Why don't people without access to clean water drink beer anymore?

Question: In the medieval ages, peasants used to drink beer instead of water because the plain water wasn't safe to drink. Why do people in countries where the water sources are polluted not do this today? Why did this practice emerge in some countries but not others?
Answer: "In the medieval ages, peasants used to drink beer instead of water because the plain water wasn't safe to drink... Why did this practice emerge in some countries but not others?"
One really good reason is this -- it's not true in the first place. It's a myth, and a very common one. People didn't use spices to cover up the flavor of spoiled meat, either!
This recent well-sourced blog post by Jim Chevallier (a food historian) discusses the topic, including quite a few period quotations, such as:

"The thirteenth century doctor Arnaud de Villeneuve said that water
  was better for quenching thirst than wine but recommended drinking it
  from a vessel with a small opening or a narrow neck in order not to
  drink too much. In the fourteenth century, Maino De Mainer (Magninus
  Mediolanensis) wrote the 'Natural [drinks] are twofold, that is, wine
  and water. These drinks are in use among us.'"

The post also links to another post, discussing the words of a fourteenth-century monk:

"It is more surprising, however, to see him warn against ale and beer
  (that is, in this period, cervoise and, in his rare term, 'hoppa',
  a fermented grain drink with hops added), two very northern drinks:
  'Avoid small and strong ale and beer, unless very old or sour. But
  wine or water and the like, however, take as drink.' Otherwise, note
  that this fourteenth century reference is yet another confirmation
  that water was a perfectly standard drink in the Middle Ages and in
  fact in this case is preferred to beer and its close ancestor."

Certainly people did drink beer, but most likely, this was because they liked it, not because the water wasn't safe.

"A young man in a tenth century Saxon colloquy is asked what he drinks
  and answers: 'Beer if I have it or water if I have no beer.' This is a
  clear expression of both being comfortable with water and preferring
  beer."

Chevallier also suggests that the drinking of water just didn't get recorded much because water didn't need to be sold, taxed, etc.
So, in the context of your question -- people don't drink beer instead of water now to avoid bad water (at least, not much) because that isn't really something that typically happened in the first place. People now often drink Coca-Cola and other carbonated beverages if they don't want to drink water, and modern beers are pretty strong, unlike small beer and the like.
Medieval people may not have understood germ theory and so on, but they did place a high value on cleanliness. There's a bit about that in Chevallier's post too, but I don't want to end up quoting the whole thing! One thing, though -- at least some medieval people did in fact know that boiling water could be helpful. Paulus (admittedly early Medieval -- 7th century) said:

"But waters which contain impurities, have a fetid smell, or any bad
  quality, may be so improved by boiling as to be fit to be drunk; or,
  by mixing them with wine, adding the astringent to that which is
  sweeter, and the other to the astringent. Some kinds of water it may
  be expedient to strain, such as the marshy, saltish, and bituminous."
Comment: +10,000 if I could...
Comment: Do you have sources for that?
Comment: These quotes show that medieval Europe drank wanter rather than beer, but what do we know about other times and places? For example, there is evideence that ancient egyptian used to drink beer because Nile was too contaminated...
Comment: Someone hasn't read his Ken Follett.
Answer: The practice of drinking beer instead of water was because people noticed that you would get sick less. Why was not understood until the 19th century, with the advent of modern bacteriology etc.
Since we now understand that it's non-clean water that makes you sick, clean water is a high priority around the world. Clean water is always cheaper than beer, so everyone who can afford beer can today afford clean water. Therefore you no longer drink beer instead of water for health reasons.
Also, the practice to drink beer was a mainly European affair. In Asia it was instead tea, which like beer involves boiling the water.
Comment: _"Clean water is always cheaper than beer"_ while it is obvious that _it should be_, traveling abroad I've noticed it's not always the case in some circumstances. Cans of water were often for some weird reason more expensive than sodas and beers at bars, and at street vendors. That being said, I assume that if you actually managed to buy water were local people do (supermarkets, tap water, etc.), then it would _have_ to be cheaper.
Comment: @Lohoris Exactly, and usually you now have at the minimum a local well drilled. If you don't it's usually sold in small plastic bags, which aren't practical to drink out without practice. :-)
Comment: So did people in the middle ages have the means to create clean water, but only did it by brewing beer because they didn't know else to do it?
Comment: @user759 They did not have the means to test if water was safe to drink or not, which in practice meant they couldn't create clean fresh water. They could make it safe by boiling it, but they did not know that.
Comment: In Hungary we have a saying about beer: Spill a glass of beer on the wall. What stays on the wall represents the cost. What flows down represents the profit.
Comment: Perhaps 19th century?
Comment: @Michael What is your question?
Comment: I thought you meant to name 19th rather than 10th century for "the advent of modern bacteriology", given that Londoners effectively [drank their own unfiltered sewage](http://en.wikipedia.org/wiki/Great_Stink) as late as mid-19th century.
Comment: In Asia most people were drinking hot water (not only tea - tea was expensive). They also had all kind of alcoholic beverages which they were usually drinking with food. The shift from alcohol to water/tea in Asia was generally caused by religion/philosophy, not health issues.
Answer: You have to boil water to maker beer, in Medieval times they didn't realize that boiling the water was what killed the pathogens and made the water safer to drink. So today, boiling the water is cheaper than making beer. Developing countries water is in more danger of chemical contaminates, which boiling will do nothing.
Answer: Certainly, as other posters have implied, cultural differences may play a role. For the much of the Islamic world (although this did vary over time and space, as it does today [compare modern Saudi Arabian alcoholic asceticism with the Tunisian belief that God had given the Tunisians special dispensation to enjoy the date wine and spirits made in the area]) it would be difficult to countenance fermentation processes as a way of sterilizing drink.
Next I think it is important to note that precisely what the brewing process was doing to beer cannot have been understood much before the 19th century. And it is also clear that over indulgence in alcohol could have deleterious effects, physically and socially, so I wonder how obvious the connection would be to the mind of someone living 1000 years ago. Alcohol had benefits, but also drawbacks. Over indulgence can even make you more susceptible to disease. We might then view the adoption of alcohol as a prophylactic strategy as comparable with something like sickle cell anemia. Given the prevalence of malaria in South America, sickle cell anemia can help to extend life. Given a different environment in which, say the anopheles mosquito struggled to survive, sickle cell anemia is likely to shorten, rather than prolong, life. Water purity, and the kind of microbes found in certain water sources, may vary between geographical locations. So, if you have a well run city, with decent drains and the whole civic architecture of aqueducts, deep sunk wells, fountains etc., would alcohol seem like a necessary prophylactic measure?
A final thought would be about the social apparatus required to brew on a large scale. Firstly, whilst it is clear that there are alcoholic drinks brewed by non-agrarian societies, a large supply of grain is usually helpful. Secondly, you have to control the alcoholic content of the beer quite carefully. You need to be able to offer the drinker a "known" quantity of alcohol, because he still had an afternoon's work before him etc. You also had to regulate the alcohol suppliers - if you can't trust your landlord, how do you know that the beer you've just bought isn't watered down (with potentially dangerous water) anyway? Finally, Europe is just one of those places around the world whose recreational drug of choice was alcohol. In societies in which opium or cannabis or cocoa leaves etc. were more prevalent recreational drugs, I suspect a fuller investigation is made into their properties in general - Europe may have been brewing high quality beer since the middle ages, but if you were about to have a serious medical intervention, the greater access of middle-eastern cultures to powerful analgesics like opium might seem a powerful incentive to live in an alcohol free culture.
EDIT:
The contrary case is also interesting, and serves to make a point about what it is that limits the spread of public health technologies like sterilizing water. Given that boiling is a relatively low cost activity, requiring only fire, a suitable pot, and some water, and that brewing is, by comparison, labor intensive and produces a result which contains a potentially unwanted side product, alcohol, why was water boiling not a prevalent prophylactic method in Western Europe? Realistically speaking, it can't be that the sterilizing effect of boiling water was somehow different a few hundred years ago. Instead, since we know that boiling water does have this effect, the limiting factor must be recognition of boiling's role in the process. Precisely because boiling was not recognized as the cause of the change in the drink, boiling as a sterilization technology is not something that Europeans could have offered. Perhaps the same is the case with tea or other boiled drinks - if you believe it is the tea leaf, rather than the preparation of the water, that makes tea better for you than untreated water, then you perhaps you already think you know the answer. Today what we regard as the correct answer, boiling, means, as my critic (see below), Lennart Regebro, has indicated, that prepared water is more commonly used, since brewing is a more labor intensive process for the same result. Boiling is often not enough, however, and there are now available various water sterilizers in various forms, including tablets etc. In general, water is also to be preferred not merely for health reasons, but also because it doesn't cause intoxication to the same degree.
EDIT2:
I guess one interesting question would also be about the role of fire in purification rituals. If heat was known to be a purifier, religiously and morally (fire in judicial practice), and alchemically, it is surprising that it was not believed that heating water produced a beneficial effect.
Comment: Fermentation does not sterilize the drink, although the alcohol acts as a preservative. The rest seems to be discussion and speculation.
Answer: Quite a few countries with bad water are also countries ruled by rather aggressive religious regimes that ban the drinking of alcohol, and will go so far as to behead or stone people found in posession of it.
In such an environment, beer is simply not openly available. Which doesn't mean people don't drink it, it just goes underground, like the production and consumption of hard liquor did in the USA during prohibition.
For example on several islands I visited in Indonesia there's not a drop of alcohol to be got, unless you know where to go (which the locals seem to know quite well). And in Indonesia they're far less violent against those caught drinking than in places like Yemen.
Comment: Indonesia, as a primarily Muslim state, does indeed ban alcohol. But that's one specific case. Could you bring more evidence to support your link between countries with bad water and countries with aggressive religious regimes, or *any* regimes that ban alcohol?
Comment: @AvnerShahar-Kashtan read again. I don't say there's a causality here, only that there are frequent cases of the two going together. Think Afghanistan, Yemen, Somalia, etc. etc.. And Indonesia doesn't ban alcohol, it's freely available in much of the countries. SOME islands or cities ban it, not the central government.
Comment: @Fitri depending on who wields the whip (and which design is used) 80 lashes is quite possibly fatal... And just because there's no official death penalty does not protect you from the lynch mobs of overzealous crowds.
Comment: Do you have any evidence of beheading or stoning for possession of alcohol?
Comment: Seconded @James request. As far as I know, the possession of alcohol isn't a capital crime under the Sharia, even in countries with the strictest implementation (unlike, say, murder, or apostasy). It can be punished up to [80 lashes](http://en.wikipedia.org/wiki/Tazir) in those countries, though.


