a counterpart on the other side - whose geometrical interpretation may be very different but whose behavior is isomorphic.
T-duality in string theory
T-duality is a duality that changes the geometrical properties of the target spacetime but that holds order-by-order in the expansion in terms of the string coupling in perturbative string theory. In particular, a T-duality must preserve the notion of the "fundamental string" (or "world sheet") and its tension and it is a weak-weak duality when it comes to the interactions between strings. The world sheet of a given shape remains the world sheet of the same shape; however, the fields on it get transformed. That's T-duality in general.
Quite generally, T-dualities that we know may be interpreted as the duality you mentioned although it may also invert the radii of several circles (a torus) at the same moment, and the shape of these circles (and torus) may depend on other coordinates of spacetime (the so-called "Buscher procedure" is a standardized method to perform such a transformation). In particular, "mirror symmetry" relating two completely different Calabi-Yau shapes may be understood as a T-duality acting on a 3-dimensional toroidal fibers of the Calabi-Yau space (a special way to choose coordinates on the 6D shape so that 3 of them look like a torus whose shape depends on the remaining 3.) However, in recent years, people have also appreciated the importance of fermionic T-dualities that mostly act on fermionic fields on the world sheet but that are otherwise analogous.
Other dualities
There are several other important kinds of dualities. S-dualities invert the weak and strong coupling. In particular, for a very large value of $g$ much greater than one, they find another theory with a weak coupling $g'=1/g$ or $1/g^2$ - which is much smaller than one - that is equivalent to the previous one. So S-dualities usually invert a small and large couplings, much like T-dualities exchange small and large radii of circles. A fundamental string doesn't remain a fundamental string under an S-duality; it usually gets transformed into another string, e.g. a D-string (also known as a D1-brane), that used to be heavy and unimportant when the coupling was weak but becomes the lightest and most important object when the coupling is strong. Again, an S-duality exchanges fundamental strings and D-strings, and similar pairs of objects.
More generally, there are also U-dualities that combine the actions of S-dualities and T-dualities: they exchange a strong coupling with large radii with a weak coupling and small radii, among similar actions. In particular, M-theory (essentially 11-dimensional supergravity with the extra "stringy" stuff needed to make it consistent at all energies) compactified on a $k$-dimensional torus has a self-U-duality group which is something like $E_{k(k)}(Z)$, a discrete subgroup of a noncompact edition of the exceptional Lie groups (for $k\leq 8$: for $k\leq 5$, the exceptional group can be written in terms of non-exceptional groups). Such an exceptional U-duality group exchanges momenta in compact dimensions with wrapped membrane and fivebrane charges (and other discrete charges such as KK-monopole charges if there are at least 6 compact dimensions).
Several other equivalences between two seemingly different theories are also called "dualities" - and the AdS/CFT holographic correspondence itself is sometimes classified as a duality as well. It would triple this article if I wanted to cover all important kinds of dualities that appear in field theory and string theory.
Why the existence of dualities was surprising
Finally, it's useful to stress that dualities - a key notion of theoretical physics of the last 20 years or so - didn't have to exist. More precisely, people thought that a theory compactified on an extremely tiny circle, or a theory extrapolated to an insanely strong coupling, is a completely new mysterious beast filled with sea dragons that behaves like nothing that the people had previously encountered. However, it turned out that the behavior of those "mysterious limits" is almost always governed by another "mundane" theory that the people have essentially known, and that the two are equivalent. It's like sailing to India and America and expecting totally new creatures over there - but finding the same animals and plants we're used to.
Whenever some radius is taken extremely tiny, coupling is made very strong, or even the number of colors in a gauge theory is sent to infinity, so that the perturbative techniques of the original theory totally break down, there always seems to be a new description that is totally weakly coupled. That's one of the lessons of physics since the mid 1990s - something that we didn't previously know but nowadays, we do know it. The fact that we seem to understand the "opposite extreme" limits of the previously known theories places an upper bound on the amount of "new enigmas" that may still be hiding in field theory and string/M-theory. If there are totally new enigmas, they have to hide at the intermediate values - like Atlantis in the middle of the Atlantic Ocean, if you wish. ;-)
Comment: +1 Sea dragons. LOL. Actually there are elephants and tigers in India and turkey and buffalo in America, neither of which are native to Europe.
Answer: T-duality involves an interchange $x_9~\rightarrow~x_9~+~2\pi R$ with the compactification of a dimension. A particle running around this circle has a momentum $\sim~1/R$, and this does connect in part with the idea of a reciprocal lattice. The mass of the particle running around the circle is $M~=~n/R$. A closed string can also wrap around a circle. The winding of a string $w$ contributes energy $E~=~2\pi wRT$, $T~=~1/(2\pi\alpha')$ the string. The mass is related to the winding number by
$$
M^2~=~\Big(\frac{n}{R}\Big)^2~+~\Big(\frac{wR}{\alpha'}\Big)^2~+~\frac{1}{\alpha'}(2N~+~w~-~2)
$$
This has the reciprocal structure as well. The connection between the lattice and compactification and string/brane wrappings will have to be brought in tighter with a connection to compactified spaces.  T-duality relates type IIA superstring theory to type IIB superstring theory, and it relates heterotic $SO(32)$ superstring theory to heterotic $E_8\times E_8$ superstring theory.  There is an interchange between mode and winding numbers with $R~\rightarrow~\alpha'/R$.
S-duality is a relationship between two similar field theories with reciprocal coupling strengths.  This is based on the Montenen-Olive relationship which is a form of the Bohr-Sommerfeld quantization rule.  Given charges $q,~g$ for a similar gauge field then
$$
qg~=~n\hbar.
$$
A low energy limit of a type IIB SUGRA with a complex field $\phi$ decomposed into an axion and diliton field as the real and imaginary parts $\phi~=~\psi~+~iexp(X)$ has an $SL(2,{\mathbb R})$ structure, where the discrete subset $SL(2,{\mathbb Z})$ has a modular (or M{\"o}bius) structure, where the field has a linear fractional transformation
$$
\phi~\rightarrow~\frac{a\phi~+~b}{c\phi~+~d},~ad~-~bc~=~1
$$
where for $b~=~c~=~1$ and $a~=~d~=~0$ we have another reciprocating relationship.
The T and S dualities may be composed in various ways to define this less well understood U-duality.

T-duality approaches


ㄹ 수(가) 있다/없다 Problems in the addition of 가

Question: I know that it definitely makes sense to add 가 here because 수 is indeed a pseudo-noun. However will there be any differences between adding 가 and not adding? It would seem that they are the same thing, because I heard them equally frequently used.
Comment: can you explain why  수 is a pseudo-noun?
Answer: They are different. But the difference is subtle and I can't find the general rule right now. Let me instead show you some examples. I never studied Korean professionally, so the following is based on my experience.
When we want to express will, we remove 가.

우리는 할 수 있어.
We can do it.
이 경기는 질 수 없어.
I can't lose this game. (I really want to win this game.)

When we want to express complaint or assert something, we put 가.

