H0w t0 prove that periodic decimal numbers are rational? $a_1...a_k(b_1b_2..b_l)={m \over n}$

Question: Given $a_1...a_k(b_1b_2..b_l)={m \over n}$ how can I prove that periodic decimal numbers are rational?
Where do I even begin?
Answer: Consider the decimal $r=0.b_1b_2\ldots b_nb_1b_2\ldots$, that is, a simple decimal of period $n$.  Multiply $r$ by $10^n$:
$$10^n r = b_1b_2\ldots b_n.b_1b_2\ldots b_n \ldots$$
or
$$(10^n-1) r =  b_1b_2\ldots b_n$$
Remember that $b_1b_2\ldots b_n$ is an integer, so that $r$ is a rational number after reduction to simplest form.
For a more general decimal $0.a_1a_2 \ldots a_m b_1b_2\ldots b_nb_1b_2 \ldots$, note that $0.a_1a_2 \ldots a_m$ is a rational number in and of itself, leaving the remainder as a rational number ($10^{-m}$) times the repeating decimal, so that the more general case also leaves a rational number.
Answer: I will just consider the case where $a = \frac{m}{n} \in \mathbb{Q}$ is between $0$ and $1$.  This is a trick taught by my high school teacher:
Rather than simply prove simply existence, I will supply a constructive method.
Suppose we have $$a = .a_1a_2a_3\ldots a_na_1a_2$$so that the period is $n$. Then we may multiply to get $$10^n \cdot a = a_1a_2\ldots a_n.a_1\ldots$$
Then try subtracting these two to get $$10^n*a - a = a_1a_2 \ldots a_n.00000\ldots$$  And divide to get $$a = \frac{a_1\ldots a_n}{10^n-1}$$You may reduce this fraction (because it need not be in lowest terms), and voila, you are done!
Comment: Very nice. Shift it on itself, to mask out everything to zero out to the left.
Answer: Suppose $x \in (0,1)$ and $x = \sum_{k=1}^\infty x_k \frac{1}{10^k}$, where $x_{k+p} = x_k$ for some $p>0$. Then $x  = \sum_{n=0}^\infty \sum_{k=1}^p x_{k+np} \frac{1}{10^{k+np}} = \sum_{n=0}^\infty (\sum_{k=1}^p x_{k} \frac{1}{10^{k}}) \frac{1}{10^{np}} = \frac{\sum_{k=1}^p x_{k} \frac{1}{10^{k}}}{1-\frac{1}{10^{p}}}$.
Answer: Suppose you want to prove that $\color{red}{a_1a_2a_3...a_n}.\color{blue}{b_1b_2b_3...b_m (c_1c_2c_3...c_k)}$ (where the red part is integer part, and the blue one is fractional part) is a rational number, firstly split it up like this:
$\begin{align} \color{red}{a_1a_2a_3...a_n}.\color{blue}{b_1b_2b_3...b_m (c_1c_2c_3...c_k)} & = \color{red}{a_1a_2a_3...a_n}.\color{blue}{b_1b_2b_3...b_m} + 0.\underbrace{00...00}_{m \mbox{ zeroes}}(c_1c_2c_3...c_k) \\
& = \color{blue}{\frac{a_1a_2a_3...a_nb_1b_2b_2...b_m}{10^m}} + \frac{\color{green}{0.(c_1c_2c_3...c_k)}}{10^m} \quad \mathbf{(1)} \end{align}$
As we can see, the blue part is already rational, we'll now prove that the green part is also rational, like this:
Let $\alpha = 0.(c_1c_2...c_k)$, then $10^k\alpha = c_1c_2c_3...c_k.(c_1c_2c_3...c_k)$ (i.e, by multiplying by $10^k$, we shift the decimal point k digits to the right), so we have:
$$\begin{align} 10^k\alpha &= c_1c_2c_3...c_k.(c_1c_2c_3...c_k) \\ \alpha &= 0.(c_1c_2...c_k) \end{align}$$
Subtract side by side, we have: $(10^k - 1)\alpha = c_1c_2c_3...c_k$, so $\alpha = \dfrac{c_1c_2c_3...c_k}{10^k - 1}$, which means that: $\color{red}{0.(c_1c_2...c_k) = \dfrac{c_1c_2c_3...c_k}{10^k - 1}}$
Example

$0.(3) = \dfrac{3}{10^1 - 1} = \dfrac{3}{9} = \dfrac{1}{3}$
$0.(71) = \dfrac{71}{10^2 - 1} = \dfrac{71}{99}$

Substituting the red part above to (1), we have:
$\begin{align} \color{red}{a_1a_2a_3...a_n}.\color{blue}{b_1b_2b_3...b_m (c_1c_2c_3...c_k)} & = \color{red}{a_1a_2a_3...a_n}.\color{blue}{b_1b_2b_3...b_m} + 0.\underbrace{00...00}_{m \mbox{ zeroes}}(c_1c_2c_3...c_k) \\
& = \color{blue}{\frac{a_1a_2a_3...a_nb_1b_2b_2...b_m}{10^m}} + \frac{\color{green}{0.(c_1c_2c_3...c_k)}}{10^m}\\
& = \color{blue}{\frac{a_1a_2a_3...a_nb_1b_2b_2...b_m}{10^m}} + \color{blue}{\frac{c_1c_2c_3...c_k}{10^m(10^k - 1)}}\end{align}$
The sum of 2 rational numbers are of course, rational, hence it's done.
Example
Express the following number in terms of the ratio of 2 integers: $0.5(2)$
We have, $0.5(2) = 0.5 + 0.0(2) = \dfrac{1}{2} + \dfrac{0.(2)}{10}$
We then try to express $0.(2)$ in terms of fraction, let $\alpha = 0.(2) \quad \mathbf{(2)}$, then $10 \alpha = 2.(2) \quad \mathbf{(3)}$. Subtracting (2) from (3) yields: $9 \alpha = 2 \Rightarrow \alpha = \dfrac{2}{9}$, so:
$0.5(2) = 0.5 + 0.0(2) = \dfrac{1}{2} + \dfrac{0.(2)}{10} = \dfrac{1}{2} + \dfrac{2}{9.10} = \dfrac{47}{90}$.
Answer: The repeating period in a decimal can be taken to be a digit in a wider base. For instance $.123123123$ can be regarded as three digits of base $1000$:  $.123,123,123$.  Therefore repeating decimal sequences are just a generalization of repeating digits, like $.333333$.
Furthermore, without loss of generality, we can factor out the repeating unit and replace it with 1. For instance $.33333...$ becomes $3\times .1111...$. Similarly, if we have $.123,123,123,...$ we can rewrite that as $123\times .001,001,001,....$.
Okay, so now we just have to show that numbers like $.1111...$,  $.010101...$, and $.001001001...$ are rational.   But these are just sums of descending powers of a base: $\sum_{n=1}^\infty{\frac{1}{b^n}}$ where $b$ is 10, 100, 1000 or whatever power of ten base we choose for the given period length (or, as it were, digit width).
For convenience let us change the sum to go from zero so that it includes the term 1, which we immediately subtract:  $(\sum_{n=1}^\infty{\frac{1}{b^n}}) - 1$.  So now we have the summation in the form of a famous power series for which we know the closed form (and we know it converges since $1/b$ is less than 1). We then use the well known identity to reduce this to $(\frac{1}{1-\frac{1}{b}}) - 1 = (\frac{b}{b-1})-1 = \frac{b - (b - 1)}{b - 1} = \frac{1}{b - 1}$.
For instance in the case of $.001001001...$, the base $b = 1000$, and so this is equal to $\frac{1}{1000 - 1} = 1/999$. Gee, we know that already from doing long division, but we have arrived at the result from the other end: sum the infinite series to recover the fraction.
We have already established that the other repeating decimals are just the product of an integer digit like 123 and a repeating decimal based on 1's. The product of an integer and a rational is rational, so all those other repeating decimals are rational also.  And of course we can add an arbitrary integer part to a rational, so that any number with a repeating decimal fractional part is rational.


Is it acceptable to heavily edit my closed question?

Question: My question was closed as not constructive, despite having some pretty decent answers. After some thought I can more or less see what was wrong with it, and I'd like to make it into something useful. Is it okay if I edit it completely, slightly altering it's meaning, if the answers given will still be relevant to the topic?
Comment: Worded right that should be perfectly acceptable.
Comment: Your English is a lot better than that of some of the people on SF for who it *is* their first language. ;)
Comment: For the record, I voted to close that question because it has nothing directly to do with system administration. How much to charge for a facility or service is a business decision. We are not here to reverse engineer business decisions, regardless of whether or not there is a technical reason for them.
Comment: Thanks for the update. I reckon that if I asked instead about the parameters in which datacenter and consumer broadband differ, it would be more relevant to a sysadmin's job, so I will probably try with that.
Comment: I had a shot at it [here](http://serverfault.com/questions/449333/how-in-terms-of-which-parameters-is-a-professional-grade-broadband-connectio). Feel free to improve the wording if needed; English is not my first language.
Comment: Instead of voting to close I've upvoted your new question.
Comment: With the new question and the edits made to it, I think this might make a canonical question on connection types, as it could be use to a sysadmin of a small business type network trying to make the step up to more professional services.
Answer: This is really three questions:

Is it acceptable to edit a question that got closed before any answers were accumulated?
Absolutely.
To be blunt, if it has no answers the question was probably closed because it was crap. The comments may even be asking you to edit it so it's a good question and we can answer it.  Fix it and let us know so we can reopen it if it's been reformed into a good, answerable question.

Is it acceptable to edit a question with a bunch of answers on it in such a way that slightly alters its meaning?
Maybe.
Like @MadHatter said we'd really prefer if you asked a good question the first time, but sometimes new information comes up (or someone asks a question in a comment that changes the way you look at the problem), so edits happen.
Editing questions means that people have to come back and fix their answers to take into account the new information.  If your edits are going to require a lot of effort from the people who answered your question it may be better to just leave the original closed and ask a new one with adequate detail.

