(This *must* have been asked before and exist somewhere in Community Wiki, but I can't find it...)


Where can you post open (math) problems? And what are the advantages and disadvantages?


* Example: This place (and Math StackExchange), duh.
* Example: The journal AMM had a corner "Unsolved Problems", but no longer. (Editor Stan Wagon told me that he doesn't know any math journal accepting unsolved problems.)
* Example: But then, probably any journal will accept a small "Open Questions" section after a research article.
* Example: USENET once had an useful group. The group still exists. (Yes, I'm *that* old...)
* Example: Other social media, e.g. Reddit has a math sub.



Places where one can post open problems
If you can motivate the problem and make some partial progress on it, you can try and publish it as a paper in a specialized journal, or at the very least upload it to the arXiv.


If you only have empirical evidence, there are journals that are receptive to this kind of this ("Mathematics of Computation" and "Experimental Mathematics" spring to mind).


If it concerns elementary mathematics and is relevant to a wide enough audience you can try a popular journal such as "American Mathematical Monthly".


Other than that, you can


* put in on your website, if you have one.
* share it with experts, as they have the highest chance of solving it, and at the very least of assessing its importance and difficulty and possibly guiding you towards relevant literature or a proof.
* share it in the problem session of a relevant conference. Problems from such sessions tend to be published in the form of conference proceedings.