모기 때문에 잠을 잘 수가 없다.
I can't sleep because of mosquitos.
이 경기는 질 수가 없어.
I can't lose this game. (There is no way that I can lose this game.)

In my opinion, it's better to remove 가 because it sounds more neutral, but they seem to be almost the same in many cases.
Answer: I reckon there are some different nuiance between each and I will show you additional cases.

수가 있다. There is/are a/some way (for specific purpose given by the context)

<code>       (Korean usually don't concern about articles.)
</code>

이길 수 있다.      We can win.
이길 수 없다.      We cannot win.

I'm afraid to explain them above like this. (Maybe some korean don't agree with me.)
But I would say only these sentences give more stress on the verb to win, in contrast, the others do so on the 'way'(수).

이길 수가 있다.     There is a way to win.
이길 수가 없다.     There is no way to win.
이길 수도 있다.     There is a less certain way to win.
비길 수도 없다.     There is no way even to draw.

For the meaning of 'even', using to win instead of to draw doesn't make sense semantically.

이길 수는 있다.     There is a certain/some way to win.
이길 수는 없다.     There is not a way to win.

As shown above, each structure of -수가, -수도 and -수는 is to mean slightly different nuiance.
Answer: I think they are the same...
Although I can't explain it, 가 is used more commonly in sentences such as 나는 잘 수가 없어 (I cannot sleep).
But sentences such as 나는 이거를 할수 있어 does not use 가 as often.
I think it's that verbs that require 를 afterwards do not use 가, as verbs that do not require it use 수.
Sorry, I only know what sounds correct in my head. :(
Source: My mom is from Korea and only speaks in Korean to me.
Comment: Just a quick addition, I just realized 가 sounds a lot better flowed with 없어 and very awkward with 있어. So my previous answer has derived to that 가 is paired with 없어 and not 있어.
Comment: Hi, welcome to the site! Are you able to edit your previous answer to add this information?


what is protocol testing and what approach can be taken to attain skills in this type of testing?

Question: I have come across this term recently "protocol testing".I would like to know about it. What skills are needed in protocol testing.
Comment: I see a lot of search results for this question.  Did none of that provide you the basic answer to your question?  Or is there anything specific that you would like to know regarding the topic?
Answer: To test that the protocols are working as expected or not is Protocol Testing. Now testing tool will depend upon the protocol which is under test. I'll share the one on which I was working on. You might have heard about RADIUS, DIAMETER etc. which are kind of protocols which enforces rules to the technology which is being used.
How data is getting transferred , How sessions are getting Authorized or Authenticated between different network interfaces, how bandwidth is allocated is covered under testing of such protocols. For Eg. We were using the tool named Wireshark which will capture the packets being transferred between two systems. You need to simulate the traffic generation in this case. So the traffic captured by Wireshark will explain when the session is getting established, when exact data travel has started and how much data is send each time etc. Which protocol comes into picture when some threshold is breached, when consumption is over, when additional bandwidth is required will depend upon the protocol you are working on.
The messages are transferred between different systems/interfaces which will adhere to the rules defined by protocols and whether they are working as expected or not will be covered as a part of testing. This is just an overview and hopefully it will clear the picture.
Answer: Protocol testing is a generic term used by companies working in communication industry for testing of different protocols in domains of Routing, Switching ,Wireless,VoIP,Security,Telecom etc...
Product companies like Cisco,Nortel,Juniper,Alcatel,Huawei... etc have networking devices like routers,switches,modems,wireless access-points,firewalls.. These devices uses different protocols to communicate ,for eg Cisco routers uses EIGRP,OSPF etc to exchange routing information ,so here protocol testing means checking whether EIGRP protocol and OSPF protocol are working fine as mentioned in the respective standards.
Communication protocols or Datacom protocol are different terms used for the protocol working in networking domain ,which are mentioned in TCP/IP stack (main purpose is for routing and switching information exchange.
Usually protocol testing is done by connecting a DUT (device under test) to other devices like routers/switches and conifiguring the protocol in it, then checking the packet structure of the packets send by the devices, checking , protocol algorithm ,their performance,scalability etc using tools like wireshark,IxNetworks,Spir


Why people of Bengal worship Durga also during Basanti Puja?

Question: There is a festival in Bengal region named Basanti Puja in which Bengali people worship Goddess Durga but why they worship Maa Durga in Basanti Puja also,whereas they have time to worship Goddess Durga in Durga Puja? Is there any reason or story behind this?
Comment: Basanti puja is Durga puja only.,the only difference being that instead of the Sharad(autumn) season it is celebrated in the spring.See this link for history and how it originated :https://www.facebook.com/DurgapujaBangladesh/posts/167171333403620
Comment: @Rickross,do you live in Bengal?
Comment: Rohit,......yes i do  :)
Comment: Rickross,I also live in Bengal!
Answer: Basanti Puja is also known as Basant Durga Puja.
It is believed that at the time of Yuddha Between Rama and Ravana. Lord Rama was finding difficulty to defeat the king Ravana. Lord Rama needed blessings of Goddess Durga. Because Rama's wife Mata sita was being kidnapped by King Ravana(known as Sita Haran). So Lord Rama changed the time of Durga pooja for seeking blessings of Mata Durga.
Also the legends of Markanda Purana describes that, King Surath has lost his kingdom and was wandering in forests where he met Samadhi Vaishya who also lost his kingdom. There they both met Medha Muni who suggested them to do Basanti Puja. So they did Basanti Puja and finally got their kingdoms back.
In this way the ritual of Basanti Puja started.
Comment: You should cite sources.
Answer: Basanti Durga Puja is the celebration of Durga Puja during the spring time. It is a huge celebration in the eastern parts of India and especially in West Bengal. Basanti Durga puja coincides with the Vasant or Chaitra Navratri celebrations. If you are confused that why Durga puja is celebrated in the spring time then let us tell you about the origin of the festival.
Scriptures say that originally Durga puja was performed during spring time. However when Lord Ram invoked the Goddess untimely (Akal Bodhan) during autumn or Sharad, people started following the suit. Hence the autumnal or Sharadiya Durga puja became more popular than the original celebration of Basanti Durga puja.Thus,Goddess Durga is worshipped during Basanti Puja
Comment: Welcome to Hinduism.SE!  You should cite sources.
Answer: For the festival of Basanti Puja, it is believed that the King Surath has performed the first ever Durga Puja in the Spring, after getting the instructions from the Sage Mesha. Also, according to the legend in Markanda Puran, it is stated that the king Surath has also lost his kingdom and wandered in the forests. His met Samadhi Vaishya has also lost his kingdom. In the Jungle they both met Medha Muni, who suggested them to perform Basanti Durga Puja for getting back their lost kingdoms. So the Samadhi Vaishya and King Surath performed Basanti Puja and got back their lost kingdoms. So this is that start of ritual of performing Basanti Durga Puja in North Bengal, during the season of Chaitra Month.
Basanti Puja rituals are very similar to Shardiya Durga Puja. But the only difference is in "Ghat" for Bodhan, which is not used in Shashti Puja. This is because this Puja is performed on time when only it is meant to be done.
I hope this post of mine will clear the facts and ritual behind Basanti Puja in North Bengal.