Is it acceptable to edit a question with a bunch of answers on it, in such a way that changes its meaning substantially?
Maybe.
If there are several answers and editing your question completely invalidates the answers, it's probably best to ask a new question.
If there's one or two answers go ahead and edit but leave comments on the answers so the authors know the question got edited and can revise or delete their answers as appropriate.
Comment: Thanks, I accepted this answer as more generally useful IMHO, as @MadHatter's refers specifically to my case.
Answer: Interesting question.  I was fairly heavily involved in your original question, both in answering it (to try to demonstrate that the entire question was founded on incorrect data) and I then voted to close it (since it increasingly seemed to be unanswerable once the error was removed).  I'm not sure how you could "heavily edit" that question without making the answers irrelevant, and my guess is that it's unlikely the old question will get re-opened, so no new information will be posted as a result of your edit.  My take is, if you think you have a better question to put to us, why not start a new question (though please do read the FAQ first!)?
But the content on this site is licensed under cc-wiki with attribution, which is posted clearly at the bottom of every page.  So if you decide to ignore me and go back and completely overhaul the original question, then whilst I think that's not as helpful as posting a good new question, I don't think I can say it's wrong.
Comment: By no means am I the Glorious One True Guardian of SF (!), so you should probably ask questions that seem good to you!  But in any case, I think there are some good, concrete answers to the question you pose above, so why not ask it, and we'll see what people have to say.
Comment: Well, I guess that what I really wanted to know when I asked that question was, "How (in terms of what parameters) is broadband service that you get in a data center different than what you get in your home or small office". (I also made a wrong assumption about the price, but that only became clear to me AFTER you pointed out that home connections don't guarantee you data throughput - so, after a partial answer). Now, if you still care ;), do you think the above would be a valid SF question? If not, would it be more appropriate for SuperUser?
Comment: @odemarken I would say asking a new question is certainly better, but be careful of simply duplicating your question, down votes can lead to you being banned from asking questions. See http://meta.stackexchange.com/questions/86997/what-can-i-do-when-getting-we-are-no-longer-accepting-questions-answers-from-th


Ableton Live Preset Resetting to Default

Question: So I've been playing around and came across a nice pad call Pad-A Snow Pad. You'll find this in 'Instrument Rack -> Synth Pad'. It'll be near the top, or should be. I don't know what I did but something has changed and I want to reset it to how it was before. Now it's just a long chord without all the nice bells and pulsating parts. Can someone screen shot a picture of all their settings as I'm clueless on what I may have deleted.
Answer: I checked the Preset in Ableton and I figured something out. The Volume of the Starshower chain (the second row) which creates the bell sound is based on the Velocity. Just click on Vel above the chains (where mute, solo, pan... for each chain is located) to display the settings. Currently it is set to Hide (orange background). If you play a note with a velocity greater than 90 you won't hear the bell sound; you'll only hear the chord. If you play the notes with your computer keyboard, you can change the velocity with the keys "c" and "v" by default. By pressing "c" several times, you will be able to here the bell sound again. Your current velocity will be displayed in the long bar at the bottom of your display.
Answer: Let me know if this helps or if anything is missing
Answer: Also- unless you have changed the default preset and actually gone as far as to save it under the original name of the default preset, I'm pretty sure you will be able to access the preset in its original form if you were to restart ableton. Perhaps you might have to do this in another project if you were pressing save in the original project file--on that I'm not so sure.


Can I add elbows to move a b-vent exhaust?

Question: I am planning out a kitchen remodel, and would like to move the main exhaust vent for the water heater about 4-6 inches from where it currently passes through the kitchen area.   The catch is that I cannot change its path through the upstairs without significantly increasing the scope of the project.  So I am trying to figure out if stepping it over through the kitchen, and then back to the original path would work.
The exhaust is 6in B-Vent.  Water heater is in the basement, has a short lateral run to the exhaust stack (which it no longer shares with the furnace), and then goes up through the first and second levels to vent through the roof.  I am thinking that rotating the lateral run about 20 degrees, and then a pair of 45s where we go into the second level should give me enough offset to do what I want in the kitchen.
Any reason this wouldn't work/be safe? Or cautionary points to make sure I account for before setting off on this path?
Here is a quick sketch of what I have in mind:
(exaggerating the movement for demonstration purposes, not to scale)

Edit: I looked up the manual on my water heater as suggested, and it referred me to "the latest version of the International Fuel Gas Code".  Looking at the 2012 version of said code, I believe this is the relevant section:

503.6.9.1 Category I appliances. The sizing of natural draft venting systems serving one or more listed appliances equipped with a draft
  hood or appliances listed for use with Type B gas vent, installed in a
  single story of a building, shall be in accordance with one of the
  following methods:

The provisions of Section 504.
For sizing an individual gas vent for a single, draft-hood-equipped appliance, the effective area of the vent
  connector and the gas vent shall be not less than the area of the
  appliance draft hood outlet, nor greater than seven times the draft
  hood outlet area.
For sizing a gas vent connected to two appliances with draft hoods, the effective area of the vent shall be not less than the area
  of the larger draft hood outlet plus 50 percent of the area of the
  smaller draft hood outlet, nor greater than seven times the smaller
  draft hood outlet area.
Approved engineering practices.

503.6.9.2 Vent offsets. Type B and L vents sized in accordance with Item 2 or 3 of Section 503.6.9.1 shall extend in a generally vertical
  direction with offsets not exceeding 45 degrees (0.79 rad), except
  that a vent system having not more than one 60-degree (1.04 rad)
  offset shall be permitted. Any angle greater than 45 degrees (0.79
  rad) from the vertical is considered horizontal. The total horizontal
  distance of a vent plus the horizontal vent connector serving draft
  hood-equipped appliances shall be not greater than 75 percent of the
  vertical height of the vent.

I would interpret this to mean that the change I am wanting to make (using 2 45 degree elbows to create an offset) is permitted, as long as my pipe dimensions fall into the correct range(which they do).  Can anyone confirm?
Answer: Do a quick search online for the brand and model of the water heater. Most manuals for install are available online, but of course this will depend on the age of the unit. The manual (in most cases) describes the limitations of exhaust pipe distances and in particular the inclusion of elbows in the flue. It is important to check as elbows reduce the ease of flow and will effectively reduce the capacity of the exhaust and distance the flue can run.
So worst case it can make the installation unsafe and it is right to confirm with a professional or the manufacturer.


Does Krav Maga or any Martial Arts helps preparing to get stabbed?

Question: You sure learn how to defend againt knife attacks, but how does it even feel to get stabbed? Would an average human stay under shock? Or Would you still keep fighting and defending after you get stabbed? Or will the pain hurt so much, that you wouldn't be able to use any techniques anymore?
Do Martial Arts or Krav Maga help hardening your mind and body against such attacks?
Would even a beginner still have a better chance to survive than someone without any fight/sparr experience?
Answer: Wow, that are plenty of questions out there ... let's try explaning it like this:
I'm in Martial Arts for a long time, was very active in Boxing for a long time and been a Krav Maga instructor and bouncer for many years. And NO Martial  Art nor Self defense system will bring you in a position to defend yourself against a knife attack - At least not without leaving you bleeding or wounded badly if trying.
There are situations where you see the knife before an attack, my personal proposal is to turn your back and run.
If you don't see it, and notice that you are stabbed, as depending on the situation and the power of the 'swing' or hit you might not even notice it as a stab, you have at least to try to avoid additional stabbs and this is where something like a martial art or krav maga can help - IF you are able to recall the movements in that situation.
I've been stabbed a couple of times and had the luck to still walk around and I can tell that the only time I have felt it was in the muscle areas. I received a stab in my leg and I can say that it felt like a very good low kick where the stab in my back, in the kidney area, I did not notice a all. Just after everything was over I felt a wet shirt- from the blood and a weaknes in my knees.
I guess it is the adrenaline that don't let you feel the pane actually.
However, my suggestion in any case, if you see someone with a knife approaching you, don't try to fight, run.
Don't believe the Marketing hype - you will NEVER leave a knife fight without injuries - weather you are successfull in defending it successfull or not. This is nearly impossible.
Comment: Forgot to mention that a stab to heart, liver or lungs will definitely make you - not being able to defend anymore!
Comment: If you try to defend someone being attacked by someone with a knife make sure you have something available that is at least longer than his arm including the knife ex. Baseballbat, chair - this one if used unproperly can be even dangerous :-(, throwing objects from a distance to distract the attacker is also good. And in any case CALL THE COPS.
Comment: I know, it's nearly impossible and I think even for the best fighter, if some one knows how to stab or even comes with 2 knifes, I had this already sometimes and it is pretty hard. in my eyes knife and gun are the most dangerous ones in common assaults
and I think if even 2 at same time would charge, then at least then everyone would run, okay except you gotta protect a person that is important to you, but I wonder if it's right to give other people time to run away while stopping the attacker, I mean you never know how fast the attacker can run
Comment: yeah that's what I would do, I would tell person near me to run away and call the cops while I try to hold the attacker, no matter if I survive 10 seconds only or 5 minutes. every second gives the person near me an advantage to get safe, at least i think like that, what would you say? If I would be alone I'd sure give up and run, just don't give them a reason to attack me.
Comment: @DeathToxic - I can not give any advice really how you should behave in such a situation, as everybody is different - I have seen people 'big mouth heroes' transforming to 'whining washcloths' when they got stabbed. Distracting for a couple of seconds helps the other person to get away. I just hope that you never encounter such kind of situation - really! But if you under any circumstances can not avoid facing an attacker and need to hold him, try at least to focus/pin his knife holding hand - MUST NOT LET GO - and shout for additional help of bystanders ... and  .. (you/they) CALL THE COPS.
Comment: I think/hope the human instinct in our mind and boy will handle it if the time comes. And I can just hope that everyone will survive. I am very paranoid regarding  this, I did MMA and do Krav Maga to built up confidence. It helps a lot actually.
I got the feeling if I would think about it it won't do it better, somehow the situation will not go as I plan anyway
Answer:
The Journal of trauma and acute care search on stabbings should be a good research starting point.(1) Wikipedia's article on stab wounds have a large reference section you should check out. Turleskin stab armour has a white paper on stabbing targets but clearly is biased towards selling armours. Finally, Prof Sarah Hainsworth has published several papers on the force of stabbing. Oh, one last thing: No nonsense self defence section on knife fighting is a must read.
Most, if not all, martial arts are fooling themselves that they can teach anything useful against a skilled knife fighter. Even a knife vs gun fight is not going to go as you think it should as Dan Inosanto demonstrates in these videos. Note that the original video showed Inosanto attacking a LEO. This has since been taken down for obvious reasons in 2017…
Even a skim read of the above should tell you one thing: Stabbing is bad news! Thus:

Do not frequent places where knife crime is common.
If attacked, run towards safety.
Never have anything in your wallet/purse that you cannot replace within 24 hours so its lose is irrelevant.
If stabbed, seek medical attention immediately. The sooner medical staff get to you, the greater are your chances of living.
Learn first aid.

