The purpose of this thread is to help focus the attention of the community on posts that may require exceptional handling. This includes requests for reopen and undeletion votes. A request should be posted as an answer below.

Please do not use this thread to engage in debates on contentious matters (e.g. reasons for closure). That should be done in a separate linked thread. The goal is to keep this meta thread free of tension, so that everyone feels comfortable posting here. Please be polite, and respect the many different viewpoints in our diverse community.

To inform readers of the current (and past) states of the targeted post, please append tags such [REOPENED,RECLOSED] or [UNDELETED] at the start of the answer.

Beware that "short" requests such as "request reopening of <link>" may be automatically converted to comments by the SE software, so you may need to write more (e.g. why you think that the question should be reopened or undeleted).

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@Bill: I saw no reason to wait with that idea. There was no actual discussion in this thread anyway. –  Asaf Karagila Oct 24 '12 at 18:34
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@Gerry: The solution would be to add a few words, I suppose. For example why it should be reopened. –  Asaf Karagila Nov 9 '12 at 12:16
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@Asaf, I opted for cursing the darkness rather than lighting a candle. Anyway, the question has been reopened. –  Gerry Myerson Nov 9 '12 at 21:54
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@Gerry: Darkness is just the light's way of proving the empty set exists. –  Asaf Karagila Nov 9 '12 at 21:55
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@Willie: I think that we should delete old reopening requests and perhaps have one post/additional thread for indexing them. I should also think that any request older than $n$ days for some reasonable $n$ should be deleted. If something has not been reopened and the initial votes expired... well, it makes sense to conclude that there aren't that many people interested in reopening. –  Asaf Karagila Nov 30 '12 at 9:49
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@Willie It's probably useful to have some nontrivial history remain so that folks $\rm < 10K$ can gain some idea about what types of questions do get reopened, and what types don't. By quickly scanning the requests it might help to convey some idea of the community consensus on marginal topics. To keep the unopened requests at the top of the active sort, they could easily be bumped if there is still interest. –  Bill Dubuque Dec 1 '12 at 1:44
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@Belgi: This is why I prefer to browse Meta with answers sorted by activity. –  Rahul Jan 5 '13 at 21:38
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@Belgi Sorting by activity solves the problem. I just bumped the only active discussion to the top with an edit. There are two requests dated by November 2012, which I guess are no longer ongoing conversations (but anyone so inclined can bump them; it's a CW). –  user53153 Jan 5 '13 at 22:07
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Thanks for the workaround, I still think theres no reasons for this to log all reopen request that were/will be made –  Belgi Jan 5 '13 at 22:10
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Per the (short) discussion in this deleted meta post, I have edited this thread to support requests for votes to undelete as well as votes to reopen. –  Alexander Gruber Jul 13 at 14:59
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@quid: Yes, yes, I see that protection triggering every time I remove it. But I gave it some more thought, and for now it seems harmless after all. –  Asaf Karagila Oct 17 at 14:06

190 Answers 190

I would like to request Motivation for the Tensor Product be reopened as I do not think it is a duplicate of and Motivation for Tensor Product.

The latter question asks "We already have Direct Product, Semi-direct products, so after all why do we need Tensor Product?", which is a question about why multilinearity is in general useful.

The former question (which was closed as a duplicate) asks "What's the reason/motivation to define the tensor product using the free vector space and that quotient to impose linearity? Can someone point me out the motivation for that definition?". These are questions about the specific construction of the tensor prodct, and NOT the general usefulness of multilinearity properties.

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I think one thing that would help is if the two questions are edited to they have more precise and more distinct (from each other) titles. –  Willie Wong Mar 18 '13 at 12:16

[REOPENED]

(Also, as pointed out by Asaf in a comment below, this was a duplicate request. My apologies for the inadvertent dupliaction.)


The question "If every convergent subsequence converges to a then so does the original sequence" should be reopened (in my opinion).

It is a request for clarification/explanation of an argument from a text.

It was closed as off-topic since it is essentially a repost of an earlier, deleted question. The thing is: this earlier question was deleted by the Community User, hence cannot be reopened by 10K+ voters. My view is that the earlier question was reasonable as well; if it could be reopened, then I agree that the current version should be closed as a duplicate, and the earlier one revised for clarity and reopened. Since that doesn't seem to be possible, I have voted to reopen the current question, and I would ask others to do likewise.