How does Erza get her armor and weapons ready for Requip?

Question: In Fairy Tail, Erza can use Requip, but I wanna know how she gets her weapons and armor there in the first place? Her ability in Fairy Tail is not to put and leave items there, it's only for switching, so how did she place all her weapons and armor there in the first place?
Answer: Lets see what wiki has to say about this.

Requip is a type of Spatial Magic and Erza's signature form of Magic, over which she possesses great mastery. It is a type of Magic that allows her to swap weapons, armor and clothes at will. Her particular form of Requip is called The Knight.

Lets look at what spatial magic means.

This Magic revolves around the manipulation of space, which can allow the user to harass the asset for battle, travel, accessing alternate dimensions, and so on. This Magic is also used in conjunction for summoning concrete material or extraterrestrial beings to their stead, such as being able to conjure up armor and weapons or allow Celestial Spirits access to the human world.

This means her ability is to manipulate that space. So you're wrong to suggest she can't put items in that space.
This means Erza can store, as well as take back armors, swords etc using her spatial magic. Her Armors disappear after being damaged supposedly back to the same space.
I would like to address that the size of the pocket Dimension is limited. So Erza can infact put weapons and the rest are stored in her house. Again from the wiki

Erza owns over 100 different types of armor, and over 200 different types of weapons.
  This Magic allows the user to store weapons and armors in pocket dimensions and summon them during battle.[...]
  The only known user, Erza, stated that there is a limit to how much the pocket dimension can store, and as a result, she leaves some of her armors in her room in Fairy Hills.

I don't know how Erza creates new or repairs damaged armors. From a different question it was answered

Erza is an user of requip magic, in other words, she can summon weapons and armors during the battles, but she does not create them.Answer Source

Other Sources on the Wiki:
Erza Scarlet
The Knight
Spatial Magic
Comment: I don't remember in which episode it was shown, but it definitely is there. There was an episode where Erza brought an armor (it's defensive capability is VERY questionable, though). This suggest that Erza might lack of forging capability, thus, she might brought the broken armor to the blacksmith/shop for repair.
Comment: @AyaseEri Yeah. The world has shown magical shops which sell items even something like clestial keys, but ones forging the armor would be very powerful mages, and since the armors are shown as unique, this has never been properly explained. Erza did say she "bought the seduction armor" though :P
Answer: Erza's Requip magic is where she stores armor and weapons in a pocket dimension and puts them on whenever she wants. Fairy Tail wiki direct quote:

This Magic allows the users to store items in a pocket dimension so they can easily summon it at any time, even during battle, which gives them a high level of flexibility in combat. However, there is a limit to how much the pocket dimension can store. Reequip can be used for simple or combat-related purposes. Some users may just choose to use it to change their clothing. More skilled users are able to Reequip different weapons in battle, and it is even possible to Reequip armors in the midst of battle, although the only Mages to ever do this are Erza Scarlet and Coordinator.

In my memory, I do remember that there was an OVA or a special episode or something where Lucy took a job where she dressed up as a cat girl and was looking for treasure for an old lady who turned out to be the person who raised Erza and some other children. In there, Lucy went to Erza's room in a scene and Erza showed her her other armor and explained her magic (my memory might be wrong though, but Fairy Tail wiki is correct). The special came out when there were only 77 episodes of Fairy Tail, I think.


What are some fun (no TV or computer) activities that a 2 year old and a 4 year old would enjoy together?

Question: The only common activity I've found that works well so far is fort building. My 2 year old boy is very active, but my 4 year old daughter loves to have books read to her. However, when I'm alone with them the fort strategy works well. Can you think of other indoor activities that toddlers of multiple age ranges would enjoy doing together?
Comment: Have you tried just letting them play and see what they like to do together? Or are you trying to create a structured activity for a reason?
Comment: They both want me to be involved, which I don't mind, but it makes it tough to find something where I can be involved with both of them at the same time.
Answer: I know I'm repeating myself here, but LEGO is such an awesome toy. For ages under 5, there's LEGO Duplo which are basically just bigger blocks that can't be swallowed. I'm building stuff with my nearly-2-year-old and it's still fun although I'm 37!
Just stacking and creating all kinds of silly constructions. My son will mostly tear them apart but also sometimes add some blocks - which in turn provides an interesting challenge To me for whatever I was trying to build until that point.
New LEGO is expensive, but we just have one small bucket of blocks that we got used (cheap!), and it's plenty. In fact I like that there are only blocks and no cars and windows etc because that encourages creativity even more.
Comment: Legos are super great.  In the same category, a plain set of blocks are great for encouraging creativity.  (Maybe not so great when your 2-year-old is more into knocking things down, but maybe in a year or so.)
Answer: I have a 5 year old boy and a 2 year old daughter. Finding something they can play together can be challenging, especially because my 2 year old doesn't have a lot of the fine motor control required for many of the things that my 5 year old takes for granted. Its also important to note, that developmentally a 2 year old will not necessarily play with others. Often what will be most fruitful will be to create activities they can do in parallel and activities in which the 5 year can assist (so that you don't have to).
Here are some of the activities I have found that my kids can easily do together:

Sand and Water Table - My kids have always loved their sand and water table and on a nice day that could play out there for hours. Many of the sand tables I have seen have various contraptions and mini-machines that are put into motion when you pour sand and water in them. Those are great, but do require very, very fine grained sand.
Forts - This is an activity that I have to setup for them. But once setup kids can play for a long time, especially if there is enough room in there that they can carve out there own space. For some 2 year olds though it might be asking a lot of them not to tear it down, so who knows...
Drawing themselves - Go to IKEA and buy a roll of their kids coloring butcher paper. Then cut out a long strip, have them lay down and trace outlines of each other. Then have them color it in.
Physical Activity - I probably don't need to give a laundry list here, there are so many possibilities. My kids love to race. I have my oldest keep time by counting, and then the other runs a distance. They encourage each other to go faster, and delight in celebrating crossing the finish line.
Sidewalk Chalk - Hours and hours of fun. Period.

