Ubuntu server 20.04 no wifi
Broadcom Wireless Drivers unclaimed after installing update 16.04
I recently made the switch to Ubuntu. Everything on my laptop works fine except my hybrid graphics card. It is Intel graphics combined with Nvidia GeForce GT650M. I tried Bumblebee, but primusrun doesn't work (segmentation fault, core dumped) and optirun works with glxspheres64, but steam games look worse with optirun than with Intel graphics (e.g. Surgeon Simulator 2013 runs fine with Intel and hangs a lot with optirun)   I tried Nvidia Prime, but it gives me a black screen on startup after install, so I can't do anything and have to power off my PC with the power button. I also tried nvidia-prime 0.5 on Ubuntu 14.04, and then I get 'low-graphics mode' and I can't get to Unity. Strikeout in math mode How can I strikeout words in math mode (similar to \sout with \usepackage{ulem} in text mode)? I've tried \usepackage{cancel} and \(\cancel{1+1=2}\) but that draws a diagonal line. How do real and effective user IDs work? When a normal user wants to make change to the passwd file, the user will by setuid be given the effective user access. User becomes root temporarily and can edit passwd.  However you can only edit your password right, and not everybody else? However your effective user access is root. So how come you're not allowed to change other passwords beside yours?  When you run a program with setuid, what does it mean actually when effective user is root, but real user id is still your name? Allow users to put their e-mail in posts? Should I edit out the (thankfully obsfucated) e-mail address from this ?  Is this something we should allow on the site going forward?  I don't care about the "thank you" or "hello", but this might lead to an answer being sent by e-mail, which would mean it wouldn't be peer reviewed (for what that's worth), and the answer would be lost to the community. Raised index of partial derivative I am having a really hard time wrapping my head around component notation for tensor fields. For example, I do not know exactly what the following expression means $$\partial_\mu\partial^\nu \phi, \tag{$\#$}$$ where $\phi$ is a scalar field. On the one hand $\partial^\nu=g^{\lambda\nu}\partial_\lambda$ where $g_{\mu\nu}$ is the Minkowski metric, and hence we could write explicitly $$\partial_\mu\partial^\nu \phi=\sum_{\mu,\nu,\lambda}g^{\lambda\nu}\partial_\mu \partial_{\lambda}\phi=\sum_{\mu,\nu}\partial_{\mu}\partial_{\nu}\phi=\partial_\mu\partial_\nu\phi. \tag{$*$} $$ On the other hand, we may think of $\partial_\mu\partial^\nu=g(\partial_\mu,\partial^\nu)=\delta_\mu^\nu;$ so that $\partial_\mu\partial^\nu\phi=\phi?$ Maybe? I am actually not sure of what this would mean. I am really confused. Any help is appreciated. Edit: To give context of where this expression comes from: I was computing the Lagrangian $$\mathcal L=\frac{1}{2}(\partial_\mu\phi)(\partial^\mu \phi) $$ considering an infinitesimal spacetime translation $x^ \mu\to x^\mu-\alpha a^\mu$. The scalar field thus transforms like $\phi(x)\to \phi(x)+\alpha(\partial_\mu\phi(x))a^\mu.$ Plugging thins into the Lagrangian yields the term I am referring to. Edit 2: The change in placement of indices are actually my doubts. I try to elaborate. I do not have any background in using indices to talk about tensors. I am used to interpret the expressions $\partial_\mu$ as the local vector field defined in some chart (local coordinates). I think about vector fields $X$ as abstract section of the Tangent bundle, which restricted to local coordinates can be expressed as $X=X^\mu\partial_\mu$. In the context of QFT, as far as I understand, the symbol $\partial_\mu$ denotes  $(\partial_t,\nabla)$ in the local coordinates $(t,x,y,z)$. So that $\partial_\mu\phi=(\partial_t \phi,\partial_x \phi,\partial_y\phi,\partial_z\phi)$. This was supposed to be my justification on why I wrote the summation on $\mu$ and $\nu$ in $(*)$, but now I note that this only applies when $\mu$ or $\nu$ appear twice, indicating the scalar product; which leads me to the last remark. I think of $g_{\mu \nu}$ as the component of the matrix $$g=\begin{pmatrix} 1&amp;0&amp;0&amp;0\\ 0&amp;-1&amp;0&amp;0\\ 0&amp;0&amp;-1&amp;0\\ 0&amp;0&amp;0&amp;-1\\ \end{pmatrix}$$ which represents the pseudo-Riemmanian metric, which by definition acts on tangent vectors, i.e. linear combinations of the $\partial_\mu$ applied to a point. This is where my doubt comes, in which was the right way to interpret the notation; in particular what is the expresion $(\#)$ in explicit coordinates? using a class defined in a c++ dll in c# code I have a dll that was written in c++, I need to use this dll in my c# code. After searching I found that using P/Invoke would give me access to the function I need, but these functions are defined with in a class and use non-static private member variables. So I need to be able to create an instance of this class to properly use the functions. How can I gain access to this class so that I can create an instance? I have been unable to find a way to do this.   I guess I should note that the c++ dll is not my code. When and where do I have the highest chance of seeing auroras? I'm wondering what time of year is best, and what locations might be best, for catching the aurora.  I'm currently thinking of February up at Sweden's ice hotel, but since the whole point of the trip is so that my wife can see the Aurora, I want to pick a location that affords the best chances to see one.  Edit: Just adding a link so you can see realtime aurora activity in the northern hemisphere    I know this is anecdotal, but I ended up going to Iceland in late february during the new moon and saw the aurora 3 out of 6 nights I was there.  There was activity every single night but the biggest problem was weather.  It was cloudy almost every single night. How can I get a reference to a method? Is it possible in Ruby to get a reference to methods of an object ( I would like to know if this can be done without procs/lambdas ) , for example , consider the following code :   class X   def initialize     @map = {}     setup_map   end    private   def setup_map     # @map["a"] = get reference to a method     # @map["b"] = get reference to b method     # @map["c"] = get referebce to c method   end    public   def call(a)     @map["a"](a) if a > 10     @map["b"](a) if a > 20     @map["c"](a) if a > 30   end    def a(arg)      puts "a was called with #{arg}"   end    def b(arg)      puts "b was called with #{arg}"   end    def c(arg)     puts "c was called with #{arg}"   end end   Is it possible to do such thing ? I would like to avoid procs/lambdas because I want to be able to change the behaviour of A,B,C by subclassing . JQuery Selector for dynamic id What's the regular expression that matches a square bracket? Is it possible to animate which characters of a text object are assigned to a material? I simply know that a single free quark does not exist. What is the reason that we can not get a free quark?  If we can't get a free quark then what is single-top-quark? How to turn cheats on or off How does Windows know whether it has internet access or if a Wi-Fi connection requires in-browser authentication? is a simple way to script small GUIs.  Can anyone give a nice example with a button and a text widget. When the button is pressed should a shell command be executed and the output piped to the text widget.  If you have other nice and clean examples for useful tasks, please add them too. How can we disable triggers, Validation Rules and workflow rules in salesforce org. [on off - switch] How can we disable triggers, Validation Rules and workflow rules in salesforce org.  This becomes really needed and important when we are doing data migration(data imports) in salesforce.  The way I handled this is create a custom label and include it in workflow and validation rule logic.   example - Custom label 'Disable workflow' = True then do not fire workflows.            if its False then fire workflow  But wanted to understand if there is any more efficient way to handle these scenarios. How to unlock gungeoneers I've seen on a wiki that there is a ghost gungeoneer and a robot gungeoneer but it didn't say how to unlock them, so how do I unlock more gungeoneers ? How do I copy to the clipboard in JavaScript? Hypernym for "audiobook", "podcast", "speech", etc.? How can I maximize the "blurry background, sharp subject" (bokeh) effect? I know that this effect occurs when there's a shallow depth of field.  My question is, what are the various ways I can increase this effect in my photos when I'm taking them?    Note that I'm not asking  how to use an editing program to achieve this effect after in post production or after the image is captured.