In general, it seems a bit unreasonable to me that the OP has been put in a Catch-22, in which their current question is being closed essentially as a duplicate of an earlier question which is unable to be undeleted/reopened.

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@AsafKaragila: Dear Asaf, Thanks, I hadn't seen this. (I somehow missed it when looking down the list.) I'll try to sort out the duplication of posts soon (although I have to rush out right now). Regards, –  Matt E May 5 at 11:31

[UNDELETED]

Request for undeletion, or explanation of deletion:

http://math.stackexchange.com/a/898051/, a "hint" answer that does substantial work toward solving the differential equation in the question Differential equation $\sin \theta \frac{dr}{d \theta}+r\cos \theta =\tan \theta,0<\theta<\pi/2$. The answer received comments that it should be a comment, but I disagree. Substantial contributions toward solving the problem are often fine as answers, even if they do not go through the full details of a write-up of a solution. (Comments included a nonsensical one selected from a review queue.) It might be relevant that the answer was posted before the other, more detailed ones.

A moderator deleted it, so regular users cannot vote to undelete. I'm requesting that a moderator undelete it, and would be interested in explanations of why people think such an answer should be deleted.

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I second this plea. An IMHO good hint started getting negative flags for some reason. I voted to dismiss the flag, but apparently that didn't carry enough weight. May be there is an anti-hint faction? It may be that the entire question were to be deleted for not showing effort, but singling out this answer sends a wrong message. For the record: it has six upvotes and no downvotes. –  Jyrki Lahtonen Aug 15 at 19:13
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I think the deeper question is why a moderator deleted it - there is likely to be something going on beside the anti-hint comments. They may not be able to give full info. Of course the comments are not in accord with the usual norms of this site - hints are perfectly fine. –  Carl Mummert Aug 15 at 20:55
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Thank you for bringing this to my attention. I don't know what I had been thinking. I see that there were a large number of flags on that post, and I suspect I trusted them too much. I've undeleted the answer. –  mixedmath Aug 16 at 7:54

[REOPENED]

This question was closed as a duplicate, but the question specifically asks not for a proof (which the OP knows) but, instead, for some intuition behind successful strategies for constructing such proofs - something that is not addressed at all in the proposed duplicate. Probably there is a good chance of further helpful answers appearing if it is reopened.

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Will you please to reopen Specific Advice/Tips to Streamline Studying Math by virtue of its upvotes and those on my Answer.

Will you please to appreciate the fact that most of the downvotes in its Meta thread (How to Adapt or Emend : Specific Advice/Tips to Streamline Studying Math) occurred before my edit to my question, and observe Lord_Farin's gracious comment underneath his answer thereunder.

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@TobiasKildetoft It may be broad, but it is also of general interest. Broad questions are frowned upon, but often are looked less critically if they are of general interest. For example: math.stackexchange.com/questions/71874/… is incredibly broad, as are many of the top-voted soft-questions. –  user1729 Oct 28 '13 at 19:11

Please reopen Intuition. If f differentiable and $\lim_{x\to0} f'(x) = L$, then $f'(0) = L$. (S.A. pp 137 q5.2.8c,d)

This question is for intuition only and no proofs. The other is for proofs. Ergo they aren't duplicates. The closers left no comments?

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May be I'm wrong, but it seems to me the pleas to reopen work better, when they come from someone other than the original poster? I rather thought that was the whole point of having this thread. –  Jyrki Lahtonen Mar 1 at 17:27
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@JyrkiLahtonen It works fine for the OP to post here if the question has either been edited to address whatever got it closed, or if the closure was based on a misunderstanding (I didn't check if either was the case here). –  Tobias Kildetoft Mar 1 at 18:38

[REOPENED. Thank you.]

Will you please to reopen Infer distance from a point to a line, from the distance from a point to a plane [Stewart P793 12.4.44]?

My question is tendered at the bottom of my question and there are no comments on why it is avowedly "unclear." I've made an ancillary edit that should only clarify the OP.

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[REOPENED] Show that floating point sqrt(x*x) >= x for all long x.

I would like to see this question reopened. I believe it is a valid numerical analysis question.

