my ubuntu is 17.04 and now I can't upgrade because of this error
Recently I have installed an older version of Ubuntu on my old machine. Whenever I try to install any software, I get an error saying it couldn't be found:  $ sudo apt-get install vlc Reading package lists... Done                Building dependency tree        Reading state information... Done     E: Couldn't find package vlc
Nilpotent matrix and basis for $F^n$ Explode string into array with no empty elements? PHP's explode function returns an array of strings split on some provided substring. It will return empty strings when there are leading, trailing, or consecutive delimiters, like this: var_dump(explode('/', '1/2//3/')); array(5) {   [0]=&gt;   string(1) &quot;1&quot;   [1]=&gt;   string(1) &quot;2&quot;   [2]=&gt;   string(0) &quot;&quot;   [3]=&gt;   string(1) &quot;3&quot;   [4]=&gt;   string(0) &quot;&quot; }  Is there some different function or option or anything that would return everything except the empty strings? var_dump(different_explode('/', '1/2//3/')); array(3) {   [0]=&gt;   string(1) &quot;1&quot;   [1]=&gt;   string(1) &quot;2&quot;   [2]=&gt;   string(1) &quot;3&quot; } prove/disprove Big-Oh I have the following function f(n), and I want to prove/disprove that it is O(n$^2$): \begin{cases}     4[sum(n/2,n)],&amp; \text{if n is even } \\     2n-1+sum(n-3,n),              &amp; \text{otherwise} \end{cases} (it should be 2 n-1, so a space between 2 and n) where sum( j,k ) is a ‘partial arithmetic sum’ of the integers from j up to k, that is \begin{cases}     0&amp; \text{if j&gt;k } \\     j + (j+1) + (j+2) + ... + k,              &amp; \text{otherwise} \end{cases} e.g. sum(3,4) = 3 + 4 = 7, etc. Note that sum(j,k) = sum(1,k) – sum(1,j-1) How do I prove or disprove that this is O(n$^2$)? When connected to the Internet, the system checks if it's in a walled garden, which is simply trying to connect to Google. If it fails, in Android 5, there'll be an exclamation mark on the WiFi icon or signal icon.   In some situations, this is annoying. Say, in China, Google is blocked, the exclamation mark will always be there, which is meaningless.  How can I disable this check, and make the system assume, connected WiFi/mobile data is connected Internet? How are we able to calculate specific numbers in the Fibonacci Sequence? I was reading up on the Fibonacci Sequence,  $\text {{1,1,2,3,5,8,13,....}}$ when I've noticed some were able to calculate specific numbers. So far I've only figured out creating an array and counting to the value, which is incredibly simple, but I reckon I can't find any formula for calculating a Fibonacci number based on it's position.  Is there a way to do this? If so, how are we able to apply these formulas to arrays? How to create meaningful relationships with NPCs How do I help the PCs to create meaningful relationships with NPCs?   Maybe it is a result of my group playing too many one-dimensional adventures trying to find whatever MacGuffin they are after each week, but most of the time my PCs don't tend to value NPCs too much. Every time they meet an NPC they quickly evaluate what immediate values the NPC can give or tend to ignore them.  What I would really like is to have some good relationships where the PCs actually go out of their way to help NPCs without selfish motivation or even just to try to think of NPCs more and how their actions affect others.  I am not looking for a specific game rules solution to this, but instead techniques to get the players to feel attachment to these fictional characters. The equals() method in Java works unexpectedly on Long data type According to preliminary estimates, the project should be completed by 2020 at a cost of about RUB 40 bn.    or      According to preliminary estimates, the project should be completed by 2020 at the cost of about RUB 40 bn.    Are both options okay, or is the first better?   We are introducing the sum to the reader, so logically it should be the first option. But there's a lot of google-hits for "at the cost of" too. Running Synaptic I get the following error message:  E: The package hl1440lpr needs to be reinstalled, but I can't find an archive for it. E: Internal error opening cache (1). Please report.   Upon accepting the message, Synaptic quits. maximum subarray of an array with integers How do you do a deep copy of an object in .NET? Beam size and material for 15 feet long swing I need to create a 15 feet span beam for kid swing (300 lbs load / 150 lbs and 2 swings assumption). One end will bolt to the tree (just for convenience).  