How can I prove that the set of all vector spaces doesn't exist? (In other words, if I gather all vector spaces, then it cannot be a set)
According to ,  The collection of all algebraic objects of a given type will usually be a proper class. Examples include the class of all groups, the class of all vector spaces, and many others. In category theory, a category whose collection of objects forms a proper class (or whose collection of morphisms forms a proper class) is called a large category.  I am aware of Russell's Paradox, which explains why not everything is a set, but how can we show the collection of all groups is a proper class?
I deleted an answer which had no upvotes or downvotes, but after some time I saw that my reputation was -2 and the reason is linked to my search engine.    I want to know why this happened and what "linked to my serach engine" means? The meaning of Bit-wise NOT in Python Short Story about interminable rain on Venus Reference sObject Fields dynamically from a string value Is it possible to reference the fields of an sObject (opportunity in this example) dynamically from a string value? For example, take the following code:  for(Opportunity o: oppList) {       for(String s : strList) {            // I'd like to set the opportunity field values based on the string             // value that contains the API name of the opportunity field       }  }  So, the variable strList contains the opportunity API field names. I'm trying to clear some fields on the opportunity in a before update trigger, but I have to access the field names dynamically in the strList. Proving a Composition of Functions is Bijective If I have $f:\mathbb{R}\rightarrow\mathbb{R}$ such that $f$ is onto and $f\circ f\circ f = f$, how can I prove that $f$ is bijective? I know that I only have to prove that it is 1-to-1 because I'm given the fact that it's onto, but how can I use the fact that $f\circ f\circ f=f$ to prove that it is 1-to-1?  Thanks! I used the jQuery Form plugin for asynchronous form submission. For forms that contain files, it copies the form to a hidden iframe, submits it, and copies back the iframe's contents. The problem is that I can't figure out how to find what HTTP status code was returned by the server. For example, if the server returns 404, the data from the iframe will be copied as normal and treated as a regular response.  I've tried poking around in the iframe objects looking for some sort of status_code attribute, but haven't been able to find anything like that.    The $.ajax() function can't be used, because it does not support uploading files. The only way to asynchronously upload files that I know of is using the hidden iframe method. Decision boundaries and Gaussian density functions Order of a product of subgroups. Prove that $o(HK) = \frac{o(H)o(K)}{o(H \cap K)}$. Sometimes I want to learn about a new programming related issue—topics such as DB ORM, search indexer, identity providers, logging, etc.  When I don't know anything about a subject, I ask for benchmarks or comparisons. I got some serious answers like links to tests, pros/cons, and Gartner's reviews. I found these very constructive and helpful. But then the question was blocked or deleted. Sometimes this happens even before I get an answer.  So where would it be more appropriate to ask questions about third party products and/or open source frameworks with which I am completely unfamiliar? Blender Nodes in Editor doesn't show preview Image I have a small problem with my blender lately. I want to make some compositing with my compositing nodes but the problem is, blender won't show me the preview Image. I have a Screenshot for you guys.    The preview Image itself is enabled, but it just won't show.  Thanks for your help in advance!  -Marten Derivative of function with 2 variables I've leart in Calculus 1 that the derivetive of $f(x)$ is:  $$\lim_{h\to0} \frac{f(x+h) - f(x)}{h}$$.  suppose $f(x,y)$ is a function with 2 variables,  does $$f'(x,y) = \lim_{h\to0} \frac{f(x+h, y+h) - f(x,y)}{h}$$ ?  If not , what is the derivetive of function with 2 variables? What is this story about 20 y.o. inexperienced "kids" fighting against a stronger foe in space? What do potential postdoc advisers seek in research statement letters? I am putting together my application for a postdoctoral position in theoretical condensed matter physics. The position is not a fellowship and is a regular postdoc position. If you have gone through the process of applying for such positions a number of times, or, even better, if you have reviewed such research statement letters, what are the main points that are sought/you seek in such letters? Specifically, should I put more emphasize on my PhD research to demonstrate my "depth of knowledge" on projects I have worked on (past research), or dwell more on things that I want/am capable to do (future research)? How do I create a nice-looking DMG for Mac OS X using command-line tools? Way to detect frozen video with ffmpeg? Is there a way to detect frozen video with ffmpeg? I only need to know if there are rather large regions of frozen video, say 10 seconds or more.   I'm familar with the  filter.   Maybe you could first difference adjacent frames, then use blackdetect to tell whether the difference has not changed? I'm not sure how to perform the difference though. Ways to give users some specific education about question quality and topicality Important: This question is being asked here, and not on the Programmers Meta,  because I believe that several SE sites suffer from this same, specific problem.    On Programmers, I see a pattern emerging.   New User sees the "Programmers" site title, and asks their incomplete, "fix my broken code" question. Community members comment that their question is off-topic, and recommend Stack Overflow.  New User says "thanks" and leaves, leaving their undeleted question for the community to clean up.    What seems clear to me is that:   New User has never seen the "What kinds of questions can I ask about here" page, or if they have, they've clicked through it without reading it. New User doesn't know how to move their question to the right place properly  (i.e. post on new site, delete from old site). Community cleanup of such questions is onerous.  The moderators on Programmers, following the principle that the community should moderate itself for the most part, are not proactive about removing such questions unless they're especially egregious. Migration is even more onerous.   OK, so we have the .  Here's what it says about topicality, about halfway down the page:    Note to those who are confused about Programmers' site scope: it's all right there, in black and white.  However, I do notice a problem.  It's circled in red.  That's our fault, the fault of the Programmers community.  What we really meant to say was "Don't ask your code troubleshooting questions here; those belong on Stack Overflow."  Coding tools deserves its own bullet.  We should fix that.  However, what are the chances that the user is actually seeing this, and evaluating whether or not they should ask their question based on this?  So here's my question, in two acts:  ACT I:  Is there a way to highlight the pain points of a particular site to new users specifically, so that we can be very clear that we don't want those questions that are clearly and unambiguously off-topic on our site, before they ask their question?  ACT II: Failing that, is there a way that we can fast-track the removal of such questions so that they no longer pollute our front page? How to check if installing a package will require a reboot? I often install packages and updates for them, although sometimes it is not convenient to do a reboot if one of the packages needs it, so is there any way of telling BEFORE installing the package/update whether or not it will require you to do a reboot? I have already looked  and the solutions there don't solve my problem, nor the more or less same one described there. How can I charge my iPhone without waking up my MacBook Pro? I'd like to create a Windows bootable USB stick, but I don't have a Windows machine with me at the moment to do so. How could I achieve the same using Ubuntu? libsensors5 or libsensors-config upgrade problem