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I am trying to get

Let $G$ be a finite abelian group. Let $a\in G$ be an element of maximal order. Prove $|b|$ divides $|a|$ for all $b\in G. A\: different \: proof\: $

reopened. It was closed as a duplicate of Prove that for any element $b$, $|b|$ divides $|a|$ (order of $b$ divides order of $a$). and Finite abelian group generated by elements of maximal order. True the question has been answered, but I haven been given another outline of a proof for this and would really appreciate it if i could get some feedback on this version of the proof for this question

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Please re-open Undefinably large algebra? . It has been substantially revised and should be sufficiently clear at this point. Also, note that you can address the question without addressing my semi-formalization of a hopeful answer. Suggestions for helping the question be re-opened can be put here: Working Towards Re-Opening (Undefinably large algebra?)

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Please reopen Prove nth root of k exists with supremum. (S.A. pp 27 1.4.6b).

My question is long but I put my questions in the gray shaded boxes. Questions are numbered too and spaced. Ergo it should not be 'unclear what you're asking.' None of the closers commented?

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[REOPENED]

My question here was said to be too broad, so I tried to edit it with a more specific example: How can you tell when you need to use the binomial coefficient for probabilities?

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[REOPENED]

Becoming Better at Math was closed on the grounds of being primarily opinion-based. It is community wiki with and tags. Though there are many possible answers (hence the CW), I do not find the question to be opinion based, as it is a call for guidance from experienced mathematics students, who in general do not disagree about most fundamental aspects of learning mathematics. (In other words, though opinion-based answers to this questions may exist, certainly not all answers are opinion-based.) This question could serve as a useful collection of practical advice specifically aimed at high school aged students preparing to major in math in college.

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[REOPENED]

The question Is there a highest order of infinity? was closed as a duplicate. Its original version asked whether "there is a limit to the orders of infinity", something that has been asked here several times already. The person asking the question has since clarified that what they are asking for is something very different,

Does there exist an infinite set of cardinality such that it can never be reached by taking power sets of a set with cardinality aleph-null?

(By the way, if reopened, an answer may as well clarify that $\beth_{\omega}$ is far from being "highest" among the infinite cardinalities.)

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[REOPENED]

Please reopen Algebraically compute $\lim_{x\to0}\frac{\sin x}{x}$ which was closed as a duplicate of How to prove that $\lim_{x\to0}\frac{\sin x}{x}=1$?. The latter explicitly asks for a geometric solution, while the former explicitly asks for a solution that uses as little geometry as possible. My own reopen vote has already expired, as have two others.

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[REOPENED] The question How to define a well-order on $\mathbb R$? were closed recently due to the confusing nature of the word "define".

If by "define" we wish to mean "explicitly describe without any appeal to the axiom of choice" then it is indeed a duplicate. However as the comments clarify, this is not the case.

I believe, if so, that it is not a duplicate of the question it was closed as a duplicate of (or any of the questions in the links in the comments I posted, as well).


Thank you Andres Caicedo, rschwieb, Belgi, Matt Pressland.

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That depends on what your definition of is is. –  Graphth Oct 26 '12 at 15:57
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@Graphth: The answer on that depends on what is your definition of definition. –  Asaf Karagila Oct 26 '12 at 15:58

[REOPENED. I cast the binding fourth vote on behalf of myself and Bill Dubuque]

This question: Prove $f(S \cap T) \subseteq f(S) \cap f(T)$ was voted to be closed as a duplicate of Is this a valid proof of $f(S \cap T) \subseteq f(S) \cap f(T)$?

However, the question asked actually was different. The latter asks for the readers to check whether the OP gave a valid proof (he didn't, and counterexamples were given as answers). The former asks for a proof. The closest answer we have on the latter to this question is this sketch of a proof. So I don't really think the two are exact duplicates of each other.

(The other proposed duplicate target is a mistake, as noted in the comments.)

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I agree, since your vote and mine bring the count to 5, you should feel welcome to reopen it. –  Bill Dubuque Nov 6 '12 at 14:33

[REOPENED] I would like to see the question on "mathematics in the movies" reopened. It asks about feature films depicting math, and/or mathematicians. I think it is at least as relevant to math, and this site, as are many other "soft questions" that still stand as open. After all, we DO have tags "math history", "education", "big-list", etc.. If questions relevant to those tags are thereby subject to closure, then the tags should be removed from this site; else, they are appropriate topics on which to post.

If reopened, I think it might very well be appropriate to "wikify" the post (community wiki), but this question has merit.


ADDED, to answer Marvis's comment below: This post is not a "strict subset" of the post to which Marvis provides a link. They are sufficiently different posts to warrant separate consideration. (See for example, Rahul's reply to Marvis).