What "treated lumber" lumber size and material I should use? Or is it better / cheaper to use metal pipe (from the plumbing department)?  I guess 4x8x16 or two (2) 2x8x16 would do it according to this website . But I do like a clean look of metal. My wife would concern about big piece of lumber running on top of the kid (that looks heavy visually). What, if anything should one do if a user's "about me" profile section contains extremely inflammatory terms (racist, etc.) I don't think anyone wants to be the thought police, and I'm particularly wary of censorship on a part of the site that is a user's personal self-statement, and is a little off the beaten path.  On the other hand, I'm also concerned that we may not want a site where clicking on a user to see their other posts would expose folks to racist or other bigoted terms that almost everyone would agree are inappropriate for civil discourse.    Don't forget the "be nice" rule".  Note that I don't want a "flag user" rule, which I know has been beaten to death already, and further think that swears and semi-inflammatory opinions, etc. should generally just be ignored - I'm only asking about terms that provoke serious emotions around racial, religious, ethnic, or sexual orientation issues.  What, if anything, is the appropriate response in such cases? I am trying to execute a web service call to our backend system when the opportunity is closed/won. I am attempting to do a  POST to our endpoint. All the api call is really carrying the account name and a few structural detail back (All listed below). My problem is that when the trigger executes I get the following error in the debug logs "System.CalloutException: You have uncommitted work pending. Please commit or rollback before calling out".  The Process Builder Passing in the company name:   APEX Class  public class Play2 { @TestVisible static HttpResponse res;  @InvocableMethod public static void sendNotification(List&lt;String&gt; name) {        HttpRequest req = new HttpRequest();     Http http = new Http();     String payload = JSON.serialize(         new Map&lt;String, String&gt; {             'companyName' =&gt; name.get(0),             'salesforceAccountType' =&gt; 'Company',             'salesforceSynchStatus' =&gt; 'Enabled'         }     );      req.setEndpoint('https://TheEndPoint');     req.setMethod('POST');     req.setHeader('Authorization', 'The Mighty API KEY');     req.setHeader('Content-Type', 'application/json');      req.setBody( payload );     req.setCompressed(true);       try {         System.debug('Point 1');         res = http.send(req);         System.debug('Point 2');     } catch(System.CalloutException e) {         System.debug('Callout error: '+ e);         System.debug(res.toString());     } } } I'm using Ubuntu 14.04 but since some weeks I'm stuck with the old 13.10 lockscreen design for some reason.  I already tried compizconfig-settings-manager but it doesn't offer me any option for changing the lockscreen design and even resetting Compiz and Unity with   dconf reset -f /org/compiz/   didn't help.  Are there any other ways to get the new lockscreen back? Hydrostatic pressure? What what I understand, hydrostatic pressure is the "weight" of the water pushing against objects. But if this is true, why is hydrostatic pressure perpendicular to the surface it acts on instead of always going down?  For example, if you placed a book on a desk, the book's weight would push against the desk, but gravity is pulling it "down". But if you put another book beside this book the first book wouldn't apply any force on it. The weight exerted from the book always goes down.  Similarly, why doesn't hydrostatic pressure always go down? I understand that if you put a plate on the seabed, the weight of the water would push down on it, but if you just had a vertical plate standing on the bed, why would force push it from the sides?  To sum up, why does hydrostatic pressure act perpendicular to the object instead of always down? Functions which are Continuous, but not Bicontinuous What are some examples of functions which are continuous, but whose inverse is not continuous?  nb: I changed the question after a few comments, so some of the below no longer make sense. Sorry. How can I detect inside a TeX file which engine (pdftex/xetex/luatex) is used to compile the file? Is there a LaTeX package that provides this functionality? I need to print an image at a certain size. What dimensions and resolution should I use? Is there a way to stream Netflix content to my Ubuntu machine?