Let $A$ be an $n \times n$ matrix. If $A$ is invertible, then $|A| \neq 0$?
How is the determinant related to the inverse of matrix?
Will I still receive OTA updates after rooting my Nexus 5? How do I set OS X to boot by default? I set Windows 7 to my Boot Camp. Windows 7 starts now by default in the normal boot. However, I want that OSX boots by default and Windows 7 only as I press ALT and then select it.  How can you set OSX to boot by default? Irreducibility check for polynomials not satisfying Eisenstein Criterion. Why does universal generalization work? (the rule of inference) SQL Index Performance - ASC vs DESC How can I distinguish between words which have the -ing in a sentence that are nouns, verbs, or adjectives?    For example sometimes -ing come with word to give us a noun, and sometimes a verb or an adjective.       That interesting snake.   That crawling snake.   That barking dog keeps everyone awake.     Is there any rule to determine whether a word with -ing is a noun, verb, or adjective? Please help me to stop auto login in Skype. There is an account that is always login again and again, although I chose to logout and sign in with another Skype name. I tried on Settings/applications/Skype to kill the account and to quit synchronise, but still whenever I open Skype again, I'm logging in automatically. Let $T$ be a linear transformation $T:\mathbb{R}^{n}\rightarrow\mathbb{R}^{n}$ such that $T^{2}=\lambda T$ for some $\lambda\in\mathbb{R}.$ Then which of the following is/are true?  $1.\|T(x)\|=|\lambda|\|x\|$ for all $x\in\mathbb{R}^n.$  $2.$ If $\|T(x)\|=\|x\|$ for some non-zero $x\in\mathbb{R}^{n}$, then $\lambda=\pm 1.$  $3.T=\lambda I$ where $I$ is the identity transformation on $\mathbb{R}^{n}.$  $4.$ If $\|T(x)\|&gt;\|x\|$ for a nonzero vector $x\in\mathbb{R}^{n},$ then $T$ is singular.  According to me since $T^{2}=\lambda T$ we have $T$ satisfies the polynomial $x^{2}-\lambda x.$ So choices for its minimal polynomial are $x,x-\lambda,x(x-\lambda).$ So $3$rd option is clearly incorrect and $4$th option is not satisfied if $T=I$ and hence correct one. I am confused in first two options . Please help me. Thanks in advance. Reprojecting raster from lat/lon to UTM in R? I would like to add texts with halo, in the composer (not in the map). Do you know a way to do it?  I'm using QGIS 2.0.1 Accessing bash command line args $@ vs $* "Email" or "e-mail"? Which way of writing the word: "Email" or "e-mail" is correct? Both variants seem to be in wide use. If both ones are okay, maybe there is a difference in contexts they have been used (one is more formal than the other)? Is it possible to transfer Minecraft PE worlds to Minecraft PC? I have a really cool Minecraft PE world, and I would like to also have it on my Computer. Is there a way to move a Minecraft PE world onto the PC Edition?  My tablet is Android and my Computer is Windows 7. Thanks! Order of finite fields is $p^n$ Let $F$ be a finite field. How do I prove that the order of $F$ is always of order $p^n$ where $p$ is prime? One Time Pad Key Distribution For multiple OTP keys, why not do this:  Key = Hash[(EC)DH Shared Secret + Counter]  The counter will increase per message, and the output of the hash will be random for each new unique input into it. What are the problems or weaknesses with this design??  EDIT: For clarification: both parties will have the shared secret from an ECDH exchange, random numbers from Pythons os.urandom for generating the ECC key How to use notify-send to notify users as root? #!/bin/bash export DISPLAY=:0 state=$(upower -i $(upower -e | grep BAT) | grep --color=never -E "state" ) stat=${state:25:32} batperct=$(upower -i $(upower -e | grep '/battery') | grep  -E "percentage:") bat=${batperct:24:26} intbat=$(echo $bat | cut -d'%' -f1) if [ "$intbat" -lt "15" -a "$stat" == "discharging" ]; then     notify-send 'battery low..! please plugin charger..... charge is only' "$intbat"     fi     The script runs fine when run as a normal user (bash script.sh). The desktop notification is displayed.  If I run it as root with sudo bash script.sh, the desktop notification is not displayed.  How can I use notify-send to notify users as root?  I am using Arch Linux. When you pass a collection like list, array to another function in python, does it make a copy of it, or is it just a pointer? Today I've suddenly received spam calendar invitation for $19.99 Ray Ban sunglasses (never subscribed, checked, searched or visited any related site, so clearly spam) and it has other visible email accounts within the invitation. I'm wondering if I should be worried whether my accounts been compromised or a simple email from anywhere just plops this in my calendar without review. Questions to be answered:  What should I check/setup in order to make sure my iCloud account is safe? How can I delete this invitation without sending a notification to the sender, so as to prevent them from knowing my email address is live? How can I prevent these notifications in the future?  Here's an image: Store pictures as files or in the database for a web app? My question is fairly generic and I know there might not be an 100% answer to it. I'm building an ASP .NET web solution that will include a lot of pictures and hopefully a fair amount of traffic. I do really want to achieve performance.  Should I save the pictures in the Database or on the File system? And regardless the answer I'm more interested in why choosing a specific way.  Many thanks, Stefan   DUPLICATE: , ,  and probably some others.    COMMENT: Thanks for many good answers. I will go for a file based solution even if I like the idea of having a 100% database driven solution. It seems that there are today good solutions to do what I want with databases etc but I have a few reasons for not doing it.   I will be on a hosted solution, I have huge amount of storage(10gb) but only 300mb for the database. It will cost a lot for extra storage in the DB. I'm not an DB expert and as well not in control of settings of the DB. A DB based solution might need custom configuration as it looks like.   If we will move to run the site on our own server I might consider a DB based solution. thanks, Stefan Using PyQGIS to transform layer of polygons into layer of holes inside bbox?