You are at a challenging age. I empathize. My 5 y.o. likes to build, but my 2 y.o. literally cannot wait to tear things down -- which is intractably incompatible. My 2 y.o. also wants to do whatever her older brother is doing. Which sounds great, but if he is drawing then she is only content if she is drawing on his paper too. So there are not a lot of things they can do collaboratively. The trick is finding something they can do independently, but side by side.
I have found though that left to their own devices, they will surprise me by entertaining themselves. And they will find fun in the simplest of things together. Be it spinning in circle till they fall over, or jumping in a pile of leaves.
Comment: Based upon your last comments we could be living in the same house :) Thanks for all of the great suggestions. With winter approaching I hate that many of those activities (that we already do) will have to wait until next year :(
Comment: +1 Very true, my sons do the same.  The oldest is the leader and the youngest only wants to do what the older is doing, and building blocks is the best example.
Comment: OMG!  Do you secretly have video cameras installed in my house?
Answer: I'm keeping an eye on this one, I have a 6 year old and 1.5 year old and I have a hard time doing anything with them where I am involved.  Together they play pretty well, so long as I keep the older one in check, but with me it's pretty much the following:

playing ball (tossing a ball like baseball or football with really soft ones for the younger)
building towers with blocks and watching them get destroyed
kick ball, but you need two as my younger likes to run off with his
slides, you will get a workout lifting one of them probably
pulling the kids on a blanket, with our wooden floors they have a great time sliding around
restaurant, we have kids food and cooking utensils, the youngest LOVES to bang the pans

I hope to get some other ideas as well!
Comment: +1 for the cooking utensils, works great with my 3 year old and 1.5 year old.  Real ones or toy ones, doesn't matter, they play restaurant or pretend to shop and cook, it turned out to be one of their favorite things.
Answer: My eldest two are two years apart. Things they loved to do together at that age (and even now for some of these):

trampoline
sandpit
lego
gardening with me (yes, really) - they loved to pull up weeds, with some guidance as to what a weed was
building towers of blocks, and then demolishing them
Comment: I love the gardening part, though I never got my oldest to understand the difference so I just give him a spare area to dig in - that keeps HIM amused.  Not so much for the youngest though.
Answer: How about puzzles?
Our 3 yr old loves doing smallish jigsaw puzzles. Whether they work independently on puzzles of differing difficulty or together on a single puzzle its great cognitive exercise and comes with a great sense of accomplishment when they are done.
We have found some great puzzles available for not very much money, in fact there are some packs of puzzles that come with multiple puzzles of differing difficulty (24,48,63 or 48,63,100 panoramics are a hit at our house) that would probably be great for the 2 yr old on the smaller end and the 4 yr old on the higher piece count end.
Comment: Yeah, this is a good one, as my daughter likes puzzles. The trick is finding something similar for the 2 year old since he isn't a big fan of "sitting". He isn't a wild man, but if he is playing he usually wants it to be something with more activity.
Answer: There are plenty of childrens cookbooks out there with simple recipes that anyone can enjoy. Even if you're not into that, something you might do regularly (like monthly or maaaybe weekly) is tube-dough cookies with your own frosting or toppings. A nice little treat for their alone time with daddy. Pizza can also be done this way . . . Generic frozen cheese especiale (or whatever) and add your own toppings. They'll love flopping pepperonis on it.
Then there's Highlights magazine. Always a winner.
I would also suggest just make some crap up. My wife told me the story of how one day her mom decided that they were going to go on an Alpine Adventure. They went to Sears and tried on coats, went to a sporting goods store and hung out near the ski display while the sales guy talked about all the places he's skied and they asked questions. they did a couple other things and capped it off with dairy queen. Along this line, you could do say a 'camping adventure' which winds up being little more than a glorified picnic in the yard. Doesn't really work indoors, but there's nothing to stop you from making up your own indoor games... you can do a lot with paper, markers (dry erase around the kiddies isn't so bad), playing cards "if you draw a mans face you have to fly like an airplane to the stairs and draw a smiley face on your page!"
So after going thru all the specifics, i think my point is that you can be as creative as you want to be. It's less important what you do, so long as you're doing it hand in hand with them, spending time with them.
Answer: A good one is stuffed animals and role playing.  You can make a house out of pillows and put an animal in there, invite another animal in for tea, and see what happens.  You can create conflicts, have a "bad guy", and then do conflict resolution.  That can be fun.  Be careful though.  Use your judgement as to what is appropriate for the animals to do, because they may take it and run with it.
We also used to have a math game which I called The Bean game, which was only playable for a short time but helps to build math skill in a fun way.  I have no beans.  I need to go to the shop to ask for some beans.  He has 10 beans.  He gives me 3.  How many does he have left?  He gives me 2 more beans.  Now how many do we each have? (WARNING: this game gets boring quickly).
Answer: one thing that my kids love when there is snow outside for the winter is really simple and cheap get a spray bottle and fill it with water and food coloring then they can go outside and paint the snow. Something that keeps them entertained in the colder weather.
Answer: Mine (4.5 year old boy and 2.5 year old boy) have a cubic metre tent that looks like a pirate ship. They stuff a blanket in it, assorted toys, fuss about taking their shoes off, optionally put on a pirate shirt, then stuff themselves in, and go ARRRRRHHH!
A big box, without the stuff, works well too.
The older one likes welcoming the little one in his home, the little one is awed that he's playing with his big brother and any excuse to shout is always welcome with him.


Disabling "symlink follow" feature on Ext3/4 mount points

Question: How can I disable the symlink feature of ext3 file system mount point ?
My server is being attacked by some sort of symbolic link hacking methods for reading other user's home directory.
There is such an option "nosymfollow" on BSD series, but I didn't find the equivalent for Linux.
Answer: There's no such option for ext3 or ext4, to the best of my knowledge. Maybe if you described the actual nature of your problem someone could propose a different solution.
Answer: I don't believe there is an option for this.  Perhaps if this is through something like Apache a Directive might solve the problem.
You could of course blow away all the symlinks under a specific directory:
<code>find . -type l -print0 | xargs -0 rm
</code>
Answer: Yep, ext3 doesn't have support for this, ufs on bsd does.  I'm not sure what context this is happening in, but I'm going to assume that some portion of users home directories is being served up by apache, which if you have the "FollowSymLinks" option enabled can cause issues.  You'd want to disable this if this is the case, but you'd really have to provide more info as to what is actually happening for a more detailed or better solution.
http://httpd.apache.org/docs/2.0/mod/core.html#options


What is the evolutionary purpose of the topology of the human ear?