(1) Clearly this is a medical journal thus contains graphical material related to trauma.
Comment: All true ... on youtube there are pleanty of examples...:https://www.youtube.com/watch?v=OaY6rxgdn4k; https://www.youtube.com/watch?v=SZqNKAd_gTw; https://www.youtube.com/watch?v=tIo9ritvcg4
Comment: Dan Inosanto's video appears to have been taken down... Maybe you can replace the link?
Comment: @Marc.2377 Yeah, no surprise there. Thank you.
Answer: There's a number of arts that deal specifically with blades combatively - that ranges from traditional to modern military styles.  There's common elements among them all, which kind of tells you there's probably some baseline truths involved if you see parallel evolution across cultures and time periods, especially since knives are among our oldest weapons and tools.
The universal strategy to all of these (aside from, "Don't get in a knife fight") is getting rid of the threat as soon as possible.  That could be jamming the arm that reaches for the waistband before the knife gets out, it could be tearing tendons with a lock, punching the head, or all kinds of thins.  There's not a single solution because knives are ugly, fast, and dangerous, and most of these styles carry a few options because you have to adapt quickly.
That said, you'll see a lot of combative knife styles spend a good amount of time on training defense, not because you're going to magically dodge/block 5 minutes of knife attacks, but because surviving the first couple of attacks is necessary for you to counter and take control.
Aside from the methods towards not getting stabbed, the other part is positioning such that injuries do the least harm - getting cut on the back of the forearms is much less risky than most other places you can get cut. You can't harden your body or your mind, you can just make sure that what gets hurt is something that you can get to a hospital with, and not something that will leave you dead.  (Well, you could wear armor, which is basically what people do when they expect violence...however that's not what you're asking.)
There are accounts from people who have been stabbed or cut and felt nothing or simply numbness.  A common danger to sharp weapons is not pain, but not realizing how badly you've been cut and not stopping the bleeding in time.  The flip side is that many arts that work with knives often teach overkill - you keep attacking because you can't assume pain will stop an opponent.
Comment: You may want to read this book to get some idea on the layers of mental/physical stuff that freezes people or slows their reactions: http://www.amazon.com/Facing-Violence-Unexpected-Rory-Miller/dp/1594392137
Comment: so it's basically true that most people get that much adrenaline in a real fight that they feel the pain after that?
I wonder how I would react or feel if I get assault charged for first time, until now I never got assaulted so I don't have that experience with the need to actually fight for my life, I am sure I won't get under shock, but I wonder if I get too nervous I wouldn't be able to concentrate myself as good as else
Comment: Adrenaline is part of it, though the other part is just how blades work and bloodloss.  A lot of the reports are just that people feel dizzy or suddenly weak, and that's because the bloodloss has kicked in, and they only thought they got punched - because they didn't know a knife was already in play.   That said, I think you're conflating a few things under "shock" - there's the surprise of being attacked, freezing up/fear/panic responses, adrenaline rush, and then shock in the medical sense.  These are different but all can impact you negatively in a situation.
Answer: From what I've seen. No, most martial arts don't deal well with knives, there is a lot of complete fantasy, wishful thinking and misinformation around.
I've never been stabbed myself, but come from the stabbing capital of the UK and know a couple who have, and in both cases they didn't realise they had been stabbed until after the event.
If we look at the past, there is a common feature that all cultures employed when they had to deal with blades. Armour. i.e. even though they had technique, they still employed armour. Put another way. Technique will not save you. It takes 1/6 of a second to throw a punch. Human reaction time is closer to 1/4 of a second. If the hand throwing the punch holds a knife, you'll see that the odds are stacked in favour of the offence rather than defence.
Armour doesn't have to be steel. Blades do very poorly against multiple layers of clothing, 3+ will often prevent a slashing attack and Gambeson worn in medieval times of 10->30 layers of linen would prevent all but heavily committed thrusting attacks. Today we have far superior fabric technologies.
Answer: There is a statistic sometimes mentioned in our school. I don't know where it comes from and therefore I can't vouch for its accuracy, but it goes like this. 1% of untrained people defend successfully when attacked with a knife. 5% of people trained to black belt standard in any art manage to defend themselves when attacked with a knife.
If the statistic is correct, the implication is that an untrained person has on average a 1 in 100 chance. A black belt or equivalent has a 1 in 25 chance of not getting seriously injured.
Or put another, if 100 untrained people get attacked with a knife, 99 will be seriously injured, 1 will walk away. If 100 black belts or equivalent get attacked, 96 will be seriously injured, and 4 will walk away.
Put still another way, if you are confronted with a knife, and running is a viable option, then run.
Comment: I can't make the edit, because it's so small, but I think you intended to write "1 in 20" not "1 in 25"
Comment: so than this means if I get stabbed it isn't like I didn't train enough?
I mean if it's true what you stated then even a pro can easily lose to a knife fight.
this means I don't need to feel bad if I fail defending myself.
of course always run away, but was just wondering.
Comment: Look for footage of trained police or soldiers dealing with a knife threat. Invariably, they try to keep their distance, and will try to use a weapon or improvised weapon in defence. These people are as highly trained as you can get.
Comment: Meant to add, in our school, we are taught that our knife defence is not to prevent getting cut, but to TRY to prevent any vital organs or arteries getting cut.


Creality Ender 3 printer power consumption?

Question: Anyone have any idea how much power it takes to run a Creality Ender 3 3D printer every day for several hours at a time? Like what does it eat up per hour? A rough estimate of power use per hour would be nice, then I can figure out how much it costs me. Can anyone help me?
Comment: Possible duplicate of [What is the power consumption of the heatbed and hotend?](https://3dprinting.stackexchange.com/questions/5537/what-is-the-power-consumption-of-the-heatbed-and-hotend)
Comment: <200W is a quick estimate. This translates to <1$£ per 24 hours,
Comment: That depends on how much heat is dissipated from the heat bed and nozzle (insulation used?), at which temperatures you print, how long, bed size, etc. Without that information it will be a guess and with the info it will still not be very accurate.
Answer: I'm currently measuring the power usage of my Ender 3. It used about 0.5 kWh for 4 hours of printing. With 2 heat ups (about 280 W each). So approximately 120 W average.
Or 0.12 kWh per hour.
Assumed your printing 12 h a day you're using 1.5 kWh a day. That translates into a cost of 0.43 € a day in electricity (0.3 €/kWh).
When you're using your printer every day for an average of 12 h. You'll be using about 525 kWh a year or 157 € in electricity at 0.3 €/kWh.
Comment: @JamesHanvey Not Fahrenheit, but Celsius.
Comment: Most of my prints are 2 or 3 hours long approximately, I am printing in pla filament and my heated bed is heated to 60 degrees Fahrenheit and the hotend is printing at 200 to 205 degrees Fahrenheit. I just was wondering what the cost would be because my mother is saying I can only print one thing per day now. I think it is kind of unreasonable but who am I to tell my mom how to make rules? Anyway I am printing at those temperatures, so how much is the power consumption, I live in new York.
Comment: So how much would that be in watts per hour?
Comment: Review the answer, are you talking to me or one of the people who answered the question. I still am kind of lost as to what my ender 3 printer consumes in watts at an hourly rate, because my mom was mad at me for printing out a print that took six hours long and she is afraid it's gonna spike her electric bill. If anyone knows how much a ender 3 printer consumes by printing pla filament at 205 degrees Celsius with a 60 degrees Celsius print bed temperature I would be able to figure out what the electric bill will be when I run it for hours at a time. Thnx everyone for trying to help me.
Comment: Why are 0.12kWh odd when you're using an average power of 120 Watts. Energy is power multiplied by time. So 120W * 1h = 120Wh or 0.12kWh.
Answer: The Ender 3 has a 360 W power supply. This is the maximum power the printer can use. However, if engineered correctly, it should never actually reach this level of consumption to prevent premature failure of the power supply. It will use the most power during initial warm up, which is a relatively short amount of time. Then the average power consumption will drop considerably once at temperature. So, an average of 120 Wh per hour seems like a reasonable estimate.
It turns out that if you are paying 11.4 cents per kWh (which is a nominal price in the US) the price of running something 24 hours a day is \$1 per Watt per Year. So if you ran your printer all year long nonstop it would cost \$120 per year or \$10 per month.
Since you live in New York, you probably pay about 1.5 times as much for electricity or 17.1 cents per kWh. So you'd multiply the figures above by 1.5, so you would get \$15 per month. Also, you don't use your printer 24 hours a day. Let's say you want to print an average of 8 hours a day. That's 1/3 of the day, so you'd divide your figure by 3 and get \$5 per month. You'd hardly notice a difference in your electric bill.
Comment: "120W per hour" is a units error. I think you mean "120W" aka "120 Wh/h".
Answer: If it is really important to you to know how much you are spending per any given print, your best bet is not to guess, but to know how much power you're using. To that end, you could purchase a power meter which monitors your power usage. Given the right one, it can even calculate the cost of the power usage all in one little package. This link, The 10 Best Electricity Usage Monitors, should provide you some ideas as to what you might find, but I'm sure there are plenty more out there (NOTE: I have no affiliation to the link provided ... it was just a random one I found through a Google search ... Go Go Google-Fu!).
As 0scar pointed out in his comment, there are just too many variables to try and guess what the power consumption might be. If you are looking for a real answer, I believe something along the lines as I've linked above is going to be your best solution to getting a real answer. Anything else is more or less just a guess.
Answer: Other answers include good estimates, and show that for one printer the electricity costs are not very significant. If you are using many printers and want a concrete answer, then it would be wise to purchase a power monitor (for continuous monitoring) or multimeter with clamp.
With a clamp-on multimeter, you can clamp the meter on to the 3D printer's plug and read the Amps being drawn. Assuming a 120 VAC single phase supply (typical for North America), the power consumption is 120 VAC multiplied by the Amps drawn by the 3D printer (P=VI), which you can read from the multimeter.
The amps drawn by the 3D printer will vary throughout your print, but for a longer print, you should be able to get a good average amps read during a middle print layer. Total energy cost of printing per day would then be:
<code> C = (V*I/1000)*t*E*n

     C, Total printer energy costs per day ($/day)
     V, AC Voltage (V)
     I, Average current draw during print (Amps)
     t, time printers are running per day (Hours/day)
     E, Energy cost from utility ($/kWh)
     n, number of printers
</code>
Comment: That will give you an estimate of +/- 6 %, which is the variation in output voltage.
Comment: @0scar, good point. The multimeter may also have a measurement accuracy listed on the device that can be factored in.
Answer: During the printing process: 110 Watts (Average)
Can be calculated as energy: 0.11 kWh (Average)
Just directly multiply it by your model's total printing time.
In short (on average) (based on 2023 electric prices)

1 hour printing
₺0.17 (Turkey)
\$0.025 (US)
€0.03 (EU)

7/24 printing daily cost
₺3.96 (Turkey)
\$0.63 (US)
€0.73 (EU)

7/24 printing monthly cost
₺119 (Turkey)
\$19 (US)
€22 (EU)

In the table below, you can see all of the actions' power consumption individually. All values are average and show isolated consumptions.

