[qsort]

AOPS is a great community but it's for competitive mathematics (AIME, AMC, USAMO, etc.)

For OP: I think the problem with your question is that "math" is too broad, e.g. Math Overflow is for professional mathematicians, things like AOPS are very specific and most online communities are about what you would call "recreational" math (think 3blue1brown or standupmaths), likely far below what you want.

[achenet]

ok, I guess from what you say Math Overflow might be my best bet then, I'd like to have the competence of a professional mathematician, even if I work as a software engineer because it pays more :)

[dash2]

The kind of maths they do on Mathoverflow is PhD level and above... they probably get paid more if they leave academia, but then they are hired to do the kind of maths on math.stackexchange.

[oefrha]

Not really, as a former theoretical physicist who also got offers from many math PhD programs and have many math PhD friends, by far the most recruiting emails we get are from quant trading firms, and you won’t find that kind of math on Math.SE.

But yeah, roughly speaking you need to at least be a math PhD student who have passed your quals to be at the entry level of MathOverflow.

[dash2]

That's interesting - what kind of maths do you use in quant trading?

[mbeex]

Significant amounts of stochastic processes. Martingales, Markov, the whole stochastic Integration theory (Stratonovitch, Ito).

[dist-epoch]

Warning: on Math Overflow they do not tolerate noobs and noob questions. They will redirect you to the Math stack exchange (which is a different one).

[achenet]

ok, thanks for the warning :)