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I would like the part about "what kind of stories would you like to see in the future" removed as it is discussiony and off-topic. I said that in my comment on the question, but the asker chose to ignore it and freak out instead. –  Rahul Nov 22 '12 at 17:50
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I believe that this question is a strict subset of math.stackexchange.com/questions/18843/… –  user17762 Nov 22 '12 at 20:28
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@Marvis: It seems to me the questions are split along fiction/nonfiction lines. –  Rahul Nov 22 '12 at 22:42
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points to comment that has been ignored yet again –  Rahul Nov 23 '12 at 20:10

[REOPENED] I would like this question:

Field Extensions of cos and sin

to be reopened. The question was closed as a duplicate of Degree of field extension. Bit IMO, the questions are different. The closed one is about whether a specific proof is correct, not just a question about how to prove it (like the other question).

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[REOPENED] I would like to see this post reopened.

It is clear that it generated a lot of thought about relations, their properties, etc. Many questions of this nature are posted to this site, and are not closed as "non-constructive." Many students are very interested in "real-life" applications/interpretations of the math they are learning. ${}$

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I don't think this post should be re-opened, although I agree that "non-constructive" might be off-putting. I think that the question generated a lot of buzz, which is good, but when you have several repeated answers and deletions it's time to close down. If we had "no longer relevant" as a closing reason I would vote that. –  Asaf Karagila Jan 5 '13 at 14:09

[Re-opened] I'd like to suggest reopening What is importance of the Bunyakovsky conjecture?. The question in the body, "how important do you consider the answer to this problem", may well be "too soft", but the question in the title seems to me to be a solid mathematical question.

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I cast the fifth and final vote. –  Willie Wong Mar 13 '13 at 12:32

[REOPENED] Find $\det X$ if $X$ satisfies the equation $8GX=XX^T$ was closed as not a real question. I have edited it in light of comments by OP. I think it's a real question now (albeit an easy one, that I have answered in the comments).

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[REOPENED] What explains this bizarre behavior?

I would like to see this question reopened. I believe it is a valid dynamical system question.

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I voted to reopen. The OP noticed a very interesting effect that has puzzled a lot of people over the years. –  Grumpy Parsnip May 4 '13 at 15:53
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@Grumpy: More accurately, we noticed an interesting effect, and we are assuming the OP noticed what we noticed. (And admittedly, I did not notice, because my eyes glazed over at the wall of digits and I didn't see the signs, although I did track the pattern of the decimal point) –  Hurkyl May 5 '13 at 1:01

[REOPENED] (Thanks.)

This question. I think that there may be insights that only working mathematicians could provide (as opposed to philosophers), and even if there are wildly differing points of view, seeing them described may be useful.

I understand that the question is not mathematical in the sense that "how do I integrate such-and-such" or "why is this number divisible by that one" are. I also believe its answers may be more interesting and useful in the long run.

Of course, it may be that answering the question in detail, considering as many of its subtleties as possible would just be too long and unfeasible. That's fine; even providing a few references and ideas that can potentially be fleshed out would be more useful that simply dismissing it.

Related.

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[REOPENED] (Thanks!)

I would like to see this question reopened.

In fact, I do not understand why it was closed to begin with: The OP posted the question, and illustrated with examples what they meant. The question does not appear trivial, and its formulation takes some preliminaries, so it cannot just be stated in two lines.

In case the question is not clear, let me rephrase it as I did in the comments (but really, looking at the examples the OP provides may be better than chasing through the formalism here):

Let $f,g$ be any linear functions from $\mathbb R$ to itself. If $h$ is a function obtained by composing $f,g$, in any order, say $h=j_1\circ j_2\circ\dots\circ j_n$, where each $j_i$ is $f$ or $g$; and $s=s_h$ is the fixed point of $h$, then we can associate a cycle to $h,s_h$ by considering the finite sequence $$ a_0=s_h,\quad a_1=j_n(s_h),\quad a_2=j_{n−1}\circ j_n(s_h),\dots,\quad a_n=j_1\circ \dots\circ j_n(s_h)=s_h. $$ (Note that the $a_i=a_i(h)$ depend on $h$.) Define $\mathrm{Sum}_g(h)$ as the sum of the $a_i$ with $i<n$ such that $j_{n−i}=g$.

Now, let $S$ be obtained by composing $f,g$ in any order, and let $T$ be the result of composing in the reverse order, so if $S=j_1\circ\dots\circ j_n$, then $T=j_n\circ\dots \circ j_1$. The question is: Is it true that we always have $\mathrm{Sum}_g(S)=\mathrm{Sum}_g(T)$? If so, how can we prove it?