Question: What is the evolutionary purpose of the topology of human ears? I understand why the ears may have a funnel-like shape but if the various "hills and valleys" do not amplify incoming sound, what purpose do they have?
Answer: Humans do not have the ability to move their outer ear in response to sound. Many animals can do that, and use it to determine the source of the sound waves. Thus, human outer ears are equipped with many "hills and valleys". It does not provide amplification (because the waves can lose their energy bouncing around the ridges), but rather gives the brain more information about the sources' location in space. The brain calculates that location by determining minute differences in the time it takes for the sound to reach sensory cells in the inner ear on both sides. The shape basically attenuates the raw information. That way you get different properties of the sound wave that enters your ear depending on the angle at which it fell on you outer ear.
Comment: Could you support your answer with references? Have the effects of different pinna morphologies been tested in humans?
Comment: Different ears are shaped differently. Does that mean the brain is aware of the topology of the ear and aware of how sound is affected by it?
Answer: Humans can localize the horizontal location of sound ("azimuth") using temporal cues (sound reaches the 'closer' ear first') and level (the head blocks some sound so it louder at the 'closer' eat) cues, depending on frequency.
For detecting the "height" or "elevation" of a sound, other cues are necessary. "Pinna cues" refer to a pattern of level distortions at different frequencies that, combined with predictable elements of natural sound (such as harmonics), allow the listener to determine elevation. The head also contributes, so these cues are grouped together as a "Head Related Transfer Function." Importantly, the two ears are never perfectly the same, so these cues can be differentiated from the azimuth cues I described earlier. One can measure these pinna cues by placing a microphone in the auditory canal.
The Wikipedia article on sound localization has some more information, as do several online course postings and textbooks, for example this one, or by searching for terms like "sound localization elevation pinna".
Comment: Yes; "spectral cues," "head related transfer function," "pinna cues," - these are all related terms with varying levels of application/specificity but they effectively refer to the same thing.
Comment: I'm guessing this is the same as spectral filtering for sound localisation?
Answer: One possible advantage is that's it's probably the best adapted to being able to hear a very low volume source of sound no matter which direction it's coming from. A horse ear on the other hand, is probably the best adapted to being able to hear an even lower volume source in only a very narrow range of directions. One reason for being able to hear well in any direction is to avoid predators. Maybe in our evolutionary history, we sometimes went into a jungle and there was a dinofelis lurking in it with silencers on its feet and we couldn't see it so we evolved ear the shape we did so that we could hear it from really far away if it started walking no matter which direction it was from us and prepare to fend it off with spears. Maybe we always had ears the shape we do then we just started evolving to have horse like ears so that we could keep moving our ears until we locate the dinofelis and then in response to that, the dinofelis evoled to stop walking a very short time after it starts when it's in the jungle so that we can't hear it from further away anymore so it became advantageous to hear as far as we can in any direction so we stopped further evolving horse like ears and that's why some people can move their ears a tiny bit. One reason for being able to hear such a low volume source in any direction is to catch prey. Deers probably evolved ears shaped the way they are so that they could keep turning them until they hear a prey animal and then keep their ears pointed towards it so that they can keep hearing it so that they can hunt it.
Answer: Evolution doesn't have a purpose. In that light, it's quite possible for an ear to be poorly adapted to hearing - which in fact it is compared to other species.
Stereo hearing (two ears, so you can better detect where the sound is coming from) is heritage from our ancestors, and I know for a fact it didn't evolve further in humans because there was no evolutionary pressure.
In any case, the topology isn't exclusive to our species and is heritage from our mammalian lineage. At some point in our lineage, the species/individuals who had this topology of the ear were able to gather sound waves better and that affected their survival so they passed on the trait to their offspring. Ear topology didn't evolve in humans.
Comment: This answer is misleading and takes the conversation in an unnecessary direction - the question doesn't include an explicit misunderstanding of evolution. I don't believe the question needs to be read as "what are the unique adaptations for hearing in the human ear." I read the question as "Ears look funny. Is there any reason applicable to survival/fitness for ears to look the way they do?"
Comment: Can you provide citations to support your assertions? What makes you certain that the ear in humans has not been subject to selection? I'm not suggesting that it has, but I would like to know if there is evidence to the contrary.
Comment: Just take a look at the ear(s) of any great ape. Clearly ear hasn't been changed much within our species and the development took part much earlier in our lineage.
Comment: But we don't know what the ear of the most recent common ancestor of humans and chimpanzees looked like.
Comment: No, we don't, because we don't know what the common ancestor is, when it lived, and what it looked like. No fossil has been found. We can only presume ears of this shape are a shared trait derived from the common ancestor, otherwise we'd be saying it evolved independently at least four times. That's next to impossible.


Why is using a password manager better than a simple passwords.txt file?

Question: Genuinely curious. What types of attack is a password manager (such as Keepass) designed to stop? If a hacker gets access to your system, it's game over anyway, so what is the added value of encrypting one's passwords?
Comment: Story time: A few years ago, my sister decided to install a peer-to-peer file sharing application on her computer. What she didn't realize is, by default, these programs give everyone else on their network access to your entire C: drive. Upon realizing this, and knowing how many ignorant users probably use this program, I searched for "passwords.txt". I found credentials to everything imaginable: EBay, PayPal, bank account, Facebook, etc. Had they been encrypted, I would had to have guessed the master password. Instead, I was granted immediate access to hundreds of sensitive accounts.
Comment: If you need more proof that this is still a huge problem and a terrible idea, I just Googled "passwords.txt" and found this in a matter of 30 seconds: https://classconnection.s3.amazonaws.com/8dcd59a2/3502418/files/1fc7-456f-a117-c21bc04896c3/large/preview0.jpg Don't be like Jenny.
Comment: If they're encrypted, how is the hacker going to decrypt them, assuming you've not saved the password manager password in a text file? Also, they tend to be capable of generating passwords, which humans aren't that good at...
Comment: Any idiot with 1 min access to my computer could open `passwords.txt`. Any idiot with 1 min access to my computer could not install malware to extract my passwords from the encrypted file a password manager store them in.
Answer: With a password manager, your passwords are not stored in plaintext on your computer. In other words, even if a malicious actor gains access to your system they will still not have your passwords.
Password managers also do a good job of helping you create long and complex passwords (vs. smashing on the keys in an Excel document), in the event that the site you are logging into becomes compromised. You can also have different passwords for each site, further reducing your risk. Additionally, you can have a USB key that is required to access those passwords, so when you aren't near your computer you take away the access key. Attackers can brute force all day long, chances are they won't get access to the passwords. This beats a password protected file all day long.
Side note: when doing penetration tests, after gaining internal network access, one of the first things I do is search all shares on the Domain for the string: "Password". I get hits every time :(  don't let that be you.
Comment: If a malicious actor gains access they can capture your master password with a keylogger then get all your passwords. I agree that a password manager is a good thing, but you shouldn't oversell the benefits.
Comment: Nowadays with everyone being in **the cloud** you can just store the password database in the cloud directory and it will be synched over all your devices, always having them with you. This defeats the argument of not be able to access them everywhere.
Comment: "gains access" means remote code execution not file access. Key loggers can work without UAC. As for the USB key, I've not looked at them in detail, but I would be highly surprised if it effectively defends against malware. Layers are good - but not if one layer makes promises that cannot be delivered.
Comment: @paj28 you're assuming they have *more* than file-level access and that they can bypass UAC (if enabled) to run a keylogger, and potentially local-admin to install the keylogger. And again, if the user sets up the pw manager such that it needs the USB key to decrypt the db, the master pw still doesn't help the malicious actor. Agreed that it's not a silver bullet, nothing to oversell, it's all about layers ;)
Comment: I don't want to be pedantic and debate the meaning of *gaining access*; as a pentester, I don't always need RCE to get access to something. But that aside, I think we're all on the same page here. Your point is received. Cheers
Answer: There are different ways of getting access to a system. Consider the following two scenarios.
Scenario 1: your apps are sandboxed, an app is compromised
Let's assume you use a system like Apple OS~X with Mac App Store apps. All your apps are sandboxed. Some apps, like text editors, have a legitimate reason to programmatically access your text documents. If one of those apps is compromised by malware, then the attacker will be able to read your password file.
However, assuming you have a password manager implemented as on Linux, where apps can use inter-process-communication to retrieve exclusively the passwords that they stored themselves, then the compromised app will not be able to extract all your passwords. It will only be able to query (and send to the attacker) passwords that relate to the compromised app itself. And the attacker could steal these when you type them inside the app anyway.
Your attacker would need to (1) break out of the sandbox and (2) either break the encryption of the file where the password manager stores your secrets, or get the password manager itself to run an exploit.
In this scenario, the password manager definitely represents an increase in security. Other attack vectors include things such as abusing virtual input interfaces to install keyloggers. Let's leave these aside for now, and just say that when sandboxed, at least you get to mediate and control which apps use such capabilities.
Scenario 2: no sandboxing, any app is compromised
Your text document is obviously vulnerable to every single app you have. More apps can be used as entry points to your system for the purpose of stealing that file.
Your password manager is barely protected at all. Attackers can trivially modify any aspect of your desktop to install keyloggers, cause you to load additional libraries on your whole user session, maybe even replace the password manager's binary. They might be able to inspect the memory of your running password manager, too, depending on the OS and how it is configured.
The attack is still significantly harder to perform than simply stealing a text file, especially if the exploit being used only allows the attacker to target and extract a single file rather than execute arbitrary code. Thus, the password manager reduces risk for more basic attacks, but not for more advanced ones.
Conclusion
Password managers provide some degree of additional security right now, as they encrypt the content of your passwords file and require attackers to compromise the password manager itself.
However, their true benefits will increase once sandboxing becomes more systematic. It will then become much harder to attack the password manager or the user session itself.
Answer: In addition to the other answers, an encrypted file is only vulnerable to malware while you are using it. Your passwords can only be stolen while you have it unlocked.
This means they can't be stolen via physical theft of a phone, laptop, tablet, or USB storage. It means they can't be stolen from whatever cloud or network storage is used to sync them. They can't be stolen from discarded hard drives, backups, or devices, even if you don't erase them as thoroughly as you should. If you use an online service instead of a locally managed application, encryption means your passwords can't be stolen by a breach of your provider.
It's true a specialized malware app on your device can defeat any password manager while you actively use it. But it's also true the same malware could steal any passwords you type in, anyway. So it doesn't make you less secure to encrypt your passwords, and it does reduce the attack surface you present, or at least enables convenient behaviors that would be reckless without encryption, such as syncing via cloud storage, or carrying around a copy in your pocket on a thumb drive.
Answer: In addition, a browser-based password manager will help you avoid phishing attacks, since it will not be fooled by the apparent domain name and simply will not auto-fill the password field in the phishing URL you may inadvertently have clicked!


