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Sometimes I am really surprised how fast are the duplicates of new question found, given that we have vast amount of questions here.

I hope posting this question here can be useful and can lead to some constructive suggestions. If it was discussed here before and I've overseen it, my apologies. (It would be, however, a little ironic.)

A few methods for finding duplicates that come to mind are the following:

  • Look in the list of related questions, which is generated automatically.
  • Sometimes it is useful to look in this list: http://meta.math.stackexchange.com/questions/1868/list-of-generalizations-of-common-questions Another useful list is mentioned in this post: http://meta.math.stackexchange.com/questions/16708/catalog-of-standard-exercises
  • If you remember that you posted such question/commented such question/answered such question, it might be relatively easy to find it in your list of comments/answers/questions.
  • Even if you only remember seeing such a question, it might make your life easier - you can remember poster, part of the title, a phrase used in the answer, simply something which helps you find the question more quickly.
  • You might search for the question - either here or using google. This might be not that easy if the search would involve mathematical formula, but:
    • If the formula has a name, it is probably mentioned in the answer or in the comments, so you might try to search for the name, e.g. "triangular number" site:math.stackexchange.com on google or triangular number on MSE. (The search on google has the advantage that it also looks into comments.)
    • If you know the correct result, you can search for some part of formula e.g "frac n(n+1)2" or "n(n+1) 2" site:math.stackexchange.com on google or here. However, the search for "n(n-1)/2", "k(k+1)/2" or "s(s+1)/2" might be reasonable in this case, too. (In general, it is not easy to search for formulas.)
  • If it is a question which was probably asked many times on Math.SE, you can try your luck on the frequent tab, which shows the questions with most links. (Perhaps it is useful to restrict this to some particular tag, like here.)

Are there some other useful tricks to find the duplicates?

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  • $\begingroup$ (somewhat) related: meta.math.stackexchange.com/questions/2871 $\endgroup$ – Grigory M Nov 27 '11 at 20:11
  • $\begingroup$ Look through your own questions. $\endgroup$ – Phira Nov 27 '11 at 22:49
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    $\begingroup$ I think you've done a good job of listing the useful tricks. $\endgroup$ – Gerry Myerson Nov 27 '11 at 23:15
  • $\begingroup$ It's a bit soft, but: I tend to remember which members tend to ask/answer which questions. One way I expedite my Google searches is to add in a few name(s) along with keywords. $\endgroup$ – J. M. is a poor mathematician Nov 28 '11 at 10:44
  • $\begingroup$ Thanks to "Questions with similar title" function, I found this question, while I was about to post a question on "Combining the Search Functionalities". It would be great to have that, because it helps to identify duplicates for the OP in spe. What do you think? $\endgroup$ – draks ... Apr 9 '12 at 11:27
  • $\begingroup$ In a recent case, I remembered the user who had answered a duplicate, and used the link to the relevant tag from the user's page to find it. $\endgroup$ – Jonas Meyer Dec 7 '14 at 16:22
  • $\begingroup$ I think it would be nice to add Approach0 tool to the list, so far probably the most powerful tool finding duplicates in my opinion. The tool was announced here math.meta.stackexchange.com/questions/24978/…. $\endgroup$ – Sil Nov 8 '17 at 7:30
  • $\begingroup$ @Sil Why not posting (expanded version of) your comment as an answer, then? (Ideally without exaggerating - I would definitely not claim that it is something which is "super secret".) $\endgroup$ – Martin Sleziak Nov 8 '17 at 12:43
  • $\begingroup$ @MartinSleziak Thought it would be more suited to be summarized on one place, but as you wish, posted. $\endgroup$ – Sil Nov 8 '17 at 17:50
  • $\begingroup$ @Sil Expecting something like this to be summarized in one place seems a bit too optimistic, considering the amount of posts about searching here on meta :-) (Just have a look at linked posts or other posts tagged (search) or the posts listed here.) $\endgroup$ – Martin Sleziak Nov 8 '17 at 18:07
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Here are some techniques that I find very useful. When composing posts I explicitly insert keywords that will aid me in quickly locating the post by searches. For example, I strive to include commonly attributed names of theorems or identities, and when such names have alternate spellings I include all of them, e.g. l'Hôpital and l'Hospital, or Besicovitch and Besicovic, etc. This has the added benefit that I can quickly compose links to my prior posts on related topics by using searches employing these keywords, e.g. the telescopy buzzword.

If we all make some conscious effort to ensure that posts contain useful keywords then that should go a long way towards making searches more effective - whether they be for locating duplicates, discovering related results, etc. Of course the built-in SE tagging system should be the first level of categorization used. But such free-text tagging by keywords proves very useful in situations where the SE tagging model doesn't work well, e.g. in answers that mention topics that go beyond that categories in the original question. No doubt others have evolved analogous useful techniques. It would be very helpful to collect such community wisdom in this thread.

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    $\begingroup$ And all this time I had thought that you just remembered that this question has been asked, and had a recollection of who asked it and when. Why kill the mystery? :-) $\endgroup$ – Jyrki Lahtonen Nov 28 '11 at 7:07
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In case someone stumble upon this question these days, there is a great tool for finding duplicates since 2016 : Approach0. It's quite remarkable, it can for example find same expression with different letters being used in it.

For more information you can see this question where it was announced by the author: Announcing a third-party search engine for Math StackExchange.

My wish is that more people know about this before asking or even answering questions.

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    $\begingroup$ This can be found also in the post you linked, but maybe it is worth adding that there is a separate chatroom dedicated both to searching on this site in general and to Approach0 specifically. $\endgroup$ – Martin Sleziak Nov 8 '17 at 18:10
  • $\begingroup$ Since you're saying that it would be nice if more people knew about this, I'll mention that I have recently made a suggestion to add mention of this search engine to help center: Should we add external searches in some help pages about search? $\endgroup$ – Martin Sleziak Nov 28 '17 at 17:28

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