Migrating to new Linux RHEL host/server -- error 150 when moving folders

Question: I've just began migration of my website to a new host. The server is Linux RHEL and I have SSH access as well as Plesk for control panel admin.
I've uploaded my files via FTP using Panic's Coda. The files are compressed as folder.tar.gz and uploaded to the httpdocs directory in the new server.
The I extracted the folder via SSH. On SSH and Coda I can see the extracted folder and the files it contains.
But when I try to save, move or delete any of these files via Coda I get
Error 150: could not rename remote file
Permissions for both the uploaded folder and httpdocs is 755.
If I do a rm -r -f via SSH then I can delete. Otherwise, anything I do via FTP/Coda causes the above error.
Any idea what is wrong here?
Answer: You might want to see if ACLs are in use.
Comment: when I do `ls -l html` (html is the decompressed folder) I see no `+` adjacent to the permissions -- does that confirm NO ACLs are in use?


How to implement reference control signal for robot heading?

Question: Some time I found that some robot controller uses a critically damped reference control signal such as in Autonomous Surface Vehicle (ASV) heading controller. My question is how do we implement this reference signal in the actual robot implementation?
See figure .
Comment: Welcome to *Robotics*, Mei. As it stands, it's not clear what you're asking. Are you asking how you implement the Laplace transfer function on a microcontroller? Are you asking where the reference signal comes from? What work have you done so far while trying to solve your problem? Please *edit your question* to include what you want to happen, what you tried, what you expected to see, and what you saw. Showing details of the work you've done already really helps everyone understand what you're trying to do and where any conceptual or implementation errors may exist.
Comment: Hi Chuck. Thank you for your response. I actually try to implement the reference signal given in the Figure. From my understanding, i have to discretize the transfer function at a specific sampling time and form a difference equation for implementation on the micro controller. But actually I also wonder why we need this low pass filter in the actual implementation? Or is it really just for simulation purpose?
Answer: The transfer function in the simulink sketch serves as a pre-filter. In general, pre-filters are used to smooth the input since step reference signals are'nt logical for tracking (tracking step command will require infinite accelerations).
Most of the time these filter are chosen as the close loop desired transfer function. Working this way creates a reference model for tracking and prevents integrator windup and derivative induced saturations in PID controllers.
If you want to implement one in Matlab/Simulink you can design a continues reference model and then use the c2d command in Matlab which transforms continues systems into discrete systems given some sampling frequency.
Good luck!
Answer: I think you misunderstood the Simulink. The transfer function is the system and the reference signal in the picture you've posted is the step function. The input for the system in the Simulink is a constant value of 20 magnitude. You can see that from the figure. The curve goes to 20 in 18 unit time and stays for the future which indicates the system is stable.
Comment: Thank you Croco. Not really, the system model is not given in the model above. I understand your point but in some application such as ASV heading control, this kind of low pass filter for reference signal is implemented.
Comment: Well the low-pass filter is a system.


How should I approach a stranger online about an internship?

Question: I was looking for a place I can intern over the summer, and I found a guy named Benjie Freund. The problem is, I don't know him, I don't know if he frequents forums, and I'm scared that I will be a nuisance if someone like me with no experience calls him out of the blue asking for work. How do you think I should go about this? Should I just look for a multi-person studio instead? Thank you for reading!
Comment: Didn't anyone ever tell you not to talk to strangers?  ;)
Answer: My first question is, how did you come to find this guy? If it was anything but an internet search: a friend, a professor, someone in the industry, anyone really, they are your key to not being a "stranger." Whatever your connection, that is how Mr. Freund came on your radar and into your professional network. You should place that information very early in any correspondence. As in:

"Mr. Freund,
My name is Mercy and I'm an aspiring sound designer. My Aunt Matilda* is friends with your Great-Uncle Larry* who mentioned that you are in the business and that I should get in touch with you."
*Names changed to protect the innocent

Ta-Da! Now you're not a stranger. Personally speaking, I will much sooner take the time out of my day for an acquaintance of a friend than some random.
But now on to a more serious matter: Doing your homework
A quick google on Mr. Freund shows there is indeed a sound designer named Benjie Freund, who was born in Memphis and went to college at the Atlanta Institute of Music. But yet, provides no info about where here currently is located. So, (A) how do you know he's close enough to warrant a summer internship?
The same google search gives us a facebook page for DJ Benji Freund  who has the "Atlanta Institute of Music '10" listed under 'Work and Education' in addition to "Audio Engineer," and "Sound Design for Games" (gotta love them internets!). Hold on to this for a second...
Facebook DJ Benji Freund shares an unflattering photo with MySpace DJ Benji Freund who says he's located in "LOUISVILLE, ??? United States." HaHA! We might have found him!
So my second question is "Is he worth all this?"
IF this Benjie Freund is in fact this Benji Freund then he has only just started his illustrious career as a Sound Designer.  This is a medium-sized "if," but if this was how I found Benji Freund I probably wouldn't stop looking. Having just graduated he hasn't had his own career opportunities to make the mistakes and figure out the secrets that are worth passing on to an apprentice. He's probably looking for an apprenticeship himself...
Reviewing your history of questions you are clearly motivated to find some kind of internship/apprenticeship, and you have received a boatload of responses from some very notable characters in the field.  I believe you can gain valuable advice by pouring over boards like this, creating projects of your own, and demonstrating your abilities here and elsewhere on the net. By making and reinforcing these connections rather than hunting down a relative "stranger" you are teaching yourself by doing, subjecting yourself to the review of professionals, and making yourself a valuable commodity that will put you in a position of opportunity when the time comes. Hope that helps.
ps- Isn't it a little late in the season to be thinking about a "summer" internship?
Comment: haha, detective Urban. You're good!
Comment: :) @Andrew when the info's so easily obtained, there is no excuse to not do a little detective work! I probably should add that I completely agree with what's been said so far, professionalism is key and that includes being polite, respectful and grammatically correct.
Comment: I was talking about for next summer. I like to think ahead ^_^
Comment: @Mercy, Bravo. I think your timing is perfect then.
Answer: I get calls/emails like this at least twice a week, and I can't speak for everyone, but...
I feel that if you send a POLITE, WELL WRITTEN email, or call and again be POLITE and well-spoken, and explain who you are and what you are calling for (looking for an internship), you will not be a nuisance. Do not call or email every day, and if he says "no" then thank him for his time and move on.
Above all else, be polite and well-spoken. Nothing is more likely to make me turn you away then an email with a bunch of grammatical and spelling errors (just got one of those), or a phone call along the lines of:
"Hey, I'm like really into the whole studio thing, and I was wondering it you, like, take in interns and stuff like that, cause I really wanna learn this whole thing, so I can be a famous sound designer/producer/engineer...".
I might suggest something along the lines of:
"Hello Mr. Freund, my name is ---- and I found your name on ----. I'm an aspiring sound designer, and I was wondering if you had any intern positions available, or coming available in the future?". Again, polite and well spoken is best (are you sensing a pattern here?). Don't waste his time giving your experience/schooling/bands unless he asks.
Folks who take this approach, I'll at least take the time to talk to...
Answer: I agree with Sonsey, If you don't ask you don't get!  The least you can be is polite and humble and if he's offended by that then he's not worth learning from.
Answer: I've been doing a whole lot of cold calling, so i know how you feel. What i do is send an email (polite, naturally), and mention in the email that i'll follow up with a phonecall in the next week. That way when you call, they have some warning and it's less of a cold call - more of a lukewarm call. Also, maybe this is overly speculative, but i've found that people are more receptive to calls in the mid morning.
If he doesn't have anything for you, make sure you ask him if he can give you any advice on who to approach. That way when you call THEM, you can mention that he pointed you their way.
Comment: Ever try to be impolite? Apply what basketball players do to Kobe Bryant: Get in a fight with him and he gets you on his team (Ron Artest and Matt Barnes). Maybe this principle will work with pro sound designers if the polite approach doesn't work!!!
Comment: @Ryan Haha, i'll let you test that one out!
Comment: @Roger IT WORKED! He said "Get Lost". I've bought the DVD of all the seasons I could and once I'm done watching them, I'm e-mailing him RIGHT BACK with the GOOD NEWS!!! I'M IN!!!
Comment: @Ryan Haha! It took me a few minutes to get that.
Answer: What do you have to offer them? What do they have to offer you? How can this be mutually beneficial? You should know this before trying to contact them. Be clear on your availability and skills. Do your research on who they are, what they have done and are doing now, and whether this fits together.
For example, I just posted an answer to another question on this site about what kind of virtual intern I would have interest in working with. If someone wrote that they'd like to hang out with me to help on my next feature film, I wouldn't bother answering because they hadn't read what I really need. But if they demonstrated interest in the educational work that I'm currently doing, then I will respond.
So do your homework and you will up your chances of getting in the door.
Answer: Unless this guy is looking for interns, he probably doesn't need them.  You'd be best to look at larger studios that might have a defined intern program.
Answer: FWIW the first question you should ask when calling someone who works in studios is: "Do you have time to talk now, or should I call you back at a better time?" If you dont ask this question & they are busy and/or stressed you may just get no to every question regardless, whereas calling back at a more suitable time may help your case...
Answer: Very short answer, but it changed my life.  Read the book "How to Win Friends and Influence People" by Dale Carnegie.  After you've read that book, put it into practice.  Then you'll know the proper way to approach people about this kind of thing.
Comment: My wife finished this book about a month ago, she now swears by it.
Comment: @Steve it's an incredible book!
Answer: Hey there,
I was just notified about this post.  I am glad to see that you know what you want to do as a career right away.  I am always open to share my experiences and ideas with people, feel free to contact me at www.neutrinogaming.com under our contact section.  We can talk more.
PS - You got a ton of great feedback from everyone on here, that is the first step.  Just listen to what people have to say.
I look forward to hearing back from you!
Benjie Freund


Logging CPU spikes

Question: I'm curious to see if there's a way to log which applications cause a spike in CPU cycles. I'm trying to figure out exactly how much processing power I use and which apps cause the spikes. Is there a program or some scripting arguments that monitor the cpu usage, and can log the application with the highest amount of cycles being used when I hit a certain percentage? Thanks for your help.  Also, I'm running Snow leopard with a 2.4 GHz Core2Duo processor.
Answer: You can try the iStats product. It is a simple dashboard to see the real time info.
Otherwise you can try setting up a cron job use the below command to grep the CPU utilization of the of the top process.
<code>top -us5
</code>

Logging CPU spikes


How to use Pentax A3000 in dim lighting?