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[REOPENED] This question should be re-opened as the OP has edited the post to try to tell us what they have done.

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[Now Deleted]

OP has put some work into improving http://math.stackexchange.com/questions/436733/trigonometry-right-angled-triangles#comment936496_436733, so I nominate it for re-opening.

EDIT: now deleted. Can a deleted question even be re-opened?

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[Now deleted]

I would like to see this big-list question re-opened. It is on the "Real-world applications of the Fibonacci Series". As I said in the comments, I do not think that this question should be closed. There is a [big-list] tag for a reason. The tag's wiki says "Please do not ask too many of these" not "Do not ever ask these".

Also, I believe that wondering about the applications of different aspects of mathematics is a worthwhile thing to do! If you ever have to write a fellowship application, then it is doubly worthwhile! However, the comments seem to be saying "How dare you ask about applications of mathematics! We do not care about such trifles here!"

I would be grateful if those who want to keep the question closed could comment here on why they have this opinion. You have heard my view, and I am interested in hearing yours.

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@Downvoter: When I wrote "I would be grateful if those who want to keep the question closed could comment here on why they have this opinion", I was talking to you... –  user1729 Jul 17 '13 at 12:15
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Sorry I just saw this. I think the question should remain closed until the OP has suitably narrowed the question down (or split in to distinct posts) to something which can be answered in a few paragraphs. As it stands, there are two 'big list' questions only tangentially related to each other, and another question which could receive some great attention as a history of maths questions if posted separately. I've added a comment to the same affect on the question. –  Daniel Rust Jul 24 '13 at 12:59

[Re-closed as duplicate about Cartesian to Polar change of variables. See comment below.]

I propose that we should reopen this question question, and posted my reason in the comments.

It was closed as an exact duplicate of a thread asking how to evaluate a Gaussian integral. But actually, the question was why, when one converts from Cartesian to polar coordinates, $dx\,dy$ gets replaced by $r\,dr\,d\theta$. That's not what the other question was about. There are other ways to evaluate the Gaussian integral than by polar coordinates, and those other ways would be appropriate answers to that older question, but not to this one. In some ways, the presence of a Gaussian integral in this question is incidental. It was really only the particular occasion for the question about polar coordinates.

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It is actually a duplicate of math.stackexchange.com/questions/37044/… –  Andres Caicedo Nov 30 '12 at 1:55

[REOPENED] Thank you.

The question was undeleted by the OP, and has now been closed. I think it does more good being open and have myself voted to reopen. As I told the OP elsewhere, "The thing is, many (oh so many!) people think that mathematics is just about computations (long and tedious and often times pointless). And many among those that know better think that the only way to use a computer is to help with these computations. But there is much more to math than that, and many more uses of a computer in mathematical research. And there are the incompleteness theorems, showing that not all mathematical practice can be automated anyway. Good answers to your question could be quite insightful!"


This question was actually deleted, but I think it is interesting and can be useful to many, not just the OP: Why are there mathematicians that do not use computers?

I was watching a video on Andrew Wiles and his proof of Fermat's Last Theorem and I quite liked the video, especially the complexity of the proof only to prove a simple concept which can be understood by most people. I also liked the graphics they used to illustrate elliptic curves and modular forms.

But then Andrew Wiles said that he never uses a computer, he only uses pen and paper and I also heard of other mathematicians that don't use computers.

Do they not use computers because there are problems only a mathematicians can solve or are there other motives? Wouldn't the proof have taken much less than 7+ years if he used a computer?

I have voted to undelete. I am posting this request here as it seems a natural extension of the intention of this post. Also, if there are issues with the question, this may be the place to address them, which may also prove useful on its own.

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@Thomas That's fine. In this case, I believe the deletion happened because of the downvote, which I consider unwarranted. Regardless, I think (as I said) that the question can be useful to many, not just to the OP, which should (surely?) be our main consideration. –  Andres Caicedo Aug 11 '13 at 19:23
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I agree that the question with answers could be useful. But, on principal grounds, I believe that it is wrong to undelete a question that the OP him/herself deleted. I think a better approach is for someone else to repost the question. I don't think that we should assume we know why someone decides to delete his/her own question. –  Thomas Aug 12 '13 at 2:42
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Ok, I didn't know that. In that case I see no problem. –  Thomas Aug 12 '13 at 12:45

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