State
Power Consumption Net (Watts)

1 Idle
7

2 Heating Bed
243

3 Heating Nozzle
41

4 Keeping Nozzle Hot
22

5 Keeping Bed Hot
44

6 Steppers Engaged
18

7 Feeding Filament
5

8 X-Y Motors with Max Feedrate - 150 mm/s
2

9 Hotend Fan
2

For more detailed information, see my Medium Post
Ender V2 Power Consumption
Answer: Just order a Watt Electricity Usage Monitor or a similar model and plug it into the wall, or better, a 3-prong extension cord, then plug your 3D printer into it, and you'll get all the power consumption detail you'll need. Show your Mom, and she'll see what to charge you, if she's so inclined. To impress her, offer to pay without her asking.


What is the best way to store carrots?

Question: The carrots that I buy come in a sealed plastic bag. In the store they are not refrigerated. I've tried keeping them in the bag in the place where I keep things like onions, garlic, and potatoes (a cabinet with metal racks, and an air opening in the back - I keep the carrots on their own rack). I've tried keeping them in the bag, but I've found that one often rots, from moisture or something similar, and causes all the rest of the carrots to go bad. I've tried taking them out of the bag, but they all quickly shriveled up and went bad. The only thing I can think of still trying is to keep them in the fridge... What is the preferred way of storing carrots? How can I keep my carrots in good condition and prevent them from rotting?
Answer: Store them in the bag in your fridge. Also, if you take the greens off they will last longer. I keep mine in the drawer at the bottom.
Comment: More specifically, removing the _greens_ from the carrots prevents them drawing moisture out of the root itself.
Comment: @MichaelHoffman The greens--and the roots (the carrot itself)--are still alive.  The continue to respire and metabolize, expending resources stored in the root since they are in the dark and without soil, so they are not photosynthesizing and producing new resources.  Removing the greens will really cut down on the nutrient draw from the root.
Comment: Why does removing the tops make them last longer?
Comment: When storing carrots or other root crops in a (usually wooden) container packed with damp sand, the whole assembly is traditionally called a _clamp_.
Answer: Store your carrots in the fridge, either in one of those special veggies containers that Bob described or in the veggies fridge compartment (usually a bottom drawer with undulated bottom to trap liquids) if you have one. (Notice the little slider that you slide to control moisture and air flow if you put veggies in there.) Only buy whole unpeeled carrots if you want them to last one or two months in the fridge being protected by their skin.
I personally keep whole, unpeeled carrots in their bag (to avoid loss of moisture), in the fridge. I never had spoilage issues, even after 6 weeks. They might look a little shriveled but will regain their moisture if cooked in liquid (making soups or using a slow cooker).
When you're saying that your carrots are not refrigerated in the store: veggies do not need doors (like frozen food) to keep them at fridge temperature. My store has a funny "storm sound" alarm (like rain is coming) just before spraying the veggies with icy water, regularly, to keep them cold. So you get your hand away for a few seconds otherwise you would get icy water on your arm (and clothes if you wear long sleeves), then (after the "storm" has passed) you can grab your carrots.
The idea of storing fruits/veggies at "room temperature" dates back to when people had basements with a constant low temperature under 60F (59F = 15C). My mother-in-law grew her own veggies and had an apple orchard; and also a huge cold basement with a lot of different rooms to store all of her fruits/veggies (separately) for months. But once you heat your house in the winter (or if you live in a hot region in the summer and have to use an AC) the "room temperature" will probably get too hot.
Answer: The best way I've found is a specialty tupperware container designed to keep produce fresh.  They have little raised trays in the bottom, and a system of vents in the lid.  The combination allows air to circulate and ripening gases to escape, retains moisture, but allows water to drip below the produce to prevent spoilage from sitting in a puddle.
Note that produce should be rinsed before putting in the container, to keep it moist, and should be rinsed every week or two to prevent drying out.
In one of the special containers, in the fridge, my carrots lasted at least a month, probably closer to a couple months.  They were still fine when I tossed them, but it just didn't sit right to keep produce I didn't remember buying.
Manufacturers of special produce preserving containers:
Progressive International and Ruppermaid both make storage containers with these properties.  I have the 4-container Rubbermaid set, and they seem slightly better designed (the tray is inside the container), but the sizes are very much not convenient.  The largest size looks like it should hold a heat of lettuce... but just barely doesn't. It's excessively large for other stuff, although it WILL hold a lot of carrots.  The smallest size is pretty much only useful for a piece of ginger or half an onion.
Comment: don't get yourself a root cellar then. Your carrots might last until early spring in one of those ;-)
Answer: It likely depends on what shape you want to store them in.  I have good luck just putting the bag in the crisper in my fridge, assuming I use them within a few weeks.  Of course, it helps to shop at a grocery store that has a good turnover, and they haven't already been sitting there for 2-3 weeks (or spent a week being shipped cross-country)
If you find that the carrots have gotten a little dehydrated from long storage, you can actually wrap 'em in a damp paper towel and leave them for a day or so, and they'll come back.  You could likely also soak them in water ... my mom used to cut up carrots, and had a tupperware?rubbermaid? container with an insert in it so you could fill it with water, then lift things up out of the water ... and they'd stay good in there for a week or more (she kept 'em so as kids we'd have healthy things easy to snack on)
Answer: You want to avoid a lot of surface moisture as it can lead to spoilage. You can fill a container with clean sand and place the carrots in there. The container can go in the fridge or a cabinet if your house isn't too warm.
Answer: You've noticed that one always goes off, so you found the reason that they sell more carrots if they are supplied pre-washed and cleaned in plastic bags!
The washing and rumbling (the peeling process) takes away the natural protection against moulds, so there is only one answer - you have to prepare all the carrots, blanch them, and freeze the ones you are not going to use immediately.
LOL it is a scam, but people do prefer to buy pre-washed and scraped carrots.
Comment: where I live, not only the pre-peeled and washed "baby" carrots are in plastic bags, but so are whole, unpeeled, and generally unmolested carrots. These keep fine if left in their bag in the fridge, best in a drawer.
Comment: Checking the batch for damaged carrots (eg any that got punctured by something hard in the same shopping bag) before storing could also help...
Answer: i think the best way to keep them from not getting mouldy is to cut the tops off the carrot itself
Comment: Does this apply to carrots that don't have greens on them, so you'd be cutting a bit off the large/round end? (Carrots in bags almost never have the greens still on, at least in my experience.)

What is the best way to store carrots?


permutation representation of $S_n$

Question: If a group $G$ acts on a set $X$, then we can speak of permutation representation on $K[X]$.
now set of all $k$-subsets $X_k$ of $n={1,2,...,n}$ is a $S_n$ - set and we can speak about permutation representation on $K[X_k]$. by decomposing it we get $[k/2]$ irreducible representations $V_0, . . .,V_{[k/2]} $  for k less than or equal to $n/2$.
     Similarly we can do this for any partition of n instead of (k,n-k) which we consider in the above case and we obtain all irreducible representation of $S_n$ with the assumption that $K[X]$ is completely reducible.
     I am looking for any other method (instead of permutation representation) which give as
all irreducible representations of $S_n$  and its character values in simpler way.
     Is there any method like this? thank you.
Comment: What is $K$?${}$
Comment: any field which is algebraically closed.
Comment: we have assumed that K[X] is completely reducible.
Comment: thanks. Is there any way to get simple representations?
Comment: Sorry, but your method is not clear from me... how do you get the non-hook irreps? Anyway, you are aware of the fact that if we fix a faithful rep $V$ of the finite group $G$, then every irrep of $G$ is a direct summand of some tensor power of $V$ ?
Comment: sorry. I have few doubts in your words. what is the meaning of non hook irreps? I know the later fact but how to extract all simple representation from tensor powers of fixed V?
Answer: Since you only care about the completely reducible case, I'll assume $K=\mathbb{C}$.
The easiest way that I know of to construct irreducible $S_n$ representations is a special case of the construction  here. In particular, symmetric group is a quotient of the (degenerate) affine Hecke algebra appearing there (obtained by setting $x_1=0$). If you take $\mu=0$ in the link, then the skew shape $\lambda/\mu$ is just $\lambda$ and the module that is constructed is the irreducible representation $S_\lambda$.
This recovers the character formula $s_\lambda=\sum_T x^T$, where the sum is over all standard tableaux of shape $\lambda$ (as described in Macdonald's text, for example).
Answer: There is a general study of representations of symmetric groups via young tableaux. In particular there is a combinatorial expression for the characters. I think that's as simple as it gets currently.
Comment: This is precisely the construction I linked to above.

permutation representation of $S_n$


Do i have to wash the undergarment?

Question: Do i have to watch my undergarments in washing machine if a little drop of urine comes on it? I was praying suddenly felt a tiny drop of urine came in my underwear, its common and i cant control it. Happens all the time. I cant be washing mt clothes in washing machine every day because of that, can i just wash it with some watee and use my clothes/undergarments to pray again? Im a male 19 years old. Thanks
Comment: Do you have only one underwear?
Comment: Salam and welcome to IslamSE the Q&A site about Islam. Consider taking the [tour] and checking our [help] for more information about our site and model.
Comment: Related [Does dripping urine break Wudu?](https://islam.stackexchange.com/q/2955/11938)
Answer: You only need to wash the parts where there is a urine stain and then once it's washed, you can pray

Alqama and Aswad reported:

أَنَّ رَجُلاً، نَزَلَ بِعَائِشَةَ فَأَصْبَحَ يَغْسِلُ ثَوْبَهُ فَقَالَتْ عَائِشَةُ إِنَّمَا كَانَ يُجْزِئُكَ إِنْ رَأَيْتَهُ أَنْ تَغْسِلَ مَكَانَهُ فَإِنْ لَمْ تَرَ نَضَحْتَ حَوْلَهُ وَلَقَدْ رَأَيْتُنِي أَفْرُكُهُ مِنْ ثَوْبِ رَسُولِ اللَّهِ صلى الله عليه وسلم فَرْكًا فَيُصَلِّي فِيهِ
A person stayed in the house of A'isha and in the morning began to wash his garment. A'isha said: In case you saw it (i. e. drop of semen), it would have served the purpose (of purifying the garment) if you had simply washed that spot; and in case you did not see it, it would have been enough to sprinkle water around it, for when I saw that on the garment of the Messenger of Allah (ﷺ). I simply scraped it off and he offered prayer, while putting that on.
 - Sahih Muslim 566
Comment: @Medi1Saif  My bad, I meant that emission of semen will require obligatory ghusl while emission of urine doesn't
Answer: All scholars are in consensus that urine is najis (ritually impure).
So to wear clothes that you know of them having got in contact with urine (See also about the case if you prayed this way without knowing it Is istinja' necessary for ablution?) while praying, you must have washed at least the parts that are najis (with water). In case of doubt -about the location of najasa- you may even need to wash the whole clothes. As Allahs the almighty says:

And your clothing purify (74:4)

Note that to clean a najasa (a ritual impurity) you must do your best effort to take away the three attributes: odor, color and taste of this najasa (see for example in Sunan abi Dawod and Ritual impurity in pants).
This means you have a couple of options:

washing it by hand you need only soap and water, if you use much water you can even resign from using soap based on a hadith compiled by imam Ahmad in his Musnad see here in Buluggh al-Maraam.
washing it in washing machine (as you suggested),
washing these parts of the underwear and praying with the washed underwear or
taking it off and praying without, there's no necessity in praying with underwear as long as your 'awrah is coated and one may even pray in one single garment. See also Exposing awrah to the ground when praying.

