Is there such a thing as purple light?
Why does light of high frequency appear violet?
Screen Resolution Problem with Ubuntu 14.04 and VirtualBox close votes vs votes remaining I am new to the review part of Stack Overflow and I am trying to understand what is the meaning of votes remaining in the Review | Close Vote.  For example: Today I was at my 39 vote and it said that there was still 21 votes remaining.        However, after my 40 vote:        So, apparently I have started with 60 votes but I can use only 40? How is canon determined for the works set in the world of ASOIAF? Reference for the subgroup structure of $\rm{PSL}_2(q)$ Other than looking at the profile, is there a way to find out someone's real name? A user on SO, Abhijit, referred to the OP of a question, acpigeon, as "he" (and then Aaron) .  I didn't see the name on the question page, nor on . Is there a way to look this up that I missed? Alternatives are Abhijit knows acpigeon, or maybe Abhijit was just making things up.  I'm still relatively new to these sites, so I'm still learning my way around. Is there a way short of the two alternatives above to know the OP's name? If so, how? Calculating slope in percentage with QGIS I'm working with QGIS, v. 2.0.1. I have several soil types. They are (amongst others) described with slope indications. I already calculated the slope from the DTM in degrees, but as description of the soil types is in percentage, I need to calculate the slope in percentage. I tried two things:  I tried the analysis tool from the raster section, but it didn't work. The result was just black with values from 0 to 0. The same happened when I tried to calculate the percentage in the raster calculator but the result was the same.  Maybe I need to say that the terrain is pretty plain. What else can I do? Update I tried the raster calculator and did what @radouxju suggested. It gave me a black picture with values from 1756.64 to 1756.64. But as soon as I click in the picture the values are &quot;normal&quot;, say 5.88 or 13.224 or something like that. The same happens when I try the gdaldem tool but with values from 0 to 0. I also tried to change the contrast enhancement (like stretch to min/max) but it didn't change anything. Update 2 I just  that maybe I have to set the scale to 111120  but that doesn't work neither (result: black, values 0-0). Render water-paint in iOS After Google Play Service update to version 13 I got an error Python dictionary: are keys() and values() always the same order? I'm currently working through the book Heisenberg's Quantum Mechanics (Razavy, 2010), and am reading the chapter on classical mechanics. I'm interested in part of their derivative of a generalized Lorentz force via a velocity-dependent potential.   I understand the generalized force   $$F_i = -\frac{\partial V}{\partial x_i} + \frac{d}{dt}\left(\frac{\partial V}{\partial v_i}\right)$$  that they derive from a Lagrangian of the form $L = \frac{1}{2}m|\vec v|^2 - V(\vec r,\vec v,t)$.  However, in the next (critical) step of the derivation, the author cites a theorem from Helmholtz saying      ...according to Helmholtz, for the existence of the Lagrangian, such a generalized force can be at most a linear function of acceleration, and it must satisfy the Helmholtz identities.   The three Helmholtz identities are then listed as:  $$\frac{\partial F_i}{\partial \dot{v_j}} = \frac{\partial F_j}{\partial \dot{v_i}},$$  $$\frac{\partial F_i}{\partial v_j} +  \frac{\partial F_j}{\partial v_i} = \frac{d}{dt}\left(\frac{\partial F_i}{\partial \dot{v_j}} +  \frac{\partial F_j}{\partial \dot{v_i}}\right),$$  $$\frac{\partial F_i}{\partial x_j} -  \frac{\partial F_j}{\partial x_i} = \frac{1}{2}\frac{d}{dt}\left(\frac{\partial F_i}{\partial v_j} - \frac{\partial F_j}{\partial v_i}\right).$$  I'm trying to understand where this theorem comes from. Razavy cited a 1887 paper by Helmholtz. I was able to find , but it is in German, so I could not verify whether or not it proved the theorem. Additionally, I could not find it in any recent literature. I searched online and in Goldstein's Classical Mechanics.   The only similar concept that I can find is in the  where we have three equations known as Helmholtz conditions. Are these two concepts one in the same? If so, how should I interpret the function $\Phi$ and the matrix $g_{ij}$ that appear in the Helmholtz conditions I found online?   If the cited theorem from Razavy does not relate from the inverse Lagrangian problem, could I have some help finding the right direction? What is the Coandă effect? How is it defined, and what causes it? Can anybody explain to me the  well? I am finding that many definitions and explanations conflict.  I am particularly confused about the following points regarding the Coandă effect:   whether the Coandă effect is the same as normal boundary layer attachment to a surface whether the Coandă effect is only defined for convex surfaces over which a jet (of the same state as the surrounding fluid) is tangentially blown what causes the Coandă effect: the jet entraining the ambient fluid or a balance of pressure and centrifugal forces, or something else? Integral $\int\!