Can you waterproof Luxury Vinyl Tile floors without grout?

Question: My questions are on the use of Luxury Vinyl Tile (LVT) flooring - my preference is to use the LVT without grout. However, I'm skeptical on how well the LVT will hold up to moisture without the grout and cleaning will be an issue.  Am I right about the need for waterproofing or would the flooring work well any way?  Is there a way to waterproof the joints on LVT without grouting? If water gets between the floor tiles as you step out of the shower will it cause even bigger problems?
Comment: Friendly reminder to anyone reading this: grout isn't waterproof, the product Ben Welborn mentioned below however is waterproof.
Answer: In some areas, you can get away without having grout, but the tiles will be more susceptible to damage because grout (or something) adds strength to the floor (and keeps the tiles in place). You may have seen or heard about how some people make really tight fittings with beveled tile, and they don't use grout. This is more for areas that are onramental or where contact with water is unlikely.
In the bathroom, I think it's safe to say that you need something between the tiles especially because there will be water (on the floor). Another purpose for grout is to protect the underlayment from water... water damage could be summarized as swelling which can cause lifting/ungluing of tiles and warping and loosening of the wood and rotten/weakened wood or just plain mold and mildew. And yes, cleaning will be an issue without grout or something between the tiles.
There are alternatives to cement grout; you should search online for grout alternatives. Epoxy and caulk come to mind. If you don't put something in there, something else (dirt) will eventually fill in the gaps.
Edit- (based on the comment "my ex-husband already laid the tiles without spacing for grout")I don't like to promote products, but I'm not aware of any other silane vinyl sealers, so try a product called Infuze. Here's the technical data sheet. Like I said, it's a silane product that I think will work for this. It's kind of messy and smelly and dries quickly (so work as quickly); it will definitely seal vinyl seams. I think it chemically bonds to vinyl to create vinyl silane. It dries clear and doesn't discolor.
Comment: Unfortunately, my ex-husband already laid the tiles without spacing for grout so I'm looking for a sealant that could go over the tiles to seal them from water and dirt. I have already caulked around the tub, toilet, and walls, but my son is a teenager that doesn't always stay on the bath mat when he steps out of the shower and I frequently find the floor wet afterwards. I know I could use a polyurethane, but the tiles are white and gray marble and I don't want them to yellow.
Comment: @DIYMommy I'm not big on recommending products, but I used something called infuze once for sealing a jagged tear in a vinyl floor. I think it could work for this. I'll edit my post with a picture.
Comment: Thank you for your suggestions. Someone also recommended Rejuvenate floor protector so I'm going to do a little more research on each product. I will post an update after I try one.
Comment: @DIYMommy Rejuvenate floor protector is floor polish... (specifically it is a chemical called Diethylene Glycol Monoethyl Ether) it's added to some brands of floor wax. Floor wax isn't actually wax, it's typically an acrylic polymer. But again sometimes they mix the acrylic polymer with polishing compounds like Diethylene Glycol Monoethyl Ether.
Comment: @DIYMommy Floor wax might work for you... but you need to keep up with it. Waxing the floor is a "regular" sort of chore.
Answer: Perhaps I don't understand your meaning. Click, one type of vinyl flooring, doesn't use grout. It is waterproof.


Why assume $R$ Noetherian for the primary decomposition of $R$-modules?

Question: When considering primary decomposition in modules, why is it often assumed that $M$ is a finitely generated module over a Noetherian ring $R$ and not only that $M$ is Noetherian? As I understand this extra assumption is not needed to prove the existence or the uniqueness properties of the primary decomposition of submodules.
Answer: I agree with you that it seems slightly more clean to simply assume that M is Noetherian. However, if M is a Noetherian module over a commutative ring R, then M is a faithful module over R/ann(M), and the latter ring is Noetherian! So in a sense, there's very little distinction between the two cases.
Answer: I think that's because in a lot of proofs the people use the fact that $M\neq 0 \Rightarrow \mbox{Ass}(M)\neq \varnothing$ and this is true only for Noetherian rings.
Idea: Let $\mathcal{S}=\{\mbox{Ann}(m)\subset R,\;m\neq 0 \}$. If $R$ is Noetherian then $\mathcal{S}$ has a maximal element, say $\mathfrak{p}=\mbox{Ann}(m)$ with $m\neq 0$. Then $1\notin \mathfrak{p}$ as $m\neq 0$ and if $xy\in \mathfrak{p}$ but $y\notin \mathfrak{p}$ then $xym=0$, but $ym\neq 0$ and therefore $x\in \mbox{Ann}(ym)$. It's clear from the definition that $\mathfrak{p}=\mbox{Ann}(m)\subseteq \mbox{Ann}(ym)$, so $\mathfrak{p}=\mbox{Ann}(ym)$ by maximality. Thus $x\in \mathfrak{p}$ and $\mathfrak{p}$ is a (associated) prime.


