What set of properties does a matrix have to have in order for all its pairs of distinct eigenvectors $x$ and $y$ to have $x^Ty=0?$
Are all eigenvectors, of any matrix, always orthogonal?
I know I can choose "Hidden in menus" and never see the devices again when I install a application. But I don't want my old mobile phones display in my account settings anymore. I have sold those phones already and never use them again!  In fact, the list look likes a cemetery in my mind now. ;( How do I convert a hex string to an int in Python?   I may have it as "0xffff" or just "ffff". Every element of a group has order $2$. Why, intuitively, is it abelian? What is the best technique for long-exposure photography in good light? A word for something that is deeper, more compelling than what is obvious or visible In a Feistel cipher structure, what exactly does the "round function $F$" do? In a Feistel cipher structure, what exactly does the "round function $F$" do to the right half of the data, before taking XOR of the output with the left half of the data? Checking from shell script if a directory contains files From a shell script, how do I check if a directory contains files?  Something similar to this  if [ -e /some/dir/* ]; then echo "huzzah"; fi;   but which works if the directory contains one or several files (the above one only works with exactly 0 or 1 files). What are the C++ coding and file organization guidelines you suggest for people who have to deal with lots of interdependent classes spread over several source and header files?  I have this situation in my project and solving class definition related errors crossing over several header files has become quite a headache. How can I make my touchpad scroll either vertically or horizontally, but not both? If I'm browsing the internet and a page has both horizontal and vertical scroll bars it becomes really irritating to scroll with my touchpad. It keeps scrolling horizontally as well as vertically!  I'm using Ubuntu 16.04  and below are touchpad settings  Device 'SynPS/2 Synaptics TouchPad':     Device Enabled (160):   1     Coordinate Transformation Matrix (162): 1.000000, 0.000000, 0.000000, 0.000000, 1.000000, 0.000000, 0.000000, 0.000000, 1.000000     Device Accel Profile (282): 1     Device Accel Constant Deceleration (283):   2.500000     Device Accel Adaptive Deceleration (284):   1.000000     Device Accel Velocity Scaling (285):    12.500000     Synaptics Edges (286):  1769, 5431, 1652, 4658     Synaptics Finger (287): 25, 30, 0     Synaptics Tap Time (288):   180     Synaptics Tap Move (289):   242     Synaptics Tap Durations (290):  180, 100, 100     Synaptics ClickPad (291):   0     Synaptics Middle Button Timeout (292):  75     Synaptics Two-Finger Pressure (293):    282     Synaptics Two-Finger Width (294):   7     Synaptics Scrolling Distance (295): 200, 200     Synaptics Edge Scrolling (296): 0, 0, 0     Synaptics Two-Finger Scrolling (297):   1, 1     Synaptics Move Speed (298): 1.000000, 1.750000, 0.036300, 0.000000     Synaptics Off (299):    1     Synaptics Locked Drags (300):   0     Synaptics Locked Drags Timeout (301):   5000     Synaptics Tap Action (302): 2, 0, 0, 0, 1, 3, 0     Synaptics Click Action (303):   1, 1, 0     Synaptics Circular Scrolling (304): 0     Synaptics Circular Scrolling Distance (305):    0.100007     Synaptics Circular Scrolling Trigger (306): 0     Synaptics Circular Pad (307):   0     Synaptics Palm Detection (308): 0     Synaptics Palm Dimensions (309):    10, 200     Synaptics Coasting Speed (310): 21.120001, 50.000000     Synaptics Pressure Motion (311):    30, 160     Synaptics Pressure Motion Factor (312): 1.000000, 1.000000     Synaptics Resolution Detect (313):  1     Synaptics Grab Event Device (314):  0     Synaptics Gestures (315):   1     Synaptics Capabilities (316):   1, 0, 1, 1, 1, 1, 1     Synaptics Pad Resolution (317): 83, 46     Synaptics Area (318):   0, 0, 0, 0     Synaptics Noise Cancellation (319): 8, 8     Device Product ID (277):    2, 7     Device Node (278):  "/dev/input/event6" I took a look at  but my question is not the same.  