Question: My Pentax A3000 doesn't have shutter speed control, only ISO and a few modes, B (bulb), 60 for flash, program, and aperture. I keep it on program, but what should I do if I want to shoot in darker lighting? Since I'm using 400 speed film, do I have to keep the ISO on 400?
Answer: In short: it's kind of a pain, because this camera was targeted at people who didn't want to worry about such things. It only has fully-programmed and aperture priority mode. As you've discovered, there's no way to set shutter speed directly (other than bulb or flash sync).
The camera's exposure program uses the built-in meter and automatically matches aperture and shutter values to give its best guess at correct exposure for the scene. (It's not smart — it just assumes what you're pointing at should be average gray.)
It has an "exposure compensation" button, but that is fixed at changing by +1.5 EV — it's meant for the common situation of a shaded subject with a bright background. More sophisticated cameras often have a range of exposure compensation settings.
So, you can, to some degree, fake this by playing with the ISO setting. That setting tells the camera the sensitivity of the film you have loaded — of course, unlike with a digital camera, you can't adjust that from frame to frame, but what you can do is trick the camera into giving a brighter exposure by setting the ISO value to be lower than it really is. Then the camera's program will pick a longer shutter speed to get in more light. (Or possibly also wider aperture, if you're in full program rather than aperture-priority mode.)
But, really, I think if you're interested in more control, I suggest you look for a different camera body. The Pentax K1000 is an all-manual model which is still highly regarded. It's kind of the opposite of this camera, in that there's nothing automatic. (But it does have an easy-to-use exposure needle.)
An aside: if your A3000 came with the 50mm f/1.7 lens, keep that — many other low-cost Pentax cameras have the f/2 instead, and the f/1.7 is far superior in every way (not just the faster aperture).


Difference between claim based authentication and classic windows authentication

Question: I have read several articles about the same but still not cleared about the difference between claim based Authentication and classic window authentication in SharePoint 2013.i Understood the claim based concept theoretically, but practically not able to see any difference while creating web application in 2013. We select claim based authentication but what we are really doing different here than SharePoint 2010. when we log in to our web application we still use username and password, so how does it really work?
Answer: The big difference with claims based authentication is that you can easily (well it is still SharePoint) add other providers.
For example you can add another provider and being able to sign in with your google account, live-id, linkedin account etc etc.

Claims-based identity allows you to factor out the authentication logic from individual applications. Instead of the application determining who the user is, it receives claims that identify the user.
In software, this bundle of claims is called a security token. Each security token is signed by the issuer who created it. A claims-based application considers users to be authenticated if they present a valid, signed security token from a trusted issuer.

Both from http://msdn.microsoft.com/en-us/library/ff359101.aspx which is a great introduction
Answer: SharePoint Foundation supports the following types of authentication and the advantage with claim based authentication is that it supports systems that are non Windows based.

Windows: IIS and Windows authentication integration options,
including Basic, Digest,(NTLM), and Kerberos. Windows authentication
allows IIS to perform the authentication for SharePoint Foundation.
This is also referred to as "classic mode authentication". This
approach has a number of disadvantages such as. This approach is
not future proof and unsuitable for environments such as extranet,
inter-organization, or situations where the domain may not be
accessible or there may be many domains in play.
Claims based authentication: The claims-based identity is an
identity model in Microsoft SharePoint that includes features such as
authentication across users of Windows-based systems and systems that
are not Windows-based, multiple authentication types, stronger
real-time authentication, a wider set of principal types, and
delegation of user identity between applications. When a user signs
in to SharePoint, the user's token is validated and then used to sign
in to SharePoint. The user's token is a security token issued by a
claims provider.

You can find the entire article here.
Answer: What I believe is the major difference is that with classic mode, you login directly, and if you need to be authenticated to a second server, it's not possible (you need to be logged in to this second server first -> new login screen). You will find that this is still often done but in that case the second connection is done under a different account (with privileges specifically for the resource, for instance).
With claims, you authenticate, you get a claims token and that is passed around to whoever needs it, and you are authenticated to this second server automatically without another login screen. Given that the server trusts your secure token service (whom initially authenticated you).
I may have the details wrong but in theory but this is generally how it works.
Claims and Kerberos are similar, but they differ in the technicalities.
Important part:

Kerberos delegation — If the client authenticates with the front-end
  service by using Kerberos authentication, Kerberos delegation can be
  used to pass the client's identity to the back-end system.
Claims — claims authentication allows the client's claims to be passed between
  services as long as there is trust between the two services and both
  are claims-aware.
Claims authentication, like Kerberos authentication, can be used to delegate client credentials but requires the back-end application to be claims-aware.

Source:
http://blogs.msdn.com/b/besidethepoint/archive/2010/05/09/double-hop-authentication-why-ntlm-fails-and-kerberos-works.aspx
Comment: Your definition in para 1 and 2 sounds more like that of what Kerberos is used for... But I believe that Kerberos is supported by classic as well as claims authentication..
Comment: You are somewhat correct as they are similar, but not exactly the same, I've updated my answer with a comparison.
Answer: You cannot configure the Forms based authentication if your web application is using Classic Mode Authentication
Answer: After converting our 2010 farm to use Claims based authentication, out ArcGIS web services stopped working and our custom WCF web services no longer got the user credentials - I believe they function with anonymous access. It turns out that WCF web services need to be Claims Aware, which they are not. WCF Services used by Silverlight inside of SharePoint are severely restricted to use Basic Binding, and it is substantial work to have them pass credentials as they did before. I am still working through the implications myself, but any clarification from someone more knowledgeable would be appreciated. As for ArcGIS, I haven't gotten the answers yet.
Comment: This would be better suited as a new question as it doesn't really get at the asker's query (re difference between claims & classic auth). As for your ArcGIS add-in difficulties, perhaps there is another forum better suited to working through those third party issues? e.g. http://forums.arcgis.com/forums/69-Esri-Maps-for-SharePoint
Answer: An important factor in claims as well is which servers talk to which servers. In Windows auth outside of Kerberos, the WFE server handles the communication to the authentication provider (generally AD) and the client talks only to the WFE server. Kerberos requires all 3 connections between the client, the SP server, and a DC.
Claims does not require SP to connect to the auth source at all. The client handles the passing between the SP server indicating what kind of properties the SP server needs to know about the user and the identity provider. This allows the SP system to focus only on authorization, not identification and thus plug into other identity providers like Google, Facebook, etc.


How is IOTA's Tangle different than classical blockchains?

Question: How is IOTA's Tangle different than classical blockchains? What are the main differences and how do they translate to real world usage? What are the strengths and weaknesses of the Tangle?
Answer: Bitcoin is based on a Blockchain approach, meaning we build a chain of blocks, each referencing the previous block. For the creation of each block, a Proof of Work (PoW) needs to be done by miners. These blocks are a means to reach consensus which transactions have taken place and can be considered safe (e.g. 6 blocks)
IOTA is based on a Distributed Acyclical Graph (DAG) approach, which they call the "Tangle". No more blocks, no more mining, instead when you broadcast a transaction, you first need to do PoW validating two previous transactions. You "attach" your transaction to those two previous transactions (see image)

The consensus about a valid transaction is not reached by number of blocks, but applying the socalled Random Walk Monte Carlo Algorithm. More details can be found here: https://forum.iota.org/t/iota-consensus-masterclass/1193
In short: your transaction is considered valid (green in the image) if you can reach it from every unconfirmed transaction (grey).
Since there is no necessity for blocks, there is no issue with the block size (see scaling debate). The theory - which remains to be practically proven - is that a DAG can hypothetically scale infinitely and that a greated number of transactions actually makes the network faster, since a transaction means confirmations.
For more info, I recommend the following videos:
https://www.youtube.com/watch?v=UwEp5cexTJE&t=251s
https://www.youtube.com/watch?v=MsaPA3U4ung&t=3s
Answer: Here are the White Papers for both data structures:
https://iota.org/IOTA_Whitepaper.pdf
https://bitcoin.org/bitcoin.pdf
Comment: Thanks for your answer but sorry, posting only links without further explanation is discouraged at Stack Exchange sites :(.
Comment: Whilst this may theoretically answer the question, [it would be preferable](//meta.stackoverflow.com/q/8259) to include the essential parts of the answer here, and provide the link for reference.


Why did Marley send titans to Paradis Island in the first place?