In what context do we write "Guten Tag" rather than "Sehr geehrte Damen und Herren"?

Question: Formal letters almost always used to start with "Sehr geehrte Damen und Herren," or "Sehr geehrter Herr Maier" if the person is known. However more and more we see formal correspondence that addresses with "Guten Tag," or "Guten Tag Herr Maier". The former may be less fashionable but is there any consent on what occasions we rather not use the one or the other?
Answer: The salutation "Guten Tag" in written communication is a more informal variation of "Sehr geehrte/r....".
Both essentially say the same thing, but "Sehr geehrte/r"  has been the accepted way to formally address a person for I don't know how many decades.
In my personal experience, "Guten Tag" has gained traction especially in industries that cultivate a more laid-back attitude - Media, parts of IT, and so on - where "Sehr geehrte/r..." is perceived as overly formal. It is becoming more and more popular among younger people (in their 20s, 30s, 40s) in general, no matter what field they work in. I have seen federal employees in their twenties use "Guten Tag" (although it's not the norm).
If in doubt - say, when applying for a position with a German Bank - you can't go wrong with "Sehr geehrte/r".
For everything else, especially if you are on friendly terms with the recipient,  "Guten Tag" (or, if you're really familiar with the person, later, "Hallo") is perfectly acceptable.
Comment: I would never *write* Guten Tag and have never seen it. (This is not to deny that other people use it, just saying.)
Comment: I find "Guten Tag..." a very elegant solution for private, but formal Emails (like asking for support online).
Comment: Online support or writing an email to a company as an individual client. My biggest problem with "Sehr geehrter Herr..." is that it is that you have to know the name, and "Sehr geehrte Damen und Herren" feels like you´re holding a public speaking, or writing a B2B letter.

I often write "Guten Tag" in my work life, but normally I use "Sehr geehrter", because it is never wrong (even if it can sound a little bit "cold" or official) and because writing "Guten Tag" 5 times a day to the same person feels strange.
Comment: @thei fair enough, but it *is* in the ranks of generally accepted salutations: See e.g. http://din-5008-tipps.de/die-anrede-nach-din-5008
Comment: And, when in Austria, stick with "Sehr geehrte..."; on the one hand, Austria tends to be more formal than Germany, and on the other hand, "Guten Tag" is perceived as typically German (as opposed to Austrian).
Answer: In my experience you usually use "Sehr geehrte Damen und Herren" to begin a correspondence, possibly when you're not entirely sure who is going to read/respond to it.
As the correspondence continues, it may be adequate to switch to "Guten Tag", as it builds up some familiarity with the correspondant.
It could easily be considered a little too formal to start every single message in the conversation with "sehr geehrte Damen und Herren" if it is already clear who's going to reply to you.
Comment: Regarding the last paragraph; when you know who is replying, it is actually better to write "Sehr geehrte Frau X"/"Sehr geehrter Herr X" instead. But other than that, I would agree that "Guten Tag" is a bit more casual in a letter (although a simple "Hallo" would be fine too).
Comment: @poke: In many situations, I would perceive "Hallo" to be way too informal, and instead switch to "Guten Tag", which I perceive as *almost* as formal as "Sehr geehrte/r ...". That applies especially when I know *who* is going to reply, but I cannot figure out from the name whether it is a male or a female person. In that situation, I tend to avoid the very unpersonal "Sehr geehrte Damen und Herren" and instead use the equally polite, but much more sympathic "Guten Tag" (either without a name, or with the with the full name, thus omitting "Herr"/"Frau").
Answer: You don't use Guten Tag in the evening or at night. ;)
No, seriously, Sehr geehrte Damen und Herren is the way to go if you are writing a formal letter to someone you barely know. If you meet someone in person, you could say Guten Tag!.
Comment: Actually, that's one reason why I often prefer "Guten Tag" over "Sehr geehrte/r ..." in letters, as the former allows for some variation ("Guten Morgen", "Guten Abend", depending on the time of the day) in the course of longer letter/e-mail exchanges.
Answer: I have never seen a letter (or an email) that starts with "Guten Tag". In what context have you seen this "more and more"?
If a letter is more informal it will start with:

Liebe Frau Müller! (Dear Mrs. Müller)

or

Liebe Mitreisende! (Dear travel group)

If it is an informal email, you can start with "Hallo" or "Hi".
Comment: I find that salutation kind of intrusive; neither would I want to judge, or imply, the recipient's personality in the salutation, nor would I want to be judged like that when receiving communication. From what I have seen (in a part of the CS sector, that is), using the less intrusive "Guten Tag" is very common whenever "Sehr geehrte/r ..." is perceived as either too formal or too repetitive, both in e-mails and in snail mail letters. Interestingly, the group salutation ("Liebe Mitreisende" in your example) strikes me as less offensive than the one to a single person.
Answer: I would use the first one if i'm writing to a group of persons and i don't know who will read my letter. Both is formal and may be used in any case.
Comment: I edited the Q to point to differences even if the person is known.
Answer: I think the difference is that 'Guten Tag' et al. are conversational (rather than written) greetings
Answer: If you are a native English speaker contacting a German media or communications professional, is it preferable to use sehr geehrte. Guten Tag or Hallo may be used in an informal setting but it may be too informal if the professional in question is not someone I have spoken to, corresponded with or even met before.
If you are an English speaker writing a request for some information about a product or requesting some data, using the "wrong" salutation may not have such negative results as when written by a native speaker. Still, in my media experience, any letter or email that was addressed to myself specifically, or the newsroom in general, that was strangely (read: incorrectly) addressed would
generally be a red flag that the subsequent letter/email was going to be garbage.
Is it wrong then to assume that a letter to a German-speaking professional with the wrong salutation will not be ignored or deleted without any response.


Mounting a drive in ubuntu within a mounted directory

Question: I am using amazon and have elastic block stores (in all respects a NAS drive) mounted on a directory /data. Now if the NAS drive I mounted becomes full I would like to mount additional NAS drives. Now there are 2 ways of doing this:

Take a snapshot of the existing NAS drive. Unmount it, mount a bigger drive and copy the snap shot back in.
Mount an additional NAS drive.

I would like to mount an additional NAS drive to keep my cost down. I will be charged for creating and storing a snap shot. And I dont want a downtime. I am not great with linux, hence the question..
Can I mount another NAS drive inside the directory /data/abc? /data is already mounted onto NAS1.. Can I mount /data/abc onto NAS2? So all other folders would continue to exist on NAS1 and the new abc directory would point to NAS2?
If this not possible then I would probably have to mount NAS2 as /data1 and create a symlink /data/abc -> /data1
Could you guys let me know which approach is better?
Comment: this question should be moved to [superuser](http://www.superuser.com)
Answer:
Can I mount another NAS drive inside the directory /data/abc? /

Sure


Grails subflow restarting to first state

Question: I have a flow that uses a subflow. Something like:
Initial State -> Subflow -> Next state
When I'm in the Subflow, I can go back and forward, even return to the 'Initial state' without any problem. However, once I'm out of the subflow and in the 'Next state', any attempt to go back send me directly to the 'Initial State' not to the last step of the subflow.
Any thoughts about that?
Thanks
Answer: Find the problem. The 'Next state' was the last state, this is why the flow was being restarted.


