Cross-posting is discouraged in general, because it can lead to duplication of effort by people answering on different sites. However, it is appropriate under some circumstances.
Most of the advice discussed by Joonas Ilmavirta in the context of cross-posting to Math Stack Exchange is applicable in general. To quote Joonas, if you're considering cross-...
Mohammad Ghomi answered
Shortest closed curve to inspect a sphere,
in a paper, Shortest closed curve to inspect a sphere, posted to the arXiv (https://arxiv.org/abs/2010.15204),
whose PDF is available here,
verifying the conjecture that the shortest curve is four
consecutive semicircles each of length $\pi$ for a unit sphere.
This doesn't quite fit the mold of the other postings, but Matt Parker
a YouTube video
that mentions MathOverflow
several times, and particularly highlights the work of
A paper by T. Chartier, P.P. Pach and myself describing the status of "Can we color Z^+ with n colors such that a, 2a, …, na all have different colors for all a?" has just been published.
Coloring the $n$-Smooth Numbers with $n$ Colors, The Electronic Journal of Combinatorics 28 (1) (2021), #P1.34. https://doi.org/10.37236/8492
Here are two kind-of dual results that have now been proved after I asked them here:
The first arose from Induced map on path manifolds: is it a submersion?, which got an answer in very general terms involving generalised smooth spaces in this preprint:
Andrew Stacey, Yet More Smooth Mapping Spaces and Their Smoothly Local Properties, arXiv:1301.5493.