I highly recommend the Outlier Dictionary, an addon for Pleco. It covers about 4000 characters, so won’t include very rare ones like 忻. Pleco itself will show basic decompositions like 忻 = 忄+ 斤.

There used to be an offline mobile app for HanziCraft, but the app seems to be gone now (a little annoying, since I’m pretty sure I paid for it).

Project Euler if you don’t mind more mathematical problems.

Oppa homeless style vibes.

Or Picard’s little theorem, which says that if an entire function misses two points (e.g. is never 0 or 1), then that function must be constant.

Does this link work for you to see the comment? https://awful.systems/comment/9163259

This is what Donald Knuth is doing with TeX :)

Wikipedia:

Since version 3, TeX has used an idiosyncratic version numbering system, where updates have been indicated by adding an extra digit at the end of the decimal, so that the version number asymptotically approaches π. This is a reflection of the fact that TeX is now very stable, and only minor updates are anticipated. The current version of TeX is 3.141592653; it was last updated in 2021.