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Is this question acceptable?: In what context is it used this symbol

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  1.  
    • CommentAuthorAJ
    • CommentTimeSep 30th 2011
     
    The Unicode character 'All equal to'
    http://www.fileformat.info/info/unicode/char/224c/index.htm

    It looks like this other character
    http://www.fileformat.info/info/unicode/char/2245/index.htm
    which I've used it in geometry to state that one triangle is congruent to another one.

    Could you give me an example, please?
    ------------------------------------------


    That was the question I asked. A moderator told me it is "off-topic"
    Why is it considered so?
  2.  

    @AJ: first of all, thank you very much for coming to meta to ask this question. I left a comment on the question, but I'll say it again here.

    For me, the question seems random, since you give no motivation or context. It is as if you asked "what is x?" or "what is the symbol æ used for?" Did you come across this symbol in a paper? If so, please provide the context. Otherwise, it looks like you're just asking about a random unicode character.

    To put it another way, I think questions about notation and convention are definitely on topic, but it doesn't seem like your question is about any notation or convention that you've ever actually seen. As another analogy, it feels similar to somebody asking, "has anybody ever made use of the binary operator xÞy=xy+y+7?"

    • CommentAuthorHenry Cohn
    • CommentTimeSep 30th 2011
     
    @AJ: One issue is that unicode contains an incredible number of obscure math symbols, most of which I've never seen anyone actually use. See, for example, http://www.unicode.org/charts/PDF/U2A00.pdf (and that's just part of the list). One could ask many questions about these symbols; for example, why is there an "integral with leftwards arrow with hook" symbol but, as far as I can see, no corresponding unicode symbol with a rightwards arrow? Most of these questions probably have no answer beyond "someone once used such a symbol somewhere, so they decided to include it in unicode for completeness, but they didn't feel a need to list all possible symbols."

    In any case, your symbol is on the list http://www.unicode.org/charts/PDF/U2200.pdf, and it appears to be the mirror image of the usual isomorphism symbol that consists of a tilde over an equals sign. The "all equal to" description is certainly cryptic, but these descriptions shouldn't be taken too seriously. (The isomorphism symbol's unicode description is "approximately equal to," compared with "almost equal to" for two wiggly lines, and I have no idea whether they are using different words on purpose.) I imagine someone once used a backwards isomorphism symbol, possibly to make some distinction from the usual symbol and possibly by accident. There's definitely no standard meaning in mathematics, so there's not much that can be said without some context.

    By the way, I'd strongly suggest not using this symbol, since the potential for confusion is too great. People are used to distinguishing mirror images like b and d, but not in tildes over equals signs.
  4.  

    While we're at Unicode guys not consulting mathematicians for the descriptions, this m.SE question is worth mentioning, I think.

    • CommentAuthorAJ
    • CommentTimeSep 30th 2011
     
    @Anton: I really appreciate your help. It's much clearer now.
    I have to say: no, I did't. Someone else asked on another site, I read the question and felt curiosity because there wasn't a tag for it in LaTeX. Then, I decided to asked on MO.
    According to the FAQs, my question seemed to be on topic. I published it.
    When the [closed] tag appeared, my first impression was: "hmm... there must be some kind of automatic filter for non-questions. I'm going to rephrase it and publish it again"

    So, it's clearly my mistake. I'll be more careful next time (Actually, I've just added meta.mathoverflow to the bookmarks toolbar)

    Thank you so much for the quick reply.

    @Henry: my mathematical knowledge is still limited. I ask myself: "How many 'widely' used symbols don't I know yet?". What if those symbols there really exist? I'd have never imagine that there were weird symbols in the Unicode set, even for advanced mathematicians.

    Well, despite of my failed question, I found your comment very valuable. I think it is what I was looking for :)
    Thank you too.



    --------------------
    PS: was it necessary to vote it down?
    • CommentAuthorquid
    • CommentTimeOct 1st 2011
     

    AJ: I did not personally downvote, but if something is posted twice it is very common that down-votes occur and the second closing is also virtually automatic; as posting twice is strongly discouraged in view of the fact that one can edit questions (even closed ones; there is a link below the text 'edit' at least if you are regeistered/loged in). In view of your description it was a natural thing to do from you point of view, and it is definitely not some big problem.

    In any case, I just now upvoted; from the point of view of points this is now better than without any votes.

  7.  

    Upvoting gives the person 10 points, while downvoting only removes 2 points. Don't upvote bad questions just because someone downvoted them! Downvoting important, and people give up their own rep to send a message.

    • CommentAuthorquid
    • CommentTimeOct 1st 2011
     

    Noah, abstractly I agree with the first part (not so sure about the last though). Yet, in my opinion this question is a special case, and moreover I think that on closer inspection the question is not so bad. It is in some sense a reasonable question, just based on a wrong -- but also reasonable -- assumption, as detailed by Henry (and cf the commen by Emil).

    [In case you want to discuss the general merits of how people vote on MO, and you create a thread, I will be happy to comment.]

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