\sqrt{\cot x}\,dx $ How to extend WIFI from ethernet Here's my current setup.   Bonded "Zone" DSL modem/router on main level serving laptop #1 via Ethernet and laptop #2 via WIFI.  WIFI does not serve basement level well, so I have Ethernet running to basement where it is divided by a switch with one Ethernet going to laptop #3 and another Ethernet going to entertainment system, which houses a Sony Playstation 3. The PS3 has gone on the fritz and is now only good for playing discs, so I recently added a Google Chromecast to the TV. The Chromecast, of course is WIFI only, and is only receiving a weak signal. Plus I need to have a decent WIFI signal to my phone, which is the interface device for the Chromecast. So what I would like to do is to use the Ethernet that was serving the PS3 and broadcast WIFI from it. What device can I plug the Ethernet into that will do this for me? And if you have a recommendation, are there any special setup instructions? What is the range of the validity of Coulomb's law? "Now" vs "by now" Consider these examples:   By now the desk should be looking significantly tidier!  By now, the failure of regulators to contain dangerous forces should be well accepted.  By now I own some forty copies of the magazine.  By now the blue of twilight is glowing.  He used leverage, but by now applied concerted discipline to contain his risks.    I wonder if I can use &quot;now&quot; instead of &quot;by now&quot; to convey the same meaning. I would think &quot;by now&quot; refers to a moment until now, but why not use past tense or present perfect tense in the these examples? How can I hibernate or sleep my Ubuntu 10.10 desktop and have it "wake up" the next day?  I have seen software that can do this on windows, so it can't be hard on Ubuntu! I was trying to install Linux on my MacBook for class when I bricked my computer, which now in an unbootable state.   I made my partition too small, so I was going to delete and create a larger one. However, Disk Utility did not allow me to delete the small partition. I followed the instructions from   However, not only was I still unable to delete it, but upon restarting I realized my primary partition (located on the same disk) changed into FFFFFFFF-FFFF-FFFF-FFFF-FFFFFFFFFFFF, and I had no options to boot.  (I am sure I performed the command on the correct disk and partition)  I saw some similar threads, but my main bootable drive is the one that got corrupt.   I don't want to lose any data. I'm not the most terminal savvy, so ANY help is appreciated, and I will kindly provide any additional information!  (Disk 0, partition 2 is my main partition, and that only one I care about)  Below are the results of  $ diskutil list and $ gpt -r show disk0.        UPDATE: :  I have followed @klanomath 's instructions. At the last step I received the following error:  Checking prerequisites Checking the partition list Checking for an EFI system partition Checking the EFI system partition's size Checking the EFI system partition's file system Problems were found with the partition map which might prevent booting Error: -69766: The partition map needs to be repaired because there's a problem with the EFI system partition's file system Underlying error: 8: POSIX reports: Exec format error    FINAL UPDATE: SUCCESS!  I just wanted thank @klanomath for helping me through this entire process. He spent 11 hours TeamViewing with me until not only my files were accessible, but until I could access my computer as could before this whole debacle (and this was after 5 hours of email correspondence).   To any future reader with this problem: PLEASE don't give up, it can be fixed! Distance from a point to a plane in normed spaces How can we calculate the distance from a point to a plane in normed spaces ? where we are not inner product, for example:  Calculate the distance in $(C[0,1],||\cdot||)$ endowed with the supremum norm $$||f||=\sup_{t\in[0,1]}|f(t)|$$ from the function $f(t)=t^2$ to the linear hull of the functions $g(t)=t$  and  $h(t)=\sin(t)$  Any hints would be appreciated. Where are the key wardens and where are my Infernal Machines to create a Hellfire Ring/Hellfire Amulet? In order to obtain a Hellfire Ring or Amulet, one has to work their way through the Uber Boss system of harvesting machines and organs to produce a legendary ring or amulet ().  To create the infernal machine required to open the portals to the Ubers, the keys are drops from mobs nick named Key Wardens.  Where are the various key wardens?  Are there any requirements for making the machines drop? What's the drop rate on different difficulty settings? I wanted to use my android phone as a proxy server, meaning that I wanted to be able to use the IP on the phone as a proxy, so all request and response are received on the phone and forwarded back to the original requestor. How do I do this?