Over 200 links in yahoo, none on google webmaster tools?

Question: I have over 200 links on yahoo site explorer, but only 2 on webmaster tools. Does anyone know why this is? My site is in the sandbox by the way, but i dont think that that effects the backlinks.
Comment: How do you know it's in the "sandbox"? I haven't heard that term for some years now...
Comment: because when i search for it using its domain name ex. site . com, it doesnt show up. It only shows up for site:site.com
Answer: Google Webmaster Tools shows a sample of the links found for your site. To get more data, it sometimes helps to verify both the www and non-www versions of your site. Generally speaking, with 200 links you can probably also check manually to see if the pages linking are indexed in Google.
Also: not ranking for your site's name is not a sign that your site is in a "sandbox" (which technically doesn't exist anyway), but it does sound like your site might not be that trusted by Google's algorithms at the moment... which is something that I'd recommend working on more than just having the links show up in Webmaster Tools.
Answer: The two sites are indipendent and so Yahoo may have discovered links on pages that Google had not crawled yet. Another question to bear in mind is that Google group the link to the same site (if you click on that page you'll see the list of the website who links to you) while yahoo shows a row for each link.
Answer: Google doesn't show all of the links, at least not immediately, and they, sometimes but not always, tend to ignore certain types of links, such as nofollow links or links their algorithm considers suspect. Also, as Aurelio noted, they consolidate links sometimes. Yahoo shows just about everything their crawler finds.


Can an external server be used to encrypt all traffic

Question: My work takes me to a number of places with WiFi connection. I want to be able to encrypt the traffic between my laptop and the internet. Can I do this with a VPN server? What is the best way to get this done? Is there a secure and reliable service that will help me do this? This is fairly important and might be shared by upto 2 people at a time. Is it better to get a server setup on a cloud hosting solution? Amazon?
Comment: I am developer so I can setup VPN's to some extent and don't mind paying for a good solution.
Answer: You could set up an end-to-end encryption solution between your laptop and another computer in a trusted network using a VPN. If you're trying to protect your connection from people who may be intercepting data at, say, a coffee house or hotel, this is your best bet.
I personally set up SSH tunnels to encrypt email connections to a system I trust, as web browsing generally has SSL certs protecting the web banking and things of that nature, and other web browsing is boring stuff that anyone watching probably wouldn't care about. If you're not system savvy, though, SSH forwarding is probably not the option for you.
Otherwise, we use a Cisco VPN solution with a Cisco client on the laptop.
If you share your wifi connection or wired connection to route someone else since you said that are 2 people at a time, they would need to protect the connection with some sort of VPN as well unless they are directly wired to your laptop and you are then forwarding traffic through your VPN.
There are free solutions available like FreeSWAN. Try googling for articles on ipsec and free VPN solutions as this will depend on what you have available to you for what options you can choose. Linux? OS X? Windows? Are you going to terminate your connection at the employer? At your home? You have to set up port forwarding and encryption on the server/router you're configuring at the end point as well.
Also remember that after your data hops out of the endpoint, it's no longer encrypted. You may be safe just from your laptop to your end server. Sniffing traffic on the end point computer or something within that network can see everything you're trying to hide. You're also imposing a processing overhead to deal with encryption as well as routing, if you're routing ALL your web browsing as through a proxy on the other side of your connection.
If you're really worried...you don't state the nature of your work or where you're going...you can look up TOR, The Onion Router, I believe it's called. Very slow, but it's made to obfuscate and protect people who are worried about things like being killed for having an opinion, such as dissidents in China. It routes traffic through multiple hops all over the world before the traffic emerges to get a web page and suck it back to your system.
Answer: There are some services which will give you a VPN to connect to for accessing from hotspots.  It should get your traffic out of the wireless hotspot securely.  But after that you would have to trust the provider.  In the long term it's probably best to setup your own VPN if you have the knowledge and equipment to do so.
Answer: https://xerobank.com/ offer a fully encrypted web connection for a monthly fee.
They also have a free trial period to check it out.


What is the difference between a Start-Up and a small company which is not a Start-Up?

Question: What exactly makes a small and youg company a "Start-Up", among other small and young companies which are not necessarly start-ups?
Comment: Right, but if "all economics is about companies", "company" is a quite useless tag, as it would have to be applied to all questions, isnt it? I removed it.
Comment: I'm voting to close this question as off-topic because it does not seem to be about economics terminology rather about business or general terminology.
Comment: Please add a description for the new tag "company" that you just created, or risk it being removed again. Right now, I personally don't know what to associate with that. Perhaps you rather want to use *firm* or *industrial-organisation*
Comment: Huh I created it? That was an accident. It's hard to belive it didn't already exist, after all economics are all about companies, is it not?
Answer: Probably, there's no clear-cut definition, however, there's an article of Paul Graham called Startup=Growth which represents the opinion many agree upon: startup is a company that plans to grow fast (... and scale). A quote:

A startup is a company designed to grow fast.
      Being newly founded does not in itself make a company a startup. Nor is it necessary for a startup to work on technology, or take
  venture funding, or have some sort of "exit." The only essential thing
  is growth. The only essential thing is growth. Everything else we associate with startups follows from growth.

That's why a new barbershop isn't a startup since

A barbershop doesn't scale.

This definition probably still leaves some space for interpretation, but not that much.
Comment: Cool. So if someone makes a company for a novel technology, but plans for the company to remain very small, then it's not a start-up even though it shares similarities, right?
Answer: There is no clear separation between what is and what is not a start-up. It's a non-technical term, and so has all the usual blurriness and context-sensitivity that goes along with that.
A start-up is usually taken to have come from nothing, with a new product or service (or new variation of existing ones). Its founders typically put in all the equity themselves, perhaps leveraged with some debt (maxing out personal credit cards, for example); and then manage it, and typically provide (almost) all the labour inputs themselves. And as a founder, it consumes all your time and energy.
Whereas some young companies are spun out of existing enterprises. Some young companies start with an existing product, and/or have separation of ownership, management and workforce. They would typically not be referred to as start-ups.
Comment: I would add that in my experience, having "the founders put in all the equity" relates to start-ups being in sectors with less capital required, typically involving IT and located at the service sector.
Answer: I might be late to the party.
A Start Up is a stage where company is still not so clear about so many things, viz

Models to profit
Clear plan about the product
Clear plan about their target/market

In simple terms they are figuring out those and many such things. Once they are done with clear product, cash flow model and market they come out of Start Up stage (if they still managed to sustain).
That's why Yahoo! was still called as world's biggest Start Up till 2015 or so because they might had not figured out so many things and still experimenting
Comment: A company with a clear business plan can still be a start-up. In fact, having no clear business plan is a rather bad sign from a start-up.