Do i have to wash the undergarment?


How do you punch out toes in ski-touring boots?

Question: I've got a pair of custom moulded Scarpa Spirit 4 touring boots where the big toes are bit too tight (I've lost the nails after a day or two of touring more than once).  What is the proper technique for punch out the toe liners without damaging the liner or overall functionality of the boot?
Answer: Pull out the liner and step into the shell - if you can fit a finger between your heel and the back of the shell you might be able to remold the liner or buy an after-market liner.  A professional boot-fitter at your local gear store can do it for you, or you can follow the directions to do it yourself in a low temperature oven.  If you have trouble fitting a finger between your heel and the back of the boot, I'd consider new boots.  They will be uncomfortable no matter what you do.
Comment: Is punching out always done with heat? I've got another pair of Solomon boots where the inner left ankle creates a pressure point by afternoon when I'm skiing all day.  What other techniques are there?


How does amount of flour affect cookies?

Question: What will happen to my chocolate chip cookies if I add more or less flour than the recipe calls for?
Comment: @Catija Let's not force the question to be too specific. It'd be sane for an answer to say, hypothetically, you won't notice anything change til you add X, from X to Y they'll get crisper, from Y to Z they'll get crumbly, past Z they won't even hold together. If the OP limits the question to certain amounts, she could easily miss out on the big picture.
Comment: Depends on what you mean by "more" or "less"... How much more or less? Half? Double? A tablespoon? Give us some more information...
Comment: The recipe used would be helpful in giving answers scaled to it.
Comment: I think it's possible to write decent answers without knowing a starting point; if the OP's recipe is already in the "more flour" direction they'll just be farther along the continuum you describe. But if you really want to know a starting point, you could assume it's the canonical Toll House recipe from the US that's on the back of basically every bag of chocolate chips (2 1/4 cups flour, 1 cup butter, 3/4 cup each white and brown sugar, 2 eggs, 1 tsp baking soda, 1 tsp salt), which makes a generic kinda soft cookie. If the OP doesn't return to clarify eventually, I'll just edit that in.
Answer: A general answer, because a concise answer would need an entire recipe.
(all ratios that follow are by weight not volume measures)
Flour gives the cookie structure. The commonly followed ratio is:

Cookie dough = 3 parts flour, 2 parts fat, 1 part sugar

That 3:2:1 ratio results in the most common cookie texture.
Adding more flour give you a hard cookie like: ginger snaps, short bread, etc.
Reducing the flour, like a 1:1:1  flour-fat-sugar will give a drop cookie and often chewy.
I Googled, and found this informative piece How to Invent a Cookie Recipe.
Comment: @Paulb That's a scan from Ratio, right? (here's [the same chapter in Google Books](https://goo.gl/y1YPZ6)) If you look at the example recipes there, the ones actually using the 3-2-1 ratio are likely crisper cookies, and then the chocolate chip cookie recipe is 1-1-1 and the sugar cookie recipe is 3-2-2. So while Ruhlman does say 3-2-1 for some kinds of cookies, for chocolate chip and sugar his recipe is pretty darn close to the Toll House one.
Comment: Is this ratio actually correct? Based on the recipe posted by Jefromi in the comments above, that's certainly not a 3 to 1 ratio of flour to sugar (by volume, anyway). 2.25 cups flour to 1.5 cups sugar is 3 to 2 ratio. Considering that this recipe is for the "classic" Toll House recipe and it doesn't match your ratios, can you please explain a bit more about this?
Comment: I edited in by *weight.* Toll House weights: 2-1/4 cups flour=306 grams, 1 cup butter=227 g, the sugars combined are 315 g. That's 3-2-3. The sugar is very high which is interesting because I consider Toll House to be too sweet. Ruhlman (the ratio guy), says rations are just the starting point.
Comment: The Toll House cookies might be a little on the sweet side, but they're not the only ones (e.g. the [Serious Eats chocolate chip cookies](http://www.seriouseats.com/recipes/2013/12/the-food-lab-best-chocolate-chip-cookie-recipe.html)) are about the same ratio, so I too have a hard time imagining 3-2-1 as a canonical chocolate chip cookie base.
Comment: This guy seems to think so. http://product.design.umn.edu/courses/pdes3701/documents/ratio_cookies.pdf Note: his ratio numbers are opposite the sequence of Ruhlman (I don't know how long that link will live)
Answer: More flour will result in a "breadier" cookie.  It will have more structure and be less chewy.  Less flour will do the opposite, resulting in a softer, flatter cookie.
http://www.handletheheat.com/the-ultimate-guide-to-chocolate-chip-cookies/


What factors affect the carbonation level of beer?

Question: Beer varies widely, and one of the most noticeable differences when drinking is the carbonation level.  What factors in the composition, brewing process, storage, etc affect the final level of carbonation?
Answer: There are two main ways to carbonate beer.
Force carbonation - This is where CO2 is forced into the fermented beer. Because of the pressure (and the temperature at which it is done) the CO2 will dissolve into the beer solution. The fizz that happens when you open a beer is the CO2 coming out of solution. (Technically you can re-carbonate flat beer!)
Natural carbonation - All beer has SOME yeast present. Home-brewers that bottle will actually pour a little bit of sugar into the racked beer before bottling. The yeast that is present will re-activate and eat the sugar. One of the bi-products is CO2. Because the beer is bottled and the CO2 has nowhere to go, it will dissolve into solution. This is why you will often see little yeast cakes in the bottom of home-brewer's bottles.
There are some beers that used forced-nitrogen to nitrogenate (if that is the appropriate word) their beers.
TL;DR - Temperature, methods of carbonating, storing beer can all affect carbonation.
Comment: Yes, the word is nitrogenate.
Answer: I have a similar question, but interpret the original question differently - probably because I am a home brewer that lets the beer carbonate naturally. If you are forcing CO2 into the beer, I imagine you have greater control over levels of carbonation. However, I understand that the final gravity, alcohol content, amount of sugar, and temperature have the most significant impact on carbonation in the bottle. The primary factors would be Lower FG and ABV will increase carbonation, and secondarily a little more sugar and time (>3weeks) will increase carbonation. This all assumes you are storing the beer in a temp range that makes for happy yeast.
Comment: The reason higher gravity beers have low carbonation is that they have more unfermented sugars. When that happens it makes for a really nice head. Lots of carbonation and unfermented sugars will give you a yuuuge head. Light beers are drier, more completely fermented allowing for more carbonation without a huge head. Think of Champagne which has a  huge amount of carbonation, yet the head does not last but a few second.


Why does my Dell 1130n printer not install?

Question: I have a fully updated Ubuntu 12.04 and my Dell laser class 1130n printer won't install via a USB port.  Does anyone have any ideas? It says that I have the wrong driver and it only prints an error page.
Comment: Nevermind, I forgot that I could use the 'generic' driver, it ended up working.
Answer: Ubuntu does not include a driver for your printer.
A google search tells me that Dell's 1130N is actually a rebadged Samsung ML-2580n with customised firmware.
What generic driver did you actually use? Paste the output of this command on a terminal: grep -H '^*NickName:' /etc/cups/ppd/*.ppd
Can you try weather the splix driver for the Samsung ML-2580N, which is included with Ubuntu, works good or better than the generic driver with your printer?
Go to system-config-printer (System -> Administration -> Printing in GNOME classic, Gear icon at the upper right of the screen -> Printers in Unity), right click your Printer and select Properties > click Change... next to the Make and Model field to see the available drivers and change it to Samsung ML-2580N.
Answer: We also have 12.04 and tried installing our Dell 1130 on it.  It had worked with 11.10 but we could not do so with 12.04.  To make a long story short we found that someone in another forum found that if you chose the Samsung ML1915 from the manufacturer list instead of the Dell 1130 that it worked just fine.


How come $32.5 = 31.5$? (The "Missing Square" puzzle.)

Question: Below is a visual proof (!) that $32.5 = 31.5$. How could that be?

(As noted in a comment and answer, this is known as the "Missing Square" puzzle.)
Comment: Compare the slope of the blue and red triangles. ${3\over 8} \neq {2\over 5}$
Comment: Everyone - further discussion on whether this is allowed can be found here: http://meta.math.stackexchange.com/questions/68/questions-about-math-jokes/70#70
Comment: To prove $31.5=32.5$, start from $88=89=90=91$ and subtract $56.5$, $57.5$, or $58.5$   http://math.harvard.edu/~elkies/Misc/rect2.pdf
Comment: See [here](https://web.archive.org/web/20100204093058/http://www.fantasticmetropolis.com/i/division/full/).
Answer: Overlay the two triangles to see the difference. This is known as the missing square puzzle.
Answer: It's an optical illusion - neither the first nor second set of blocks actually describes a triangle. The diagonal edge of the first is slightly concave and that of the second is slightly convex.
To see clearly, look at the gradients of the hypotenuses of the red and blue triangles - they're not 'similar'.
gradient of blue triangle hypotenuse = 2/5
gradient of red triangle hypotenuse = 3/8
Since these gradients are different, combining them in the ways shown in the diagram does not produce an overall straight (diagonal) line.
Comment: The word you call "gradient" is more familiar to me as "slope", but I guess it's just regional variation.
Comment: It's not exactly a coincidence that these are ratios of Fibonacci numbers. That's a good way to end up with ratios that are quite close to each other. In this case, both are approximants to $\frac{3-\sqrt{5}}{2}$, the limit of $\frac{F_n}{F_{n+2}}$.
Answer: Since this question has been bumped up, and some people find pictures helpful, may as well supply illustrations of the explanation.
The area of the red triangle is $\frac12(8)(3) = 12$ and that of the blue triangle is $\frac12(5)(2) = 5$. The area of the yellow and green regions are clearly 7 and 8 respectively, so the total coloured area is 12 + 5 + 7 + 8 = 32 which is less than 32.5, the area of a 13×5 triangle. Indeed, in the figure below you can see that the hypotenuses of the red and blue triangles "dip" below the actual hypotenuse of the large triangle; this accounts for the 0.5 difference in area.