Question: We see later in the series that the nation of Marley have been turning people into titans and dumping them on the shore of Paradis. But why? King Fritz warned the world that if the citizens of the wall were ever threatened by an outside force he would send the colossus titans inside the walls to flatten the Earth. So why send titans? I assume its a kind of revenge for the great titan war, but why risk the wrath of the king?
Comment: Actually the king didn't intend to use the rumbling, Tyler  told this in the episode "Declaration of War" episode 4 of final season. So there wasn't this problem, they wanted the founding titan since the technological powers were growing, and hence the chances of losing to enemy countries. This was the reason they told too. Though it's not directly stated, but to me it may imply that they themselves intended to use the founding titans powers to create more controlled titans (Zeke's ability is limited) and threaten the neighbouring countries. Either way Marley has used titans this way only.
Comment: To get the Founding Titan. I believe this was already explained in the manga and in the anime clearly.
Comment: Is it not just a convenient way to get rid of troublesome Eldians without executing them. Titanize, no need to worry about anything after. They can't turn back/cause more trouble. The pure titans never threatened Paradis - they had walls and the king has the power to control. That's how I interpret it.
Answer: High-ranking people in Marley nation such as the Tybur family probably knew about the king's lie. As Willy Tybur admitted knowing this in the episode "Declaration of War" episode 4 of the final season.

Also, the beast titan (the predecessor of Zeke) knew about the oath. I don't remember the manga chapter.

However (perhaps) hiding this secret was used as a tactical strategy to motivate people to attack the island (not everyone is going to buy that " one gazillions years ago their ancestor killed your ancestor so turn them into zombies and lock them behind bars").
So because high-rank people knew about the bluff and hated Eldians for their "sins" and they wanted their resources and the founding titan, they keep harassing the island while used that bluff to motivate people against them at the same time.


How do I use gels to make my flash match the color of the ambient light?

Question: When using fill flash to supplement ambient light, how do you determine the flash color that will match the ambient light?
Answer: That comes down to color temperature of the ambient light. Flash always has something similar to daylight (5500-6500K), so you need to use conversion gels from daylight.
Most useful gel is CTO (color temperature orange), which will color daylight to tungsten (3200K). Usage is as follows:Stick CTO gel on flashSet color temperature to tungstenShoot
This has two possible effects:If ambient light is tungsten, everything will look just normalIf the ambient is normal daylight, you foreground will have proper color, and everything else will be toned to blue. This can provide nice color separation effect (example)
Other usual gel is window green, that converts daylight (e.g. flash) to fluorescent-like green. Usage is similar to full CTO.
People also use half- and quater CTO, which convert daylight to 3800K and 4600K. These can be used for less-visible separation, or to warm up light for portraits. (Usual scenario: light some stuff with ungelled flash and the person with 1/4 or 1/2 CTO-gelled flash; an example with a bit more complicated setup can be seen here.)
Full description, examples and links to much more can be found in article at Strobist, overview of various gels can be seen at Rosco.
Comment: Tungsten indeed seems to be 3200K; fixed. You're right about 3/4 CTO, but I had quater CTO in mind, which really goes to 4600K, at least per Rosco. Anyway, the point is that there is really wide range of color correction gels available for tuning the light. Full gel set for small flashes can be bought for around $10. (Google for "strobist gel collection".)
Comment: The Rosco link shows Full CTO as being 3200K, and 3/4 CTO as 3500K, might want to clarify your explanation.
Comment: I found early on that hslogen lights are higher temperature then classic light bulbs, not matching tungston ballanced film.
Answer: If you're using flash as fill light outdoors on a sunny day you shouldn't need anything.
If you want to get a bit funky though, on a bland day with a grey or white sky, you can gel the flash with a CTO to warm up your subject, then set the WB on you camera to cool the shot back down again. This has the effect of lighting you subject correctly while having a cool blue sky that was never there.
-Matt


How does LastPass store my passwords on their website?

Question: LastPass advertises that they do local encryption of passwords before they are transferred and stored on their website. However, when I log in with my past password, I can access all my passwords in clear text there. Doesn't this imply that they also have access to all my passwords? How can I verify that they do not have access to my passwords?
Comment: [Podcast on LastPass encryption mechanism](http://www.grc.com/sn/sn-256.htm)
Comment: Steve Gibson says it's on the up-and-up. That's good enough for me. http://blog.lastpass.com/2010/07/lastpass-gets-green-light-from-security.html
Answer:
All encryption/decryption occurs on your computer, not on our servers. This means that your sensitive data does not travel over the Internet and it never touches our servers, only the encrypted data does.
[...]
Your encryption key is created from your email address and Master Password. Your Master Password is never sent to LastPass, only a one-way hash of your password when authenticating, which means that the components that make up your key remain local. This is why it is very important to remember your LastPass Master Password; we do not know it and without it your encrypted data is meaningless. LastPass also offers advanced security options that let you add more layers of protection.

Source: http://lastpass.com/help.php?topic=whysafe&nw=1&fromwebsite=1
In other words, your computer encrypts your passwords with your email and master password and sends that data to Lastpass. When you authenticate with your master password at Lastpass.com, Lastpass.com returns all your encrypted passwords, which are decrypted locally on your computer with your email and master password. Every communication happens over SSL, so anything intercepted is doubly useless (since everything is encrypted with not just the SSL keys but with your email and master password).
The best way to ensure this is to set up a script to monitor network activity and see if anything that is decrypted (including the master password) goes to lastpass.com. Based on what I've seen on forums, it seems other users have done this and found nothing suspicious.
Comment: @chovy You see them in your browser - that is different then seeing them on the server.  Yes, the raw data comes from the server, but it is decrypted using data local to your computer before it is shown to you.
Comment: How are the passwords stored locally on the machine though? In plaintext?
Comment: How come I can login to lastpass.com in Safari (w/o Lastpass firefox plugin) and see all my passwords then?
Answer: I just verified what waiwai said, and only a hash was transmitted to the lastpass server, and only encrypted passwords were returned. It looks like a key derived and stored in local storage.
To steal your master password, a vulnerability or compromise of the server would (or at least should) be needed for someone to modify the way the application behaves. My opinion is that lastpass is safe for normal web usage, but should not be used for high-value targets that skilled attackers may be after.


Is is possible to render ONLY Compiz with Unity3D using llvmpipe?

Question: Basically, I have very poor 3D performance on everything OpenGL when using Unity3D. GLXGears gives me an average fps of 33FPS on Unity3D, while using Unity2D gives me an average of 58FPS. So, I was wondering if it's possible to load just Compiz and Unity3D using the llvmpipe software renderer, while still being able to have hardware accelerated OpenGL for everything else, to keep my FPS up, so I can play my games. My graphics hardware is an ATI RS480. I'm using the stock Ubuntu drivers with the extra DRI package installed, as I've found out that using Oibaf's Graphics PPA screws everything OpenGL up for and makes all of the games I play (urban terror, WINE games, OpenArena) not render correctly. In the case of WINE applications, they ALL crash.
Answer: LLVMpipe is an especially interesting Gallium3D driver since it allows accelerating the state trackers atop any modern CPU, but for any close to decent level of performance when using OpenGL you need a hefty multi-core CPU (here's some LLVMpipe benchmarks just from last week) that supports the latest SSE4 instructions as well. While some OpenGL games will run with LLVMpipe and the performance of this driver that leverages the Low-Level Virtual Machine is much faster and better than Mesa's old software rasterizer or the Gallium3D Softpipe driver, Compiz nor the GNOME Shell (and most other compositing window managers) yet work with this driver.
Sorry, but that means no.
Answer: which driver are you using? best idea seems to install your drivers with jockey. as far as i know llvmpipe isnt supported yet. i had heard rumors that it will be introduced in ubuntu 12.10. But i wont expect too much from llvmpipe. It is most likely that llvmpipe will be much slower than using the properity drivers.
Comment: There are no drivers to install with Jockey. The card that I have has not had proprietary drivers available since Ubuntu 9.04. It is an ATI Radeon Xpress 1150. As stated in the question, I'm using the built-in drivers from the repos, and installed the extra-dri modules package. The drivers from the PPA, as stated above, cause more issues than they fix.
Answer: As a curiosity, I did try compiling the Mesa LLVMpipe driver manually on a 12.04 install, then using it with Unity 3D via "UNITY_FORCE_START=1" and -- while the performance is pretty decent with LLVMpipe and a modern CPU in a game like Chromium, with Unity 3D a highend machine was slowed to a crawl, with compiz using 25% to 75% of a core @ about 1/2 fps, and gobbling up between 1 and 3 GB of RAM (which, when it peaked, started getting swapped out leaving the machine completely unresponsive).
In other words, it's a neat idea, but sadly not practical.


Not able to send mail from solaris server

Question: I'm not able to send mails from my server using MAILX command,have found that some of them,sendmail.cf has no entry for DS.Is there any other things which can disable mail functionality in server?
Answer: The absent DS entry might not matter.
The sendmail daemon needs to be running, twice (at least). Since version 8.12 of sendmail, it uses a split queue model: ps -ef should show one sendmail daemon process owned by smmsp which takes the mail you sent and hands it over to the one owned by root for delivery.
There are many ways to configure mail on Solaris. Often both inbound and outbound mail processing will be centralised on particular servers (for ease of spam filtering, virus checking, alias expansion, and so on, and so that users have only one mailbox each). You should speak to your system administrator about how mail is set up on your site.
There is a very clear description of the three most common configurations in this short article at Sun's BigAdmin site.
You probably want the last one: outgoing only.


