in no particular order:

https://matlas.ai

https://www.mathdeck.org

https://portal.mardi4nfdi.de/wiki/Portal

https://www.theoremsearch.com/search

https://ansumandas441.github.io/mathematical-discovery-engin...

https://www.wolframalpha.com/

+1 to wolfram alpha. But just like Chegg, I thought wolfram alpha would be harshly affected by the AI disruption.

I used it a lot in college but never since. Are current college folks still using it?

Slightly related: https://oeis.org

The encyclopedia of integer sequences can be quite useful: https://oeis.org/

If the formula generates an integer sequence, then searching that sequence on OEIS should give a lot of good information.

Wolfram Alpha

Claude