For example, can I set?  gb = cd /media/Dan/evolution   so that everytime I execute gb in bash , I can cd to that particular directory?  I find something online, 'alias' command. But It seems can't do the work above.  Is it possible to do it? How?  # I'm using Ubuntu 14.04
I would like to create an alias to rm command in order to have a confirmation message after executing this command. So I am creating an alias like this alias rm='rm -i'. But as far as I know this is a temporary alias and it lives until you close the terminal.  As it is explained  to save alias permanently I need to execute ~/.bash_aliases or ~/.bashrc commands in terminal and add my alias there. But when I execute ~/.bashrc I get following error message :  bash: /home/bakhtiyor/.bashrc: Permission denied   When I run ~/.bash_aliases I get another error message like this:  bash: /home/bakhtiyor/.bash_aliases: File or directory doesn't exist.   What is the actual problem and how can I solve it?
How to calculate $\int \frac{x^2 }{(x^2+1)^2} dx$? I'm trying  to calculate $\int \frac{x^2 }{(x^2+1)^2} dx$ by using the formula: $$ \int udv = uv -\int vdu $$  I supposed that $u=x$ s.t $du=dx$, and also that $dv=\frac{x}{(x^2+1)^2}dx$, but I couldn't calculate the last integral. what is the tick here?           the answer must be: $ \frac{1}{2}arctan\ x - \frac{x}{2(1+x^2)} $ + C biblatex ieee style same authors Why is a T distribution used for hypothesis testing a linear regression coefficient? In practice, using a standard T-test to check the significance of a linear regression coefficient is common practice. The mechanics of the calculation make sense to me.  Why is it that the T-distribution can be used to model the standard test statistic used in linear regression hypothesis testing? Standard test statistic I am referring to here:   $$ T_{0} = \frac{\widehat{\beta} - \beta_{0}}{SE(\widehat{\beta})} $$ EDIT:  The vector space for the $(\frac{1}{2},0)$ Representation is $\mathbb{C}^2$ as mentioned by Qmechanic in the comments to his answer below! The vector spaces for the other representations remain unanswered.  The definition of a representation is a map (a homomorphism) to the space of linear operators over a vector space. My question is: What are the corresponding vector spaces for the   $(0,0)$ Representation $(\frac{1}{2},0)$ Representation  $(0,\frac{1}{2})$ Representation $(\frac{1}{2},0) \oplus (0,\frac{1}{2}) $ Representation $(\frac{1}{2},\frac{1}{2})$ Representation infinite dimensional Representation? Why was M87 targeted for the Event Horizon Telescope instead of Sagittarius A*? The first image of a black hole has been released today, April 10th, 2019. The team targeted .  Why didn't the team target  at the center of our own galaxy? Intuitively, it would seem to be a better target as it is closer to us. #1115 - Unknown character set: 'utf8mb4' When I Import Mysql from local server to web ser it is showng me error 1115 - Unknown character set: 'utf8mb4' in Creating wordpress wp_comment table CREATE TABLE IF NOT EXISTS  `wp_commentmeta` ( // // ) ENGINE = INNODB DEFAULT CHARSET = utf8mb4 AUTO_INCREMENT =1;  #1115 - Unknown character set: 'utf8mb4'  My mysql version is Version information: 4.0.4, latest stable version: 4.4.10 (local version)  Version information: 4.0.10.7, latest stable version: 4.4.10( server version)  Please help me to solve out the error How confident is my model? Is there a way to know? I often use  to keep my sentences gender neutral. Many of my friends (none of them are native speakers), however, consider this to be grammatically incorrect, and suggest me not to use it in formal English exams.  My questions are:   Is singular they widely accepted in English speaking countries such as America and Britain? Is it formal enough to be used in formal situations, such as a public speech? Is it casual enough to be used in informal situations, such as gossiping among friends? How to prove that eigenvectors from different eigenvalues are linearly independent How can I prove that if I have $n$ eigenvectors from different eigenvalues, they are all linearly independent? How to implement an abstract class in ruby? I know there is no concept of abstract class in ruby. But if at all it needs to be implemented, how to go about it? I tried something like...  class A   def self.new     raise 'Doh! You are trying to write Java in Ruby!'   end end  class B &lt; A   ...   ... end   But when I try to instantiate B, it is internally going to call A.new which is going to raise the exception.  Also, modules cannot be instantiated but they cannot be inherited too. making the new method private will also not work. Any pointers? How to perform hard / cold reboot of iPhone or iPad? I understand that when I press and release the power button on the top of my iPhone or iPad, it stays on but turns off the screen.  Whereas if I press and hold the power button, a "slide to turn off" control appears on the screen.  Sliding this, as I was told, does turn the unit off, but it does so by suspending the RAM to disk (flash memory, actually.) So, when turning on the iPhone or iPad, it isn't so much rebooting the OS as it is resuming it from flash storage.  How to force the OS to completely reboot on these devices? What is Electromotive force (EMF)? How is it related to potential difference? What is  (EMF)? How is it related to the potential difference? Is it created by the potential difference in any conductor? Is it a process? Why is it called force?  Does writing EMF instead of voltage make any difference in AC circuits? (Studying alternating current I found some books using EMF while others used voltage.) using gedit through ssh connection - should I have gedit installed on the server? I wanted to try out using graphical tools over ssh. Just to learn a new thing :)  so my understanding is this, I connect like this: ssh -port -x user@server  But I'm running ubuntu server so it has no gui and thus no gui tools. My understanding is I've to install gedit on server. But also xserver?  Can someone clear up this concepts to me? How to adjust the breaking in the bibliography? Let $A$ and $B$ be $n\times n$ matrices such that $ABAB=0_{n\times n}$.  Can we conclude that  $BABA$ must be  $0_{n\times n}$?   I fail to give a counterexample when $n=2$， so I guess the answer is "yes" Add a note about downvoting on Meta Should I use the first or last bits from a SHA-256 hash? Can someone please give me the prerequisites and mathematics required for string theory? Are there some good references to study it, both online and in a book? Please consider I am a newbie in string theory, and only about up to the level explained in Hawking's Brief History of Time and Penrose's Road to Reality. I would prefer something which does not omit rigor, but builds it up.Also, are there good online lectures on this?  (I have covered most of GR, but I am not really comfortable with advanced Quantum mechanics. I have studied QM only from Griffith's book, and a few McGraw hill lecture notes.) Downloading from the main Ubuntu archive is slow even when it's not release day, how can I get apt-get to automatically use a mirror that is close to me? Any nice, stable ways to keep a window 'Always on top' on the Mac?