Process gets stuck on futex_wait_queue_me()

Question: I have a process that uses Groovy+Java and works fine on a windows machine, but on Ubuntu 11.10 it hangs every a couple of minutes for a few minutes (!!) when I look at the resource monitor I see there is no CPU or Memory usage and the process is in futex_wait_queue_me() mode. The process is a simple loop feeding data to a DB (again, works fine on windows).
How to resolve this?
Thanks
Answer: Regarding the thread
futex_wait_queue_me hanging Firefox under Ubuntu, here is how I solved it.
As someone mentioned turning off asst. technology is suppose to work for the bug, but, for many it does not.  It is because of a feature still turned on by default from the newer v11.04 download.
Go to
assistive tech.
keyboard access
keyboard pref
acc.
keyboard acc. audio feedback
(unclick) beep when acc. features are turned on/off.
Do a new reboot of the system. Problem fixed.
Now, you will see that gvfs-fuse-daemon still has the futex_wait_queue_me listed under channel, but, Firefox is no longer hanging or crashing.
In the FWIW category, I noticed it mostly crashed with pop-ups blocked when the browser went to fetch whatever from google analatics, so, there may be something left over from Chrome's tracking acting as a bug., I don't know. I just started using Ubuntu so I can say without a doubt, I am not an expert. Also, it improved the playback of videos under VCT too where the video went blank.  This probably has to do with the flash player.
The other thing that might be common are people using the Sis630et chip set on older Pentium 4/Celeron systems.
Posting this here because I don't know how to effectively get the word out to save people the hassle, but, this came up near the top of my search for a solution and I wanted to help better great software.


Can a countered creature be regenerated?

Question: After a creature is countered by a spell such as Counterspell can it regenerate. I was trying to counter my friend's Mortivore, but he said it could regenerate from that. So I need help trying to figure if he is lying or not.
Comment: On a tangent, unless your friend is a certified judge (or something like that) then it could simply be that he is mistaken.  Being wrong doesn't make someone a liar.
Comment: Not sure what you are asking but if a creature is countered with a spell that has nothing to do with regeneration.
Answer: No, a creature cannot use Regenerate to avoid being countered.
When your friend casts Mortivore, it goes on the Stack as a Spell.  It is not a Creature until the spell resolves and leaves the stack.
Counterspell uses the Counter keyword action, which removes a spell from the stack.  This means the spell never resolves and does not have any effect.  In the case of a creature spell, the effect is to put a creature on the battlefield; if it is countered, the creature is never created.
The abilities on a creature card (including regeneration) cannot be used until the card has finished being cast and become a creature on the battlefield*.  Therefore, Regenerate cannot be used while Mortivore is on the stack.
Since Counterspell gets rid of Mortivore while it is still on the stack, your friend cannot use Mortivore's regeneration ability to avoid your Counterspell.
Note: Even if you could use a regeneration ability from the stack, it still wouldn't apply here.  Regeneration only applies when a permanent would be Destroyed, which is different than being countered.
*Except in certain exceptional cases, none of which apply here.
Comment: Doesn't this apply to any card being countered? Until it's in play, a card only has a casting cost, color(s), and a type; the text box is n/a until it's a permanent - *unless it says otherwise*.
Comment: @Mazura I'm not quite sure what you mean.  Are you pointing out that this applies to enchantments, artifacts, etc. as well as creatures?  Or do you mean something else?
Comment: *Counter removes a spell from the stack. This means the spell never resolves and does not have any effect.* This is about how counters work, not regeneration.
Comment: Re "*More broadly, it can't use any of its abilities to avoid being countered. [Except in certain situations where card text overrides the game rules.]*", This line is wrong. "Can't be countered" doesn't override any game rule, yet it's an ability that can be "used" avoid it being countered. What was suppose to be the point of this line? Maybe I can help you come up with better wording.
Comment: @ikegami "Can't be countered" overrides the game rule that says that cards can be countered.  But after reading it again, I agree that my second sentence is a bit awkward.  I was trying to emphasize the difference between a *creature* and a *creature spell* w.r.t. abilities, but I think the rest of the answer covers that with more clarity.  Would it be better to just remove my second sentence altogether?
Comment: Re "*Would it be better to just remove my second sentence altogether?*", Yes. Spells can have abilities that can be used on the stack. Even activated ones.
Answer: There's no possible interaction between Counterspell and regeneration.
You can't use Counterspell on a creature (a creature card or token on the battlefield), only on a creature spell (a creature card or copy thereof on the stack).
Yet Mortivore's activated ability can only be activated when it's a permanent, not a spell. This is the case for most activated abilities. One can only activate an abilities of object in other zones if they are in the only zone in which it makes sense to activate the ability. (e.g. Cycling on cards such as Akroma's Blessing can only be activated from one's hand since only cards in hand can be discarded.)
In fact, regeneration can only be given to permanents (cards or tokens on the battlefield), not spells (cards or copies thereof on the stack).

109.2. If a spell or ability uses a description of an object that includes a card type or subtype, but doesn't include the word "card," "spell," "source," or "scheme," it means a permanent of that card type or subtype on the battlefield.
110.1. A permanent is a card or token on the battlefield. [...]
112.6. Abilities of an instant or sorcery spell usually function only while that object is on the stack. Abilities of all other objects usually function only while that object is on the battlefield. The exceptions are as follows: [No exceptions apply to Mortivore's activated ability.]
Answer: You cannot regenerate a countered creature.
As a general rule, unless an ability only makes sense when applied in another zone (e.g. "return this card from graveyard to hand" clearly must work in the graveyard and "cannot be countered" must function on the stack) then that ability only applies while the card is a permanent on the battlefield.
Regenerate is one such ability. Countering a creature spell prevents it from entering the battlefield, so it never had an opportunity to be regenerated.
Furthermore, Regenerate prevents the destruction of a permanent, but countering a spell doesn't destroy it.   So even if you could use regenerate while it was on the stack the ability wouldn't do anything.
Answer: Regenerate means "The next time this creature would be destroyed this turn, instead tap it, remove all damage done to it, and if it is in combat, remove it from combat." Countering does not destroy, partially because  "destroy", like many words in magic, is a word with a very precise meaning and countering doesn't say that it destroys, and partially because a spell is not on the battlefield, and destroy is exclusively relevant to things on the battlefield. So regeneration is not applicable, the same way you can counter a creature with indestructible.


Can you cap a chimney to reduce moisture and still use the chimney with a gas fire?

Question: Or does capping a chimney always remove the ability to use a fire ?
Answer: I'm pretty sure that you can get caps that allow you to still use a gas fire.
Consult an expert though.
Answer: Yes, you can, but definately consult an expert like ChrisF stated. I would start with a fireplace and chimney store. I once made a cap that covered the flat space on top of the chimney and then had a raised center with a roof and screen to keep out rain and birds.
Comment: If you mean the moisture form rain, yes.  More problematic is the condensed moisture from a gas fire
Answer: NO
The moisture is a result of lower flue temperatures allowing the water vapor in the exhaust gas to condense (dew point). A cap will reduce the flow, but does nothing to prevent condensation.
The only practical solution is to line the flue (with a metal liner) and remove the damper (if this was a wood burning fireplace, originally.  The liner is sealed at the bottom with a fitting and an appropriate mortar.  The liner is then attached to a cap for a end-to-end seal.
The water in question is acidic and will attack the mortar, potentially causing leaks and joint failure, over time


Tutte's theorem and independent edges

Question: I'm trying to prove that a graph $G$ contains $k$ independent edges iff $q(G-S) \le |S|+|G|-2k$
for all sets $S \subset V(G)$ (where $q(G-S)$ is the odd components). I'm having trouble with verifying whether the following direction is correct though, so, does this look valid for showing that if a graph contains $k$ independent edges, then $q(G-S) \le |S|+|G|-2k$?
Let's assume that the graph $G$ with order $n$ contains $k$ independent edges, and add $n-2k$ vertices that all are adjacent to the vertices in $G$, call this graph $G'$. Then, we can find a perfect matching in $G'$ by matching each of these $n-2k$ added vertices with the remaining $n-2k$ vertices in $G$ that are unmatched. Then, by Tutte's theorem, we have that $q(G'-S) \le |S|$ for all $S \subset G'$. Take a subset $S$ of $V(G)$, and also include all the $n-2k$ added vertices to $S$, and call this subset $S'$. Then
$q(G'-S') = q(G-S) \le |S'| =|S|+|G|-2k$.
Answer: Yes that looks correct to me.
Essentially, $G'-S' = G-S$.

Tutte's theorem and independent edges


What Properties Matter For the Bound on Generalization Error of Hypothesis Set?

Question: As I know, there are several upper bounds on the generalization error of hypothesis set with respect to sample complexity. For hypothesis space $V$, if the learning algorithm output the set of hypotheses that are consistent with the training data, i.e., $\sum\limits_{1 \leq i \leq n}{\frac{I[h(x_i) \not= y_i]}{n}} = 0$, the VC theory gives the bound on generalization error. If we consider the "margin" or confidence of classification, PAC Bayes theory gives tighter bounds on generalization error with margin incorporated.
My question is: what other properties or structure of the set of all possible hypotheses output by the learning algorithm matter for the bound on generalization errors? Name a few.
Answer: Rademacher complexity is also a tool used to bound generalization error.


How do I as an application sys-admin monitor Verified by VISA and MasterCard SecureCode?

Question: We integrate with 3 payment service providers in multiple countries, and sporadically we will get customer reports that the 3DSecure-check part of the payments-process is not working.  We've yet to get adequate reports to troubleshoot the problem or proactively warn customers that this may happen (or disable the check for the duration of a problem).
How can we directly monitor the VISA and MasterCard service-dependencies so that we can get better insight into our payments-process status?  Our PSPs do not expose the problems in a proactive way; we get problem-reports direct from customers, as I mentioned - not the best place to be in.
We're after:

An API-call we can periodically make to get red/green status for each system
A web-page we can hit to see whether the system is expected to be up/down
A web-page we can hit to see whether there's upcoming maintenance/down-time
A way of running through a test payment, but against a real live payment-card, but that won't then affect the credit rating of the card's account since we'll essentially be setting up and cancelling payment-authorisations on a several-times-a-day basis if we do that

Where can we find some/all of the ab
Answer: I recommend some browser-based transaction monitoring. We are using AlertFox with good success  for our online store (monitoring product search, shopping cart and credit-card/Paypal backend). If you google for Keynote or Gomez, they also provide this service, but I never used them.
As an unrelated site note, it is unbelievable how often Paypal has sporadic issues...


