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...

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.

ProofSouq.com! https://proofsouq.com

8M+ search index entries over Lean and Rocq corpora and growing; propositions only for now, so more oriented towards premise retrieval at the moment.

Claude

Wolfram Alpha