Similarly, in the second figure, the hypotenuse of the red and blue triangles are above the actual hypotenuse, which accounts for the 0.5 difference in area again.

The difference between the two figures is a thin parallelogram with vertices at the endpoints of the hypotenuses in both figures (coloured pink below), which has area exactly 1. (If it looks much less, that's the nature of the optical illusion.)

Also, there are some links to similar figures at Wikipedia's article on this missing square puzzle.
Answer: Surprisingly, the general case remains to be explained.
Consider the recursion $F(n+2) = F(n+1) + F(n)$, with $F(1) = F(2) = 1$
producing the Fibonacci numbers 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89 ...

Let the blue triangle have horizontal and vertical sides of length $F(n+1)$ and $F(n-1)$, respectively.
Let the red triangle have horizontal and vertical sides of length $F(n+2)$ and $F(n)$, respectively.
So that
in the upper figure the two triangles create a rectangle with an area of $F(n)F(n+1)$
and
in the lower figure the two triangles create a rectangle with an area of $F(n-1)F(n+2)$
By the Fibonacci identity $F(n-1)F(n+2) - F(n)F(n+1) = \pm1$
we can see that the two rectangles are bound to differ in area by exactly one unit.
As n increases the gradient/slope of each triangle tends to the square of the Golden Ratio $ \phi$,
one from above and one from below.
Thus, as $\phi^2$ = 0.3819660112...
2/5 = 0.4 > $\phi^2$
3/8 = 0.375 < $\phi^2$
5/13 = 0.3846... > $\phi^2$
etc.
Comment: As an aside, it's relatively harder to dissect the F(n)×F(n+1) and F(n-1)×F(n+2) rectangles into two shapes (which leave a "hole" in the second case), even though they differ in area by 1. (At least, I couldn't see something simple.)
Answer: The red and blue triangles are not similar (the ratios of the sides are 3/8 = 0.375 and 2/5 = 0.4 respectively), so the "hypotenuse" of your big triangle is not a straight line.
Answer: No, this is not a proof of your statement. If you look very closely, you will see that the hypotenuse of the triangles aren't straight. You can also verify this algebraically by calculating the internal angles of the triangle using trigonometry.
The other way to demonstrate the non-straightness of the hypotenuse is in the fact that the red and blue triangles aren't similar, which can be seen by the difference in the ratios of their width and height. For the blue triangle, this is 2/5 and for the red triangle this is 3/8. As they are both right angled triangles, these ratios would be the same for similar triangles and the fact that they aren't means the interior angles are different.
Answer: I never looked at this, and thought that it was the same shape. You can't just change the number of angles in a shape and call it identical to the original. If you were to sketch only the outlines of each colored block on a blank piece of paper would the resulting "Shapes" be Identical or Even Similar? The answer is most definitely not. The illusion is looking at it on a graph VS in a Tangible form of any type, Imagine that you have 2 completely identical blocks of wood. You remove a identical portion as shown in figure 2, You didn't just make that hole appear out of nowhere because it can only be achieved by the removal of a block and thus a massive difference in the amounts of Angles laying within the "Shape" therefor making figure 2 nothing more than a combination of different shapes rather than being one definable shape of any type. I've yet to hear a better explanation than my own!
Comment: good efforts! Now, why not try hand at some newer questions?
Answer: In the top diagram, the areas of the two smaller right triangles are (3)(8)/2 = 12, and (2)(5)/2 = 5 which total 17. The area of the rectangular region is (3)(5) = 15. The total area is therefore 17 + 15 = 32 [not 32.5]. In the bottom diagram, the same sum holds for the smaller triangles of 17, but now the rectangular region is (8)(2)=16, but wait! We have to subtract 1 because one of the little squares is missing. So the total area of the bottom diagram is 17 + 16 - 1 = 32. Therefore the areas of the two shapes are identical. The large "apparent triangle" of base 13 and height 5 and area 32.5 is not really a triangle at all because the two smaller triangles that form its hypotenuse have different slopes. It is a four sided figure.


Windows Error logs codes

Question: In windows system error logs I see many error codes like 11, 9, 5... I need to know what are the reasons for these errors and how to troubleshoot them. Also every time this error occurs in the description it has something like device ....\adpu320A2. I know that adpu320A is the SCSI card but what is this 2 after that?
Answer: I mostly use EventID.net to check event log entries, have you used EventID.net already?
Answer: Your question is far too general and there are so many possible error codes, many of them device specific, that it's just not possible to hand you that information ready to use. You need to put some effort in to help yourself.
When you view those event log entries there's a link for more information. Start by clicking on that. It won't always produce the information you need but it's a start.
Another thing you can do is to Google for the event ID and error codes.
After trying those things, if you still have a question about a specific error ask that question.
Comment: The error code that I see mostly is 11. I tried googling already and eliminated the possible causes like driver and bios update. But still the problem is there. I posted it here to see if I have missed possible causes.
Answer: The problem is that the error codes written to the event log are internal to the application, process, program, service, technology, product that generates the events.
My own applications sometimes write to the Windows Event Log. The first kind of thing i want to log is called:

Event ID 1

as i add things, i keep adding numbers; maybe i mix it up, or skip some:

Event ID 1337
Event ID 69
Event ID 10404

There is no documentation for these events, except for when users report error. i ask them for the codes, search my source code, and try to figure out what happened.
But i think you are limiting yourself to only Microsoft products, and probably only products created by the Windows group. i think you want documentation for all possible event id codes that can be generated by all Windows products.
Such a comprehensive singular list does not exist. So use eventid.net.
Comment: What you describe for your own applications shows extremely poor management and understanding. This is hated equally by admins and users. If we give you an error code for your software you should be able to instantly look that up in the documentation and then explain to us what caused it. The only reason to go back to the source code should be to fix the problem.

Windows Error logs codes


Why does this happen in the end of Suspiria?

Question: In the 2018 movie Suspiria, we see that Mother Suspiriorum kills everyone in the school at the end. Aren't the witches in the school worshiping the three mothers?
If that where the case, what is Mother Suspiriorum enraged about? Why does she kill her own worshipers?
Comment: I've seen Suspiria few months ago so I don't know if I misunderstood but, if you are referring to the final scene when everything turns red, the mother killed everyone who had worshiped Mergo instead of Mother Suspiriorum. Though I don't know if you are mentioning this scene or another one.
Comment: Yes that's the scene. Marko and all the witches worship the three Mothers. Suspiriorum kills Marko and her followers, I'm not sure why they are killed nevertheless.
Answer: She killed them because they were loyal to Marko only (I wrote Mergo because of Bloodborne ahah). Hence, during the film there is a scene where there is a sort of vote, and the majority vote for Marko, making Mother Suspiriorum lose. In brief, Suspiriorum killed those who were not loyal to her, but to Marko.
Comment: Yes, that's the problem. Marko won the vote, but she was selfish and detained all the power for herself, pretending to be Mother Suspiriorum, this is the reason why at the end, those who were loyal to Marko were killed, while those who were faithful to Blanc were spared.
Comment: I thought it was a vote between Madame Blanc and Marko and Blanc lost. Suspiriorum is one of the three mothers that every witch in the school worshiped.
Comment: At the end, right before Susie evoke the Death, can't remember the exact words that she uses but I'm quite sure that's the scene.
Comment: didn't realize she was pretending to be Mother Susperiorum. Is there a conversation or scene in the movie that suggests this? Can you please point me to it?
Comment: I watched that scene again. Markos only says death to any other mother. But doesn't claim to be Suspiriorum as such.


Was the tribe of Shimon part of the Kingdom of Israel or Judah?

Question: It's clear from Sefer Yehoshua (Perek 19) and elsewhere in Tanach that the tribe of Shimon lived in an area scattered throughout the tribe of Yehudah. However, when the kingdom is divided between Malshus Yehudah and Yisrael, it appears as though only two tribes make up Malchus Yehudah, and Shimon is not one of them (Melachim I 12:21). Did Shimon join the kingdom of Yisrael? How did they manage to do that while living among Yehudah?
Comment: What about Leviyim, they were also scattered in the northern 10 tribes
Comment: @ShmuelBrin they were scattered among all of the tribes, and didn't fight at all probably. They didn't count for many of these things
Answer: Ralbag notes this in his commentary to Melachim 12:23, and he writes that the verse does mention יתר העם in addition the tribes of Yehudah and Binyamin, which is meant to be a reference to Shimon.
From Divrei Hayamim I 4:31 it appears that at least some of the members of Shimon had movd out, with their cities being given over to the tribe of Yehudah (see Rashi and Radak there), so it could be that by the time of Rechavam there wasn't so much of Shevet Shimon left in the Judean lands.
According to Tosfos Yom Tov (Sotah 8:1), Shevet Shimon did indeed break off from Yehudah politically while remaining in the same place geographically, and so the constant infighting between the two kingdoms often took place between Yehudah and Shimon.
Sefer Shalsheles Hakabbalah (pg. 234 in the edition published in Jerusalem, 1962) writes that after successfully exiling the Northern Kingdom of Israel, Sancheirev waged war against the Kingdom of Judah, and captured several cities of Yehudah along with all of the cities of Shimon. The implication of this story is that the author (R. Gedalyah ibn Yihya) understood Shimon to belong to the Kingdom of Judah, at least with regards to the exiles of the two different kingdoms.
Answer: The tribe of Shimon was scattered through the land of Israel over time as punishment for Shimon's role in the deceit and murder of the Shechemites (Genesis 34) and for the Shimonite men committing harlotry and idolatry to Ba'al of Peor (Numbers 25). This scattering was prophesized by Jacob on his deathbed (Genesis 49).  There is supposedly a midrash (maybe someone else can give a link) that states that some of Shimon's inheritance in the land passed to other tribes when the widows of Shimon remarried after their husbands were killed after the incident with Ba'al of Peor.
As you correctly point out from Joshua 19, the tribe of Shimon was given cities that were surrounded by the territory of the tribe of Yehudah. The book of 1 Chronicles 4:31 states that the Shimonites had these cities until the reign of King David. After that 1 Chronicles 4:39-43 has some of them moving to Gedor and others to Mount Seir in Edom.
Basically when 1 Kings 12:21-23 lists Yehudah and Benjamin as the two tribes who followed Rehavam and formed the Kingdom of Yehudah (Judah), it is referring to the only southern tribes that still had significant land holdings. There were probably individual members of the tribe of Shimon who still lived in the area and there were almost certainly Levites that lived in the area, but neither of them held a recognizable piece of territory.  There were likely also minority populations of Gibeonites (see 2 Samuel 21) and Jebusites (see 2 Samuel 24:16) that still had a few families within the southern kingdom, but the bulk of the men and land that made up the southern Kingdom of Yehudah were of the tribes of Yehudah and Benjamin.