Bijection between binary trees and plane trees?

Question: I would like to describe a bijection between binary trees and plane trees. A binary tree has a root node and each node of the tree has at most 2 children (left and right). A plane tree has a root node and the children of each node are ordered from left to right. Can someone enlighten me on this? Thanks!
Comment: There is a way of encoding n-ary trees into binary: the left is a son and the right is a brother. Are you looking along those lines, could you provide some context?
Comment: @dtldarek: Indeed. Left-child right-sibling binary tree, in wikipedia terminology. Can you make that an answer, so we can +1 it?
Answer: There is a way of encoding n-ary trees into binary: the left is a son and the right is a brother; here is the Wikipedia's entry.
P.S. This was a comment, made an answer on request.
Answer: The tree is a totally repeating structure, so we need to try for a bijection between a plain tree with one root node plus whatever number of children and a particular binary tree.
I think the following algorithm will work:
Lets suppose the Root node of a plain tree has n children, then we count $min (k): 2^k <= n $
Next we build the 3-level binary tree with Root node = Root node of the plain tree and intermediate nodes also = Root node of the plain tree.
The binary tree itself is a subclass of a plain tree


Algorithms for community detection for bipartite graphs?

Question: Are there any algorithms for community detection for bipartite graphs (2-mode networks) implemented in igraph, networkX, R or Python etc.? In particular, is there such an implementation in which one would be able to restrict the detection of communities just on one of the two modes?
Comment: How would one "restrict the detection of communities just on one of the two modes" without knowing in advance which nodes make up the modes?  It seems circular.
Comment: In a [bipartite](http://en.wikipedia.org/wiki/Bipartite_graph) network you already know the two modes. So for example if half the nodes that belong to mode "A" link with a node that belongs to mode "B" then you have a community there.
Comment: If you know in advance which nodes belong to each mode, then that answers my question about how to restrict the detection.  However your example and its implied notion of "community" is unclear.  If a vertex in a bipartite graph does not link to any vertex of the opposing mode, then it doesn't link to any vertex (it would be isolated).  In a connected bipartite graph every mode "A" vertex links to some mode "B" vertex and vice versa.  "Community" would typically mean something more than a connected subgraph.
Comment: On reflection I suspect your "link with a node" meant to link with a single common node, giving a clique in the projected graph (see Answer), and thus "a community there".  Apologies for not grasping your point at first reading.
Comment: No apologies needed. My English was not so clear anyway.
Answer: The phrase "community detection" is loosely defined as partitioning the vertices of a graph into "communities" such that each has members more densely linked to one another than to members of other "communities".
Our first task is to ascertain what this should mean in the case of a bipartite graph, which by definition consists of two "modes" such that members of one mode are linked only to members of the other mode.  It may be expressed, at least for simple graphs, as having an adjacency matrix of special block structure:
$$A = \begin{pmatrix} 0 & B \\ B^T & 0 \end{pmatrix}$$
It seems to me that the most pertinent interpretation of "restrict the detection of communities just on one of the two modes" would apply said algorithms to the "projected" graphs corresponding to blocks of $A^2$, i.e. the first mode with adjacency matrix $BB^T$ and the second mode with adjacency matrix $B^TB$.  Note that even if the original bipartite graph is simple (so that $A$ is binary), the projected graphs will generally be multi-graphs.  Fortunately igraph has a method to construct these for us.
We are equally fortunate in that the igraph community detection algorithms and related have been "updated to handle weighted graphs" (such as multi-graphs).

S. Fortunato (2010) surveys community detection criteria (Community detection in graphs) and their use with bipartite and multipartite networks.  The interpretation I suggest above is articulated on page 8:

Multipartite graphs are usually reduced to unipartite projections
  of each vertex class. For instance, from the bipartite network of
  scientists and papers one can extract a network of scientists
  only, who are related by coauthorship.


What causes swelling after impact?

Question: Why does the head swell after getting hit by something hard? What is the liquid that forms after impact?
Answer: When physical damage to the cells occur a biological process called inflammation occurs. When inflammation occurs, white blood cells are rushed to the area in order to fight off any bacteria or harmful waste leakage from damaging the cells further. In order to get the white blood cells to the site of damage faster, the body induces a process called vasodialation where the arteries are expanded and blood is rushed to the site. This causes the site of injury to swell and redden, two signs of inflammation. The white puss that forms comes from the white blood cells. The white blood cells release chemicals that kill of bacteria and waste in the area and forms a white pus that will eventually harden or break through the skin to be exerted. If the site of impact begins swelling within a matter of hours, it is likely acute inflammation is occurring and the body is rushing blood to the site. However, if swelling occurs on a more rapid time scale it is more likely that this fluid is from the damage of the impact itself and the cells and capillaries are leaking blood and extracellular fluids (What is Swelling?).
In short, when you bump your head your body is trying to fight off further injury and infection by inducing the inflammatory response. It is also leaking fluids due to the impact and may result in rapid swelling at site of impact. The symptoms you discuss are due to inflammation and are a normal process of your body's defense mechanism. Hope this helps!
Comment: I'm not sure you're correct (at least I'd appreciate seeing evidence for your assertions.) Inflammation takes time. The bump that appears after hitting one's head takes moments to minutes. I think this is much more likely to be simple extravasation of serum from damaged cells and capillaries, etc.
Comment: Thanks for the rebuttal. The question offered no information when regards to time scale, but vasodilation and rush of blood to the site of impact can occur within a matter of hours (http://www.nationwidechildrens.org/swelling-bodys-reaction-to-injury). If swelling occurs on a shorter time frame, @anongoodnurse 10  is correct in saying it is likely due to the peripheral damage to the capillaries and cellular structure. Thanks for the input, I will be updating my answer!

What causes swelling after impact?


Why can't I sing high notes as a young female?

Question: I am 14 years old and want to become a singer.
I have seen that most female singers can usually hit these amazing high notes with no problem whatsoever. Like, for example, Hasley can do somewhat high notes, and you can actually hear that she is doing it effortlessly.  I have even seen that some male singers can hit the high notes that I can't, which is really embarrassing for me.  But I feel that with my super small vocal range, I just can't.
The lowest note I can do is an F3 with a lot of force, and my highest is probably a B4. I have tried so many exercises, but I have no voice coach because money is tight.  So I need to try to teach myself, but I find YouTube videos useless – I do what they tell me for weeks and months, and it's the same, I strain so much.  I need help getting better at singing higher notes without sounding like I'm a dying sheep because my strains are so big, and I really don't like them!
Comment: You are 14 and are much younger than the people you listen to. Your voice has just *begun* to change and will keep developing until full maturity in your 30's. If you reach beyond what you can vocally do, you'll ruin your voice permanently. Focus on singing songs that feel comfortable to sing. Look for exercises by CLASSICALLY TRAINED vocalists. Too much garbage floating around online.
Comment: Perhaps you're a low-voiced young female. Not all women sing as high as others; not all women sing as low as others.
Comment: Also note that the females on YouTube may not be actually hitting those notes...
Comment: Most of the female singers _who are experienced, talented and famous enough for you to find on youtube_ can hit these high notes. Beware the statistical sample you are using does not imply _most female singers_ can hit these high notes.
Comment: There are good books on voice training, which can help a lot.  But definitely be on the lookout for a voice coach of some kind because it can really help.  There are some things that are simply hard to communicate in writing, that a coach can see/hear/explain.  Community colleges often have classes for the public.  Maybe someone has a web-based approach that is affordable.  I bet someone has a Patreon page with youtube lessons that could help.  Meanwhile, avoid doing anything that makes your throat hurt.
Comment: My experience is that voices get **lower**, not higher, as you mature and age, including for females. Keep that in mind - if you're anything like me, your voice could keep slipping lower well into your 20s.
Comment: I really feel for the OP, training in anything nowadays is very expensive.  If both parents are working basic jobs just making ends meet is difficult...sometimes saying it is easier than doing it - it is just plain expensive.  Guitar / Piano lessons are very expensive, my daughter had 20 minute sax lessons that were like 40 bucks a lesson.  That is expensive.  Something like this should have some federal backing behind it so people who want to learn get to learn without the burden of that much money.  I know harder said then done.
Comment: "I strain so much" - that might  be part of the problem. Reaching high notes requires you to relax. Take things light and soft.
Comment: Ask your voice teacher about it. If you don't have a voice teacher, well that's why you can't hit those notes. It's not natural, it's training.
Comment: I can't afford one and there isn't one nearby so I think that's not an option
Comment: Well in any case that's why you can't sing as higher than you can right now.
Comment: i don't think i need a vocal coach to reach higher notes it's not like they sell a medication i just need to know the exercises and teach myself
Comment: thankyou guys so much i just feel like i can train my voice but i ont know how
Comment: @Dekkadeci - voices get lower, but developing technique can extend range.
Answer: You could be not reaching those high notes for any of these reasons:

You are young and your voice is still developing
your vocal technique is bad
you're doing the wrong exercises and over-straining your voice
you have a naturally low voice, and you'll never hit those notes

