You can't go faster than light, and light can't be additive (if you shine a light from a spaceship, the light is not going $c$+"speed of spaceship", it's just going  like it always does).  But what happens when two spaceships, each going 75% the speed of light, are traveling directly towards each other?  Aren't the two spaceships now traveling, in reference to each other, at 150% the speed of light?
Let's say we have $2$ particles facing each other and each traveling (almost) at speed of light.    Let's say I'm sitting on #$1$ particle so in my point of view #$2$ particle's speed is (almost) $c+c=2c$, double light speed? Please say why I am incorrect :)    EDIT: About sitting me is just example, so in point of view of #1 particle, the second one moves at (almost) $c+c=2c$ speed?
How do I connect Android devices to a WiFi AP and mobile data network simultaneously? How do I know whether to use the definite (the) or indefinite (a, an) article, and when to omit it altogether? underline omitting the descenders I'm very well aware that it is considered best practice not to use underlining other than on typewriters and in handwriting. But sometimes it is needed nonetheless.  In my opinion the typographically least offending way to underline in print is to leave out the descenders. This is also how I learned to do it in handwriting. Very few fonts offer a pre-made underlined variant, but some text-processors (e.g. OS X text-engine) do -- depending on the font used -- an acceptable job at emulating this effect:    How would I tweak any of the existing ways/packages for underlining to do the same?  As a bonus, I'd like to be able to tweak the behaviour (extent of the omitted part, apply to which letters) per font and text style, preferably working with XeLaTeX. Can "Thanks for your edit!" banner be linkified with the Suggested Edit Link? How do I run a sudo command needing password input in the background? I have recently disabled  so that it now prompts me for a password every time.  And though this is good for security, it has caused one slight problem which I haven't been able to figure out a solution for, I am unable to run commands which go along the lines of:   sudo &lt;command&gt; &amp;   In the past I would have run a sudo command before that, it would have cached my password and allowed me to run sudo commands without prompt for the next few minutes, and thus it would allow me to run the command.  But when I run it now as there is no caching before hand and as it starts a new thread immediately and sudo doesn't even prompt me for a password, I am unable to run it in this way.  So unless I run sudo -i before it I am unable to run a command in this format which is becoming rather annoying. So I was wondering if there is any way to get around this and still run programs and commands in this way?  I am running Ubuntu GNOME 15.10 with GNOME 3.18, and specifically the program I want to run in this fashion is etherape if that makes any difference, but I would really like the solution to work for all programs and commands. Selecting the size of a System.Drawing.Icon? Consider a string under tension, for example, a string on a guitar. When a guitar string is plucked, it vibrates at a certain frequency. When the tension on the string is increased by twisting the tuning peg at the end of the neck, the frequency of the string increases. According to , increasing the tension on a string also increases the speed of the wave moving through it. According to  the speed of a wave is higher in a more dense medium.  Does this mean that increasing the tension also increases the density?  This does not seem very logical to me, since density is the "compactness" of a substance, or how close together the individual molecules are. By twisting the tuning peg on a guitar, you rotate the cylinder that the string is fixed to, wrapping it around more tightly. This pulls more of the string around that cylinder, similarly to when you reel in a fishing rod. Since more of the string's molecules are now wrapped around the peg, there should be fewer molecules over the same distance between the bridge and the nut (the two ends of the vibrating portion of the string), therefore bringing the density down. Why does the division algorithm work for converting between number bases? Why is there a difference between urls submitted to a sitemap and urls in the google index? How can I improve Ubuntu overall system performance? What are your tips for improving overall system performance on ubuntu? Inspired by this  I realized that some default settings may be rather conservative on Ubuntu and that it's possible to tweak it with little or no risk if you wish to make it faster.  This is not meant to be application specific (e.g. make firefox load pages faster), but system wide.  Preferably 1 tip per answer, with enough detail for people to implement it.  A couple of mine would be:   Install  (via Software Center or sudo apt-get install preload); Change  value - "which controls the degree to which the kernel prefers to swap when it tries to free memory";   What are yours?  PS: Since this is not intended to have a unique answer but rather, several useful tips, I'm making this community wiki out-of-the-box. 8 GB RAM installed, only 7.5 GB usable. Windows 7 64-bit I just upgraded my computer's RAM, adding another 4 GB to the computer.  Windows 7 acknowledges that 8 GB is installed, but says only 7.5 GB is accessible.  Any idea why? I thought since I had the 64-bit Professional version this wouldn't be an issue. I want to find the $nth$ derivatives of the function $e^{f(x)}$ with respect to $x$, the first derivative is $$e^{f(x)}f^{\prime}(x).$$  The second derivative is $$\left( {\frac {d^{2}}{d{x}^{2}}}f \left( x \right)  \right) {{\rm e}^ {f \left( x \right) }}+ \left( {\frac {d}{dx}}f \left( x \right)   \right) ^{2}{{\rm e}^{f \left( x \right) }} .