How can I share a private YouTube video with someone so they can edit and save it?

Question: I need for someone else to add things to videos that I upload to YouTube.  How can I share a private YouTube video with someone so they can edit and save it?
Answer: You can't share a private video with anyone who doesn't have access to your account. Someone mentioned posting it as 'unlisted' but that isn't private since anyone with the URL can see it. If you trust the person to not share the URL then post the video as unlisted.
Since the video is already completed it can't be "edited" in the true sense of the term, but rather add additional features YouTube provides (annotations, changes to title, etc.
Comment: You **can** share private videos with specific e-mail addresses. https://support.google.com/youtube/answer/157177
Answer: You can connect your YouTube channel to Google+ page and then you can add managers.
In this way you can share private o non listed videos with your collaborators.
Comment: technicality: You can't connect a channel with Google+ pages anymore, you'll need to connect them to Google Brand pages now.
Answer: I believe just setting the video as "unlisted" works.
Unlisted means that the video is not listed on the site, but available for view for anyone with the link !


Migrating from Exchange Server 2003 to Sendmail in Linux (squirrel mail)

Question: I want to know that how do we migrate the emails from exchange server to Linux sendmail.
What are the precautions that we need to take to avoid the data loss?
Thanks,
G.Gopi Krishna
Answer: Migrating from exchange to sendmail?
Hmm... Strange.
Most people migrate from sendmail/postfix to exchange. Exchange is more than just only email. You have calendar, notes, contacts, tasks, Global Catalog with all contacts from your domain, centralized privilege an account management from AD, users can set autoresponder and give other users authorization to their emails and calendar.
Exchange is piece of collaboration platform from Microsoft application stack.
When you want to migrate only email, you can write some script, that will query AD/LDAP for existing email accounts and create mailboxes on Linux box. And then Another script, that would get all email from exchange using IMAP or POP3.
Maybe it is possible to use some Exchange connector, but I haven't seen this kind of solution. Connectors are frequently used for migrate data to Exchange, not from Exchange.
Are you loose anything. YES, you lose Exchange functionality.
I found this on the web:

SUSE Openexchange Server has MS
  Exchange Server migration method. But
  you need to buy this SUSE Linux
  product.

Maybe this will by usefull for you.


What's with all the questions involving tears?

Question: Are we somehow attracting a certain type of worker that breaks down into tears in the workplace? (due to criticism or code reviews - not something bad happening in their life)
I think I've never seen this actually happen. Is this common?
Comment: possibly related: [Copying questions that had a misfortune of being widely exposed in the hot list](http://meta.workplace.stackexchange.com/q/2445/168)
Comment: At least one of these questions looked really fake to me ("Rain man"), there was little to do though because even if mocking it met our standards well enough.
Answer: The audience of The Workplace is biased toward people with problems in their professional (and sometimes personal) lives, problems that are difficult enough to interfere in their daily activities that they seek out external help.
Let's say only 1% of the people we know in our work environments fall into that category. Now take that 1% from all workplaces in the English-speaking world. There's a good chance workers who break into tears find their way to this site.
The chance of any one of us actually experiencing this may be small, but in the larger context it may be more common.
Comment: Agreed. And once someone reads a question about crying and thinks "Hey, I cry too!", they are more likely to post their own similar question.
Comment: Not to mention the Hot Network Questions makes it even more public and expands the likelihood of someone having a similar question finding this site.
Comment: I deleted my answer as I hadn't considered this angle.  This is an excellent point, thank you for changing my perspective.
Answer: It's not common in that it's a daily event, but I've seen plenty of people have emotional outbursts including tears. People can be sensitive to different things and react in many different ways
I'll admit it even happened to me when I got an email stating my mother had passed on. I was physically unable to talk and just forwarded the email to my manager and walked out the door for the rest of the day. And my eyes are watering up right now just thinking about it.
So if you busted into my office right now without warning you might see a grown man in tears (you'd have to be fast though).
Comment: Hugs. I was at work when I got the phone call from a concerned neighbour who had gone in to find my mother had died (living by herself on a farm, widowed and fighting leukaemia). That was four years ago and sometimes still feels like yesterday.
Comment: I cried at work every day for year after my beloved died.
Comment: @AndyDent a decade ago and can still evoke powerful emotions in me, I don't mind, it's good to remember her and it's healthy to feel vulnerably human once in a while (in private), cuts me down to size.
Answer: The Workplace covers all kinds of workplaces, doing all kinds of jobs, all over the world.  I've seen a 16-year-old fast-food worker cry; I've seen a new grad having that first brush with nasty corporate politics cry; and I've seen others cry for specialized, personal reasons like a health issue or personal loss.  It happens -- not often in most of our experience, it appears (not in mine, anyway), but unusual situations are more likely to prompt questions on a Q&A site about the workplace.  After all, the stuff that's routine and common is stuff that people are more likely to find an answer to before coming here.
Answer: Someone who cried a lot made it to HNQ, and scored a lot of points. Then a lot of other people started crying too in the hope of making some easy points. Don't read too much into it.
Answer: Yes people frequently cry in the office for a variety of reasons. Often teh reason they are so sensitive may have little to do with the actual event because it might be teh last straw in a series of events. Someone who cries in a code Review for instance may have previously lost a job and is frightened that it will happen again. Someone working through the loss of a spouse or child may cry easily at a certain tone of voice or a phrase that reminds them of the person who may have died months earlier. People new to the workplace often find it a much harsher environment than they have experienced before. Someone who is insecure might find that especially difficult and be more predisposed to cry over things that most of us would ignore.  Sometimes there are clashes of cultural expectations as well. Someone who was encouraged as a child to cry and show emotions (usually as a girl) may find it difficult to turn off what they learned was appropriate behavior when they are first in a  professional environment.
Answer: Yes its common.
In my nearly 30 years of professional experience, I have seen it. Its less common for a company to allow a single co-worker to continue on with the behavior over an extended time.
When someone suffers a death in the family or something like that its normal. Hell I cried when my grandfather passed while I was at work.
I think there is a human part of this as well as an over tolerance of this behavior of bring personal drama into the office.
Comment: I guess I meant more of the "ongoing crying jags" or "crying every code review" type of sob story. Not the normal human reaction to bad things in the world.


Review on DPA lavaliers 4061, 4060, and 4071

Question: People have been recommending DPA's to me for the longest time. I'm in the market to buy two at the moment and probably wireless. I wanted to know if anyone had experience with them and maybe recommend another company besides countryman that can compare to them. My main concern is capturing the best possible dialogue on set in all types of locations. I also will be running my boom with this set up.
Answer: Within the standard limitations of lavs, they're awesome. I was pretty happy with my Countryman B3 lav, but the DPA 4060's never cease to impress. About the only thing sweeter is the Sanken COS-11, which seems to be the lav of choice when inter-cutting with Schoeps mics. But I'm a huge fan of the DPA modular connector system, especially as a sound designer who sometimes needs to just run them into a field recorder, instead of a bodypack (so nice to be able to do both).


Multiplayer game principles

Question: I am developing game using libGDX libraries . I want to create multiplayer mode , where several players can play at the same time in the same room , each player can create his own room and other players can connect to the room they want , I thought a mechanism like this
In this mechanism the left client is the one who has created the room and other 3 clients are connected to that room . The game physics runs on the room creator and sends the positions of physical bodies to the server , server sends that positions to other clients , other 3 clients send their input to the server and the server sends that inputs to the room creator and that inputs influence on the physic. But I think this cycle is not good because if the device of room creator runs slow then the game will start lagging and the fps will drop down and other clients will see the game with the same fps which is in the creators device . So can anyone offer me another mechanism for multiplayer game? I am using Node.js for server-side program and Java for client side .
Comment: Just for learning more about game networking, parallel simulations (server/client) and getting in touch with the problems of this topic you might be interested in this great presentation: [Video](http://www.gdcvault.com/play/1014345/I-Shot-You-First-Networking), [Slides](http://downloads.bungie.net/presentations/David_Aldridge_Programming_Gameplay_Networking_Halo_final_pub_without_video.pptx)
Answer: I guess the reason why you were choosing this architecture was to keep the computation costs and computation effort low on the server, allowing it to be lightweight. (Another advantage of this approach is that the usually low upstream of the clients are relieved by a server acting as a broadcasting intermediator.) Of course the downfall is that it now depends on the clients performance. There is no way around this connection.
So you have to decide on what to prioritize:

Better network performance of the hosting client.
Better performance of the game on the hosting client.

If you have slow handheld devices which should act as clients you might prefer the first solution, using a strong, scalable server that hosts EVERY game/room.
If you always have strong clients with a good connection you might keep your infrastructure low in cost by choosing the second.
Another solution would be to do peer-to-peer communication using the server only for initialization of a game, eg. register as hosting client. Thereby you are able to lower the impact on the bandwidth of the server and you don't have to limit the number of games because of a limited bandwidth: The clients do the work.
I would stick with your approach but with a dynamic part. Since every client can start a game/room in your approach it has the ability to run as hosting client. So why not let the clients provide information about their current performance (FPS/CPU/RAM/Bandwidth/Latency) on which the current host decides if another client is a better host. If this is the case the game migrates from the bad to the new, better host.
What you get is:

Low impact of running games on the server in aspects of network, CPU and RAM.
Much simpler and better scalable server implementation.
Always get the best host for every game session, even if my client accidentally starts an antivirus scan or my damned brother a download of pr0n in the middle of the game.
Low performance handheld devices profit from strong game partners.