Only a qualified voice teacher will be able to tell you which of these is the case, or if you have other problems. Otherwise you're just wasting your time and risking damage to your voice.
If your parents really can't afford lessons, your school might be able to help you.
Comment: This is pretty much the answer to every voice question on here.
Comment: @BobsaysreinstateMonica Unfortunately these questions keep coming, and they're not quite duplicates, so this needs to be repeated. It would be sad if someone ruined their voice because of questionable exercises they found on the internet.
Comment: And risking damage to your voice.
Comment: Note that even if you have a naturally low voice and will never hit those notes, you probably can learn to hit them.  Just not the same way.
Answer: To be realistic, different people have different ranges. If everyone had the range of Mariah Carey or Axel Rose they would use it. However Taylor Swift only has just over 2 octaves and she has managed quite well.
As others have said; don't strain but remember that falsetto or even whistle registers might be available to you. This is a way of getting extra octaves but with a different tone from your normal head or chest voice.
Here's a tip that I haven't seen elsewhere but discovered by accident. When we talk with a lot of expression in our voices, we naturally change pitch a lot, and without strain. Try saying, "WHAT?" as though you've just heard something completely unbelievable and your voice may naturally go much higher than you're used to. If that happens, say it again but instead of stopping, continue singing that high note.  A lot of strain comes from forcing the muscles rather than just letting them do what they do naturally.
Good luck and enjoy your music!
Comment: Axl is a natural bass who hits what would be considered crazy high notes for a bass by doing very weird things with his voice. Its very raspy sounding, not a normal falsetto, and a lot of people don't like the tone. I'm no expert on it (just a fan who has all his music), but it sounds painful to me, and no I wouldn't suggest anyone try it themselves without professional supervision. For an example of a proper male voice with redonkulous range I'd instead suggest using Myles Kennedy, who supposedly goes Bb1 - D6.
Answer: Keep straining only if you want no decent voice - ever! At your age and stage of development, it's gently gently. My baby's learned to walk - time to teach him how to jump over hurdles. Absolutely not.
It may just be that you have a low voice with a low range. Nothing wrong with that. Except we all yearn for something we don't have. It's called being human. Keep on singing whatever you can, in the key you can. And be well aware that that means you're not going to sing along with some of your favourite singers, in what's essentially their key - not yours.
Practise scales, along with an instrument if possible, so you keep a tab of where you are. Don't expect your range to enlarge overnight - we're talking years here. Which might equate to a semitone each three or four months! Not world shattering, but realistic - if you don't strain. You are standing up, aren't you? Your chest is out, shoulders back, and head up, aren't they?
On the realistic side of things, don't hold out hopes too much. Even with a vocal coach, your voice may never have a huge range - you are what you are, but what you'll end up with in your late twenties, can still be a voice worth listening to. Not all great and famous singers had that much of a range, by any means.
Answer:
I have been seeing that most of the singers that are females can usually hit these amazing high notes with no problem whatsoever.

they worked hard to make it seem easy,
voice of every person is different.

Find a good teacher and take singing lessons. There is no better way to address your specific issues.
Answer: As a classically trained, Major Label (all of them) Music Producer, I would like to reinforce much of the good advice hear.  Specifically the advice about not straining.  If you have any discomfort while singing, you should stop immediately (mid note if needed) and assess what you are doing and your motivation for singing in that way.  Again being a good and compelling singer actually has nothing to do with the singer's range.  It hinges much more on your ability to emote and communicate.  The point about your being 14 and most female voices not maturing until mid to late twenties is also important, being patient with yourself and allowing yourself to enjoy the process of singing.  If you are not enjoying singing, no one is going to want to listen to you.  So take the time to enjoy what you're doing when you sing.  The two main take aways is don't strain… EVER, and always take the time to enjoy what you are singing.  I'll leave you with one other piece of advice.  I personally, and some of my artists have had a lot of success with Roger Love's "Set You Voice Free" (full disclosure Roger is a friend and colleague of mine).  Best to you and enjoy your journey of being a life long singer!
Answer: Apart from all the good advice already given: keep in mind that music is art, not a sport!
There are great singers with all sorts of different ranges. Actually, I love the contra-alto voice of legendary singer Kathleen Ferrier (check out her version of "Blow the wind southerly", for instance). I don't know what her range was, but as a contra-alto it can't have reached very high: though that is about as relevant as, for instance, a president's shoe size. What matters is that she was a phenomenal artist.
I hope you'll keep at it ... go and make wonderful music!
Answer: We all have naturally a "head voice" and a "breast voice". Little children naturally "know" to use their head resonance.
When starting school many children (girls and boys) are losing or forgetting their head resonance and lose singing in higher ranges - consciously or unconsciously the mean it is uncool speaking or singing in higher ranges!
You can win back your higher range by singing on vowels like ihh, uuh, oooh, sighing, whistling and imaging you have a bird in your head that is singing there. Sing through your nose n, ng, hm. Always piano.
Stand on a chair or on the table and howl like a wolf. Don't mind what others may be thinking. You will feel that your head is resonating and you will be able to sing a fifth or an octave higher.
Answer: I trained for classical singing at Juillard and can tell that your singing range is not simply set.  I started at a bass-baritone, but instructor worked with me (a lot) to increase my range so I could preform pieces previously "too high" for me to sing.
Training and practice can increase your range, both up and down. That said, not all women can be sopranos no matter how much they train. As other posters have said, you'll need to find a teacher who can help you find the ideal range for your voice as well as strengthen it to increase that range to some degree.
Answer: Are you taking lessons at all?  Or just trying to sing along with recordings?
It takes proper technique to hit these notes.  As others have stated you might still be developing but even if not you need to learn (1) how to support you voice with your diaphragm, (2) get the correct shape in your mouth and palate to generate resonance in the right place, (3) understand where the resonance should be felt in the head.
For many people singing in their speaking range comes naturally but this is only a fraction of the possible notes you can hit.  With training you can hit notes both below and above your speaking range without force.  Good volume and tone come from technique and once you've learned that technique it will be effortless.
My cousin was a vocalist and vocal coach for her entire life and occasionally gave voice lessons to pop and rock singers.  They would usually search for training only after hearing from a doctor that their career was over if they kept singing with bad technique.  The people you idolize might be properly trained, or might be damaging their voices and in a few years not be singing any more.
I think taking lessons is the first step.  If you have been taking lessons for a while and not progressing that is more of an interesting question.
EDIT:
I missed the comment in your question about money being tight.  If things are not working with YouTube then take a break because you could damage your vocal folds.  Correct singing technique needs to be felt internally and that is hard to convey in a video.  I takes classical voice lessons and this is a real struggle for me.  Trying to understand what my coach is talking about and how to convey to her what in inside of my head and chest feel like.  When it clicks it works.  Try looking for something online where you can join a group.  The rates might be better.  Or you might consider some kind of community group where professional teachers donate time for lessons.  Just to get started on the right track.


What's the purpose of yawing?

Question: I've never understood the purpose of yawing, i.e., rotating around the vertical axis by activating the rudder. Provided that airplanes have ailerons that allow banking (used for turning), why do they actually need to yaw? Furthermore, I've read that yawing induces an undesired banking because of the different lifts generated by each wing, due to the fact that one wing "feels" a higher airspeed than the other (a similar effect as a car's wheel spinning faster than the other during a turn). So, if yawing is so "useless" or bothering, why do airplanes have rudders? Moreover, could an airplane takeoff, fly and land without rudder?
Comment: I read it as "yawning" and was confused why this was asked on Aviation.SE.
Comment: @Claudix the ailerons induce yaw in the opposite direction of your intended turn.
Comment: @Claudix: Yeah, I was just praying it wasn't a pilot issue.
Comment: No.  Yaw can be caused by the engine, by wind, by the ailerons, ground conditions when taking off and landing, by "weather cocking", etc.  You use the rudder to induce opposite yaw to prevent turning around the Z axis.
Comment: (continued) **The reason rudder yaw induces a bank is because the tail is not aligned with the center of mass.** The CoM is in the middle of the fuselage, and the tail (and center of the rudder) sticks up rather high above it. Therefore, you get this slightly diagonal torque-arm acting on the body of the plane instead of a nice straight, perpendicular line.
Comment: related: http://aviation.stackexchange.com/questions/3864 (see in particular the comments)
Comment: @Mehrdad, in such case that would be a "passengers.stackexchange.com" question
Comment: I think yaw'ing relieves the pilot of tire'ness... :)
Comment: Take off and fly? Sure. Land? Not in a cross-wind.
Comment: `yawing induces an undesired banking because of the different lifts generated by each wing` - and that's why you use rudder to prevent the yaw so that you do not get the undesired banking.
Comment: @Simon, but isn't yawing only induced by the rudder?
Comment: Well, this is a simple question. When you are sleepy, bored or tired, you breathe more slowly. Your body requires oxygen and needs to throw out the carbon dioxide. So, your brain makes you take an extra breath, which is deep and long called a yawn, so that you can take in more of oxygen and give out carbon dioxide fully.

