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While editing @Ruy's answer to How can one define a kind of "determinant" on a reduced group $C^*$ algebra?, I tried to insert an \eqref to the equation labelled star3 in the main post. Although equation labels seem to be required to be unique within a page (because they are implemented by anchors; for example star3 is at https://mathoverflow.net/questions/347474#mjx-eqn-star3), the MathJax parser was baffled by my reference to star3, since it was not defined in the post itself. (I'm not sure if the reference would have rendered correctly if I submitted the edit; I know that sometimes the preview renders differently from the posted content.)

It seems quite likely that this is not a bug—it's of a piece with the requirement that macro definitions only extend through the post that they define—but, since equation labels really are global (per-page) objects in ways that macros are not, perhaps it is not the correct design decision; so I guess I'm calling this a feature request for the capability to refer in one post to an equation labelled in another post on the same page. It is not a particularly urgent request since it can currently be solved by manually editing in the appropriate anchored link, but that is fragile in the face of possible future changes to the implementation of equation references.

I couldn't find any other relevant tags, but please feel free to edit them in if you know any.

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    $\begingroup$ Unrelated to the issue with begingroup..endgroup - are you sure that it does not work? Maybe it was wrong only in the preview. Could you have a look at the same post after my edit to see whether this is what you wanted to get or whether I misunderstood what you mean? $\endgroup$
    – Martin Sleziak
    Apr 21, 2022 at 16:07
  • $\begingroup$ @MartinSleziak, you are right. I suppose it does only fail to work in preview. Would you post that as an answer so that I can accept it? $\endgroup$
    – LSpice
    Apr 21, 2022 at 17:13

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Originally posted as a comment - expanding to an answer, following the request from the OP in a comment.

After a recent edit, the post renders correctly with $\eqref{star3}$.

The most likely explanation is that this renders incorrectly in the preview - but it should work after the edit is saved.

It is known that there are some problems with \eqref and \label in preview: MathJax preview broken when equations contain \labels. (Although the issue described in that post is different from the one mentioned here.)

I will explicitly mention that my original suggestion that validity of label is limited by begingroup..endgroup was incorrect. (The comments about this are now deleted, but they were saved in chat. And 10k+ users and moderators can see a deleted answer here - which was intended as a test how begingroup..endgroup behaves w.r.t. labels.)

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