As far as I know Awesomenauts uses such dynamic host approach in a peer-to-peer network.
Comment: I think I should choose the way I thought, because if I run physics on the host it can't support several rooms (dual core 2.8ghz )
Comment: I'm not sure if I got you right or you misunderstood me. I propose to have a physics simulation per room (game session). This simulation is hosted at the creator at beginning of the game. This host is responsible for only this game session and its physics. If it is not strong enough it passes the ownership of the game session to another, stronger client in the same room (game session) which from now on plays the local server for physics and coordination: **For all clients in the same room there is only one of them which acts as the server and only for this room and no other.**
Comment: I understand what you say , I`ll try to do in that way, thank You
Answer: It's usually not a good idea to couple the framerate to the update rate of the network. The internet is just too unreliable for this. Your clients will feel every single lag and jitter.
A better method is to have the server only send changes and have the clients interpolate with them.
So when an object is moving from position A to position B over the course of 50 game ticks, you don't send 50 messages with 50 intermediate positions. You only send two messages. When it starts moving, you send a message "Object O starts moving from position A into direction X with speed V" and when it stops the second message "Object O stops at position B".
This allows the clients to start showing a smooth movement the moment they receive the first message. Any short-time lags during the movement will be invisible. When there is a lag just when the object stops, they might experience a slight jump to the end position, but that's it.
Comment: The correction of the position (when the message about the final position comes in) can be smoothly done over time. Imagine a rubber band connecting the correct position with the character on a wrong calculated position. The greater the distance the bigger is the force toward the correct position and so the way the player is moved toward the correct position per tick.

Multiplayer game principles


Were cavalry used in the First World War?

Question: Due to the nature of trench warfare, a cavalry or a horse-back squad would've been rather useless in the war. But, I also know that countries had not anticipated trench warfare – they didn't know that their troops were going to dig holes in the ground to defend themselves from enemy machine guns. So I was just wondering if any countries  deployed their cavalry.
Comment: How about http://en.wikipedia.org/wiki/Horses_in_World_War_I#Cavalry ?
Comment: Remember that generalised trench warfare was a West front phenomenon. In the East front, where the lines were longer and the number of soldiers proportionally smaller, trenches played a less important role.
Comment: Did you ever read the book and/or watch the movie "War Horse"?
Answer: British Cavalry was surprising successful on the occasions it was employed by local commanders in small scale attacks exploiting gaps in the German defensive lines after the Germans had withdrawn to the Hindenburg Line from late 1916 onwards. Despite what many ill-informed commentators say, many citing quite erroneous accounts by eye-witnesses who get the fundamentals wrong because they did not see what happened but thought they knew what should have happened, death tolls were not always high.  Charging horses, moving fast did manage to avoid much of the machine gun fire directed at them  Additionally, British Cavalry Regiments contained a section of machine guns and horsed field artillery that could be employed to suppress German fire.  High Wood is a good example although most of the casualties cited above were sustained well after the original charge and occupation of the High Wood hill.
The true value of cavalry emerged as a means of exploiting a gap and pushing forward to occupy ground that the infantry were too tired to reach.  It worked!
Many knowledgeable historians will also theorise that the 1918 German Offensive fails in part because strong cavalry forces were not available to German generals to push the British hard enough to completely break them.  The absence of cavalry at brigade and divisional level meant that German infantry were advancing without being able to adequately protect the flanks of their attacks with cavalry.
Don't forget that all modern cavalry carried rifles and were trained to rapidly deploy them.
Comment: This could be an interesting (and rather heterodox) answer. Right now it badly needs sources.
Comment: Do you have any citations? Particularly for events like "High Wood", which is apparently familiar to you, but not to an average history enthusiast.
Comment: @MarkC.Wallace - the Battle of High Wood Hill was part of the Battle of the Somme. More particularly, the Battle of Bazentin Ridge, where dragoons (mounted rifles) and honest-to-gosh *lancers* defeated and held temporarily German machine-gun and artillery positions after being softened up by artillery and infantry. It *is* a very interesting answer and needs more citation.
Answer: Probably the only large cavalry charge is the one Australians performed during the battle of Beer Sheva. The ANZAC forces were in fact mounted infantry and the charge was performed with infantry weapons (no lances but rifles with bayonets), this was surprising for Turkish defenders. It was so quick that the Turks could not destroy wells.
On other fronts (especially in Poland, Russia and Romania) cavalry performed reconnaissance tasks, but played no major role in any battle. Many cavalry men changed forces; one of the most notable examples was Manfred von Richthofen, the best pilot during the war. He had the rank Rittmeister, which is cavalry captain. His brother, Lothar, also a famous ace, was a cavalry man too.
Comment: The linked Wikipedia footnote says there are even more after this
> Some claim that this was the last successful mounted cavalry charge in the history of modern warfare, but this overlooks the 4th Light Horse Brigade Charge at Sheria on 7 November 1917, the 5th Mounted Brigade Charge at Huj on 8 November 1917 and in 1918, the 10th Cavalry Brigade Charge at Irbid on 26 September, the 14th Cavalry Brigade Charge at Kiswe, and the 4th Light Horse Brigade Charge at Kaukab on 30 September, the day before the Capture of Damascus during the Sinai and Palestine campaign.[Falls 1930 Vol. 2 pp. 95–595]
Comment: @user69715 Please check [this Italian charge at WW2](https://en.wikipedia.org/wiki/Charge_of_the_Savoia_Cavalleria_at_Izbushensky)
Answer: Yes. See Wikipedia.

All of the major combatants in World War I (1914–1918) began the conflict with cavalry forces. The Central Powers, Germany and Austria–Hungary, stopped using them on the Western Front soon after the war began. They continued to be deployed in a limited fashion on the Eastern Front well into the war. The Ottoman Empire used cavalry extensively during the war. On the Allied side, the United Kingdom used mounted infantry and cavalry charges throughout the war, but the United States used cavalry for only a short time. Although not particularly successful on the Western Front, Allied cavalry did have some success in the Middle Eastern theatre, possibly because they faced a weaker and less technologically advanced enemy. Russia used cavalry forces on the Eastern Front, but with limited success.

Although trench warfare is the image of WWI, not all the war was fought from or in trenches.
For example,

One of the last cavalry charges of the war came at the Battle of the Somme in 1916. The attack was on July 14th on High Wood – a German strongpoint that was holding up the British advance. Men from the 20th Deccan Horse, an Indian cavalry unit, attacked the German positions. Armed with lances and despite going uphill which slowed down the charging horses, some of the men reached the woods. Some Germans surrendered when confronted by cavalry in woodland – something they could not have expected. However, the attack, while brave, was very costly with 102 men killed along with 130 horses.

That is just from 10 minutes of cursory research; a more skilled scholar could doubtless pull up other examples.
Comment: Not worth adding another answer for this, but the two main reasons cavalry wasn't used as much despite most of the armies still having a cavalry branch was 1) Highly vulnerable to long range rifles and artillery.  For this reason, they were kept so far behind the lines they could rarely get to the front in time to make a difference, and 2) Horses had trouble navigating the countless shell holes at the front without breaking their legs.
Comment: Such a large link, but still helpful... upvote :)
Comment: It seems Germans had mounted voluunteers on the East African Front too: https://en.wikipedia.org/wiki/African_theatre_of_World_War_I#East_African_campaign.2C_1914.E2.80.931915
Answer: Cavalry were certainly used during the First World War.
It is worth remembering that the First World War was a world war, and wasn't limited to the attritional trench warfare of the Western Front, which is often the first image that people call to mind when they think of that war.

To give just one example from another theatre of the war, the Battle of Beersheba, in Ottoman Syria on 31 October 1917, included what has often been called the "last successful cavalry charge in history". The 4th Brigade of the Australian Light Horse charged Turkish positions in the town (supported by British artillery which successfully suppressed Turkish machine-gun positions).
It seems appropriate to be answering this today, on the centenary of the charge of the Australian Light Horse, which was instrumental in the capture of the town of Beersheba.
The capture of Beersheba broke the Gaza-Beersheba defensive line, and the Ottoman Seventh and Eighth Armies were forced into retreat. Gaza would fall a week later, and on 9 December 1917, British troops entered Jerusalem.
Answer: At the risk of flogging a dead horse here, I think we need to mention another aspect - the attitudes of the top commanders. The most glaring example I have in mind is Haig who famously told young officers (the remark may be apocryphal but certainly reflective of his recorded opinions) in July 1914:

I hope none of you gentlemen is so foolish as to think that aeroplanes
  will be usefully employed for reconnaissance purposes in war. There is
  only one way for commanders to get information by reconnaissance, and
  that is by the cavalry

Well, that may be understandable, but amazingly the man persisted in this opinion and this is what he had to say in 1926 (yep, nineteen twenty-six, eight years after the end of the war):

I believe that the value of the horse and the opportunity for the
  horse in the future are likely to be as great as ever. Aeroplanes and
  tanks are only accessories to the men and the horse, and I feel sure
  that as time goes on you will find just as much use for the horse—the
  well-bred horse—as you have ever done in the past.

(quoted from a page that does a really admirable job of skewering Haig).
However, not all cavalry commanders of WWI were bloody cruel butchers like Haig. An example of a cavalry general who both did his job well (on the Eastern Front) and learnt enough in the process to realize that cavalry was over is Mannerheim.
One more thing worthy of mention: Celaya.
Comment: Why in a quote if you do not think it's accurate? Haig made aircraft top priority when was CinC.Hiag was one of those for British Army reform writing the field service regulations. Haig was a innovative CinC he had his faults but bloody cruel butcher is just emotive.
Comment: "the remark may be apocryphal"certainly implies the quote is untrustworthy. Haig is more complex like most actual people, and characterisation of Haig as a cavalry general opposed to  change is misleading. just objecting to over simplistic version Haig ,who was a great proponent of innovation both before and during the war. InJuly 1914 cavalry more important than aircraft and a lot more reliable as scouts.
Comment: @pugsville Sorry, what quote?
Answer: Cavalry was used only sporadically in World War I. On the Western Front, there were only a handful of divisions used for "special services" such as scouting and transport. http://en.wikipedia.org/wiki/British_cavalry_during_the_First_World_War
In the Eastern front, where distances were larger, cavalry was used as "spearheads," e.g. by the Germans at the battle of Tannenburg, and by Russia's General Brusilov.
Cavalry was also used in "peripheral" areas such as the invasion of Iraq (then held by the Ottoman Empire).
Comment: i upvoted this as I cannot see why it is unhelpful
Answer: They were certainly used. But see John Terraine. He maintained it was irresponsible to plan an offensive without bringing up cavalry to exploit a success. Tanks from 1916 could break in, but not exploit. Cavalry couldn't fulfil their traditional role of exploitation, barbed wire and a machine gun stopped them.
They did not have what they needed - the Blitzkrieg tank. Warfare was at a hiatus in technology. World war 1 - also the only war fought without voice command.
Comment: Sources would improve this answer.  Who is John Terraine and where would I find the information you reference?