$$ The third derivative is $$\left( {\frac {d^{3}}{d{x}^{3}}}f \left( x \right)  \right) {{\rm e}^ {f \left( x \right) }}+3\, \left( {\frac {d^{2}}{d{x}^{2}}}f \left( x  \right)  \right)  \left( {\frac {d}{dx}}f \left( x \right)  \right) { {\rm e}^{f \left( x \right) }}+ \left( {\frac {d}{dx}}f \left( x  \right)  \right) ^{3}{{\rm e}^{f \left( x \right) }} $$  The fourth derivative is $$ \left( {\frac {d^{4}}{d{x}^{4}}}f \left( x \right)  \right) {{\rm e}^ {f \left( x \right) }}+4\, \left( {\frac {d^{3}}{d{x}^{3}}}f \left( x  \right)  \right)  \left( {\frac {d}{dx}}f \left( x \right)  \right) { {\rm e}^{f \left( x \right) }}+3\, \left( {\frac {d^{2}}{d{x}^{2}}}f  \left( x \right)  \right) ^{2}{{\rm e}^{f \left( x \right) }}+6\,  \left( {\frac {d^{2}}{d{x}^{2}}}f \left( x \right)  \right)  \left( { \frac {d}{dx}}f \left( x \right)  \right) ^{2}{{\rm e}^{f \left( x  \right) }}+ \left( {\frac {d}{dx}}f \left( x \right)  \right) ^{4}{ {\rm e}^{f \left( x \right) }} $$  My question is: Is there a general formula or a pattern for the nth derivative of $e^{f(x)}$. You may use the maple command diff(exp(f(x)), x$5) to do some experiments. Thanks a lot ^^ Immigration check between Northern Ireland/Republic of Ireland and Great Britain Do immigration checks exist between Northern Ireland and England/Scotland? (not just for flights but also ferry/bus/train) Do immigration checks exist between the Republic of Ireland and England/Scotland? Redirect console output to textbox in separate program I'm developing an Windows Forms application that requires me to call a separate program to perform a task.  The program is a console application and I need to redirect standard output from the console to a TextBox in my program.    I have no problem executing the program from my application, but I don't know how to redirect the output to my application.  I need to capture output while the program is running using events.    The console program isn't meant to stop running until my application stops and the text changes constantly at random intervals.  What I'm attempting to do is simply hook output from the console to trigger an event handler which can then be used to update the TextBox.  I am using C# to code the program and using the .NET framework for development.  The original application is not a .NET program.  EDIT: Here's example code of what I'm trying to do.  In my final app, I'll replace Console.WriteLine with code to update the TextBox.  I tried to set a breakpoint in my event handler, and it isn't even reached.      void Method()     {         var p = new Process();         var path = @"C:\ConsoleApp.exe";          p.StartInfo.FileName = path;         p.StartInfo.UseShellExecute = false;         p.OutputDataReceived += p_OutputDataReceived;          p.Start();     }      static void p_OutputDataReceived(object sender, DataReceivedEventArgs e)     {         Console.WriteLine("&gt;&gt;&gt; {0}", e.Data);     } Example where $f\circ g$ is bijective, but neither $f$ nor $g$ is bijective Can anyone come up with an explicit example of two functions $f$ and $g$ such that: $f\circ g$ is bijective, but neither $f$ nor $g$ is bijective?  I tried the following: $$f:\mathbb{R}\rightarrow \mathbb{R^{+}} $$ $$f(x)=x^{2}$$  and  $$g:\mathbb{R^{+}}\rightarrow \mathbb{R}$$ $$g(x)=\sqrt{x}$$  $f$ is not injective, and $g$ is not surjective, but $f\circ g$ is bijective  Any other examples? Scala 2.10 + Json serialization and deserialization I lock my computer a lot, and was wondering if there was a way to get the lock screen when I hit Ctrl + Alt + L. Right now when I use that key combination it turns the screen black, and I have to hit a key or click to bring up the screen where it shows the option to enter your password or switch users, as well as displaying the desktop background behind it.   Is there anyway to avoid having to hit that extra key, to bring up the screen I want? And is there anyway to keep it that way? It's really a matter of personal preference, and I don't see any reason why not to have it stay that way as my monitor eventually is turned off and/or my computer suspended. How to disable grub menu at boot time in Ubuntu 20.04? I have installed Ubuntu 20.04 alongside Windows. When I boot the system, it shows the grub menu for 10 seconds to choose between Ubuntu and Windows. I don't want that menu, just want to boot it up right after pressing the power button. I have looked into  thread and added GRUB_HIDDEN_TIMEOUT_QUIET=true  and even changed GRUB_TIMEOUT to 0 and then ran sudo update-grub. But it didn't work, menu is still being shown at boot time. This is my /etc/default/grub file-   # If you change this file, run 'update-grub' afterwards to update # /boot/grub/grub.cfg. # For full documentation of the options in this file, see: #   info -f grub -n 'Simple configuration'  GRUB_DEFAULT=0 GRUB_TIMEOUT_STYLE=hidden GRUB_HIDDEN_TIMEOUT_QUIET=true GRUB_TIMEOUT=0 GRUB_DISTRIBUTOR=`lsb_release -i -s 2&gt; /dev/null || echo Debian` GRUB_CMDLINE_LINUX_DEFAULT="quiet splash" GRUB_CMDLINE_LINUX=""  # Uncomment to enable BadRAM filtering, modify to suit your needs # This works with Linux (no patch required) and with any kernel that obtains # the memory map information from GRUB (GNU Mach, kernel of FreeBSD ...) #GRUB_BADRAM="0x01234567,0xfefefefe,0x89abcdef,0xefefefef"  # Uncomment to disable graphical terminal (grub-pc only) #GRUB_TERMINAL=console  # The resolution used on graphical terminal # note that you can use only modes which your graphic card supports via VBE # you can see them in real GRUB with the command `vbeinfo' #GRUB_GFXMODE=640x480  # Uncomment if you don't want GRUB to pass "root=UUID=xxx" parameter to Linux #GRUB_DISABLE_LINUX_UUID=true  # Uncomment to disable generation of recovery mode menu entries #GRUB_DISABLE_RECOVERY="true"  # Uncomment to get a beep at grub start #GRUB_INIT_TUNE="480 440 1"   How do I disable it? Preventing robots from crawling specific part of a page Limit of a stochastic integral