All these wrong answers... tsc.
Comment: `I've read that yawing induces an undesired banking because of the different lifts generated by each wing, due to the fact that one wing "feels" a higher airspeed than the other` **This is incorrect.** The effect you mentioned is indeed similar to differential rotation of car wheels during a turn, but it's negligible for a plane. Yaw is a much more shallow rate of turn, so differential airspeed over wings is almost negligible, easily corrected with slight aileron action.
Comment: @DrZ214: Unless you have a _really_ high yaw rate (for instance, due to a rudder hardover), in which case the airspeed differential _does_ become significant (but, in which case, you have worse problems to deal with).
Answer: From a totally practical standpoint, let's say you're in a small tricycle gear aircraft with a freely castoring nose wheel (or a tail dragger) and you're starting your takeoff roll in a calm wind. When you bring the power up, the aircraft is going to want to turn left (why is another question), in other words to yaw about its vertical axis in a counter-clockwise direction, and head for the grass off the left side of the runway. You counteract that adverse yaw by using right rudder to keep the airplane straight. Initially you might have to use some right brake as well until there's sufficient air moving past the rudder. If you had a steerable nose gear, you wouldn't need the right brake.
You reach flying speed, apply up elevator, and lift off. As the airplane rotates about its lateral axis, there'll be some additional adverse yaw as a result of that rotation (again another question). Even if you had a steerable nose gear, that would now be useless as the nose gear is no longer on the ground. Your means of continuing to fly straight ahead is to counter the adverse yaw with rudder, and you would continue to use right rudder as you climb out. If you don't, you'll start turning left.
You reach the altitude you want, level off, and trim for level flight straight ahead. Then you decide you want to turn left. You start the turn briskly with left aileron, but you forgot to use any rudder. If you were looking at the ball on the bottom of the turn and bank indicator, you would see it shoot left. If you had used left rudder while initiating the turn, that wouldn't have happened. You would have started a coordinated turn.
Now you're coming back to the airport, and there's a crosswind from the left. You decide you want to counter that crosswind by slipping into it. In other words, you want the airplane to travel sideways at a speed that matches the crosswind while still having your nose pointing straight down the runway. So, you lower the left wing and to keep the airplane from turning, you use right rudder to keep the airplane aligned with the runway. You match the crosswind by adding or decreasing your bank.
There are other uses of rudder, and especially in aerobatics.
I once flew a few loads of skydivers. The jumpmaster sat behind me, and to help him in positioning for the drop zone, he wanted flat turns. In other words, he wanted the airplane turned while keeping the wings level. So, if he wanted a left turn, that meant left rudder and right aileron. Uncomfortable from a pilot's standpoint.
Finally, back in the 1960s when I was working on my instrument rating, I had an instructor who taught that the best way to track a localizer in close was to use rudder only for small corrections. Personally, I don't like that method, but it's one way of doing things.
A useful thing, the rudder. You need to be able to directly control movement about all three aircraft axes.
Comment: @Claudix You're free to ask those other questions as other questions. It just fits this site's format better, though, if they're asked as separate questions rather than all answered in the same place. It's easier to find what you're looking for from a search engine and the answers are more concise and readable that way (among other reasons.)
Comment: Great answer, sir. Missed the answers (short, at least) to those "another questions", though :-)
Comment: I know :-) However I'll try to find the answer on the Internet first! And by the way, thank you for your detailed answer!
Comment: Is "why" because of the right-hand rule?
Comment: @JFA, the direction depends on the direction of rotation of your propeller. Most propellers turn clockwise, leading to left yaw
Comment: @JFA It's been 55 years since my last Physics class, but as I remember the right-hand rule is simply a convention for representing torque as a vector. You put your right hand around the axis of rotation with your fingers pointing in the direction of rotation. Your extended thumb 90 degrees to the plane of rotation is the direction of the vector. For why the airplane wants to turn left as you bring the power up, check out http://wiki.flightgear.org/Understanding_Propeller_Torque_and_P-Factor.
Comment: @Terry I suppose the right hand rule is not a "why" but a demonstration of, as you say, representing torque as a vector. The "why" is the torque. I mostly said it that way because there about a thousand right hand rules, all relating to the motion of objects or particles in a circle.
Answer: The purpose of yawing is, as you note, to provide a way to rotate the airplane around a vertical axis.  Ailerons can roll the airplane around a longitudinal axis and produce a slipping turn, and turning with the rudder alone produces a skidding turn, but proper turning is a combination of both aileron and rudder inputs and produces a coordinated turn.
Turning with aileron alone creates adverse yaw when the rising wing provides more lift and more drag than the lowering wing, causing a yawing moment away from the intended direction of turning.  Additionally, if you want to maintain effective gravity downward in your reference frame, you need a yaw moment and rudder is what provides that.
There are many other situations where rudder is essential to flight

Correcting for p-factor turning tendencies
Takeoff and landing in crosswinds
Spin recovery
Controllability in stalls to prevent spin entry
Engine failure in multi-engine airplanes

If rudder were useless, we wouldn't have them.  There are airplanes without rudder pedals, e.g. the ercoupe but this plane still has rudders that act in accordance with the ailerons through turning the yoke to provide the necessary forces for coordinated turns.
Comment: There are also situations where yaw alone is *directly* useful to introduce intentionally _uncoordinated_ flight, for example, a slip to landing. Fly a tailwheel airplane, maybe even an old one with no flaps, into a short field and this is _de rigeur_ for achieving a steeper descent profile without gaining energy and airspeed. It's also how to correct for a crosswind on approach and landing. As a CFI, that's the first answer I'd think of when reading the OP's question.
Comment: Stopping the people in the back puking is also a good use of rudder.
Comment: @Claudix no, I meant what I wrote.  See the link and go ask a CFI to demonstrate this to you in an airplane if you are skeptical.
Comment: @casey, I think you meant, in the second paragraph, "Turning with rudder alone creates adverse..."
Comment: Sorry, I didn't mean to seem skeptical. I didn't know ailerons induced yawing.
Comment: don't forget the takeoff/landing *roll*. Without yawing (whether induced by rudder or nosewheel steering) we'd all have a very bad day :)
Answer: The primary purpose of yawing is to stop yawing.
Airplanes with long wings have a tendency called adverse yaw. Adverse yaw causes the plane to yaw away from the direction of turn when attempting to turn using ailerons and elevator. The more you try to turn left, the more the nose point right.
The big problem with adverse yaw (apart from it being potentially confusing to pilots) is that while the lift from the wings held at a bank angle is trying to turn the plane in one direction, the nose is going in the wrong direction eventually causing the plane to stall. This can make turning impossible.
Early aerodynamicists didn't fully appreciate adverse yaw because experiments with model planes tended to have the plane fly straight. It's not until you attempt a turn that you'd encounter adverse yaw. Even early glider pioneers didn't fully appreciate it much because a lot of the early hill hopping glider flights were also in straight lines.
Among the first people to encounter and attempt to solve the problem of adverse yaw was the Wright brothers. Taking inspiration from other early glider pioneers and large kite fliers their early gliders didn't have a rudder. They found that it was unstable when the wings tip to either side (bank). What they experienced was adverse yaw. Their solution was to add a vertical stabilizer and it solved the problem for the most part. Later they made the rudder movable to allow the pilot to further correct for adverse yaw manually. We still teach this "correction" maneuver - it's called the coordinated turn.
Comment: I'd like to note that these days there are other ways to overcome adverse yaw including (but not limit to) aileron differential, drag rudders, spoilerons (technically a variant of drag rudders when used this way) and low aspect ratio wings. It's just that the rudder was invented as a solution to adverse yaw.
Answer: A turn performed with just the ailerons and elevator will not be "coordinated". This is to say, the apparent G force will not go straight "down" through the aircraft's floor, which is desirable for various reasons. Among them is passenger comfort (being pulled down is less unpleasant than being pulled sideways), but there are other advantages too.
In addition to making coordinated turns, the rudder is useful to correct yaw induced by gusts of wind, to set up and maintain a wing down attitude in a sideslip crosswind landing, to quickly lose altitude without picking up lots of speed (somewhat notably used by the Gimli Glider), and for various aerobatic maneuvers.
Answer: To my knowledge, at least during combat/ww2 Using the rudder to yaw  in that way was often applied by pilots who were weary of being attacked from behind. Adding a  small amount of rudder/aileron to yaw, meant that instead of flying straight and level, the aircraft would sideslip through the air, therefore throwing off any enemy fighters aim/Bullets. Any combat airman will tell you, never ever fly straight and level in a combat zone...
Comment: Welcome to aviation.SE! This is an interesting answer but yawing also increases drag and reduces speed, which would presumably be a disadvantage in a dogfight. Do you have any sources you can link to that mention this technique? And if you're new to the site, the [tour] may be useful.
Answer:
Moreover, could an airplane takeoff, fly and land without rudder?

Absolutely.  I have an electric ducted-fan model of an F-16 that has ailerons, elevators, and a steerable nosewheel, but no rudder.  It takes off, flies, and lands just fine, and this concept could certainly be scaled up to a full-scale airplane.
In many light airplanes, much of the time the rudder is used more of a "trim" control to optimize streamlining by cancelling out effects like p-factor, but is not absolutely essential.  It becomes more important in crosswind landings, but some light airplanes like the Aercoupe have been built with no independently controllable rudder.  Such aircraft invariably have tricycle landing gear rather than tailwheel landing gear, for greater directional stability on the ground.
In sailplanes (gliders) with long wings of high aspect ratio, adverse yaw is very pronounced and it is very important that the rudder be applied along with ailerons when entering and exiting turns.  There are various design features, like differential aileron deflection, that can reduce adverse yaw.
Answer: There are two main reasons: control on the ground and efficiency in flight.
When an aircraft is rolling on the ground, the only way to control its direction is with the rudder. When a pilot both takes off and lands an aircraft they use the rudder to steer. If the aircraft had no rudder, it would likely roll off the runway.
In flight, the rudder is used to keep the fuselage parallel to the line of travel. For example, if the pilot makes a turn without using the rudder air will hit the side of the aircraft and the drag on the plane will be significantly higher.
Another cause of inefficiency in propeller aircraft is what is known as P-factor. This is a yawing of the aircraft due to asymmetrical force in the propeller causing drag. The pilot must hold down the rudder constantly to prevent this. It can become quite tiring to do so in long flights.

What's the purpose of yawing?


How to remove wet mud from leather shoes

Question: My Leather shoes are covered in wet mud. They are Ahnu, Karma sandal shoes, with a stitched pattern. If I let it dry, they will be permanently disfigured. Do I hose them down, or wipe them and bring them in to my shoe guy?
Answer: let it dry then brush it off. Any residue can be dabbed off with a wet cloth again leaving it to dry
Answer: Generally, it can be hosed off.
However, these are suede shoes, which require a little more care than smooth leather. I would try wiping the mud off as gently as possible.
If that's not possible, hose them down, patting the water off right after. Then stuff them full of paper towels to absorb the moisture and use a suede brush (finger nail brush, if you lack the suede brush) to remove the water stains with more paper towels. Inside the shoes is the same treatment, but the sole may be irrecoverable from stains.
If there's any kind of mud stain, vinegar may help to remove it, with a suede brush.
Answer: Hose them down. Stuff them with old newspapers, to keep the form in shape and soak up some water. Than put them near a radiator or some heat source, to speed up the drying, but not too close. If you can't hold your hand there.. it's too hot. Good luck