I have unique keys encrypting each plaintext (in CBC mode, AES-256) and I do not use a key to encrypt more than one plaintext. Is it insecure to use null IVs in this case? If it is insecure, why? I want to remotely control my Android device from my PC without rooting it. Is this possible? Collage of six photos, not Facebook Lookback movie After running a VM and logging out, I got a black screen. I powered off and re-started but got the same after the bios option in start up passed. Several attempts went bye with the same result. So I fiddled with bios boot options a bit and was able to get to a different looking Ubuntu start up screen which, after entering pass key would run a series of commands that were too numerous and frantic to make out. They appeared to be overlapped/layered (the text that is) and seemingly repeating endlessly. I had to power down to get it to stop and re-booting just had the same effect. Finally, I hit the down arrow key when prompted for "disk sda5_crypt" at the pseudo Ubuntu log-in screen I mentioned before. This brought me to an all black screen which read something like:  lvmetad not active cannot process volume group "umbuntu-vg" unlock disk sda5_crypt**   From here I entered a password and Ubuntu start up ran.  That was an unsettling hour or so and I'd like to avoid that in the future. Any ideas what caused it and how to avoid it going forward?  Should have said this before but Ubuntu is the only OS on my machine and my optical drive cannot be opened manually. Show that $R/I\otimes_R R/J\cong R/(I+J)$ I know that the finite sum of Bernoulli i.i.d. random variables is a binomial distribution, but what is the distribution of $$\lim_{n \to \infty}\sum_{k=1}^{n} \frac{x_k}{2^k}$$ where $x_k$ is a Bernoulli random variable with parameter $\frac12$? Disable automatic activation of Gnome Shell activities on mouse over Incorrect bookmarks and page number in table of contents I have problem with table of contents and with bookmarks. In pdf file   table of contents shows wrong page number of Introduction and Bibliography, Bookmarks: introduction and bibliography don't bring me to the correct page.   Part of my code  \documentclass[12pt,a4paper]{book}  \usepackage[polish]{babel} \usepackage{polski} \usepackage[cp1250]{inputenc} \usepackage[pdftex,pagebackref=false,draft=false,pdfpagelabels=false,pdfstartview=FitH,pdfstartpage=1,bookmarks=true,pdfauthor={LiN},pdftitle={PD},pdfsubject={Titlei},pdfkeywords={words},unicode=true]{hyperref}  \usepackage{tocloft}  \begin{document} \tableofcontents \addcontentsline{toc}{chapter}{Introduction} \chapter*{Introduction} \chapter{Chapter1} \section{Section} \addcontentsline{toc}{chapter}{Bibliography} \begin{thebibliography}{9999999}% \bibitem{1}Author, \emph{Title of book}, Publisher. \end{thebibliography} \end{document}   How can I change that? Is there a bulletproof way to know a file has been successfully decrypted? I am wondering whether there is a bulletproof way to know if one is successful in decrypting a file.   As an example, say I encrypt a text file and protect it with a 3 letter password. I use brute force to decrypt it, but two different passwords lead to a file that contains English intelligible text. Is there a way for me to know which one is the original one? In other words, is there an infallible way to know I've entered the password correctly? I read that     A typical stream cipher encrypts plaintext one byte at a time, although a stream cipher may be designed to operate on one bit at a time or on units larger than a byte at a time.   (Source: , William Stallings.)  A block cipher encrypts one block at a time. The block may be of size one byte or more or less. That means we can also encrypt a block of one byte by help of a stream cipher as a stream.  So, what exactly is the difference between a stream cipher and a block cipher? I read this book a few years ago which was part of a series, but I only found the first book and now I've forgotten the name.  The first book featured 3 children/teens from different places that end up on a quest to search for magic gems/stones together and the story takes place in a medieval kind fantasy world. One is a girl, while the other 2 are boys. One of the boys actually finds a stone/gem while he is on a school field trip (I think the gem/stone is red in colour), and when he picks it up, he gets transported to the fantasy land but ends up in some sort of military training ground/camp somewhere in a desert type area.  He isn't familiar with the fantasy world. The other 2 characters each start out on their own on a quest to collect some magical gems/stones which once collected, can be used to fight an evil ruler (I guess).  But halfway through the book, the girl and boy meet up and continue the quest together while the third boy(transported one) doesn't know that the stone he picked up is actually one of the magical stones others are looking for.  A few other details I remember:   The stone the boy finds is red in colour I think  The cover of the book had a border depicting the stones  It's part of a series or trilogy    Some books with similar storylines, but not the one I'm looking for:     Deltora Quest The Seven Towers Sword of Heaven series Westeria series Diadem series