I broadly work in decision making under uncertainty - so a bit of control theory, a bit of optimization, a bit of machine learning, and AI planning (MDPs, POMDPs).

I definitely don't do math say at the level of a math grad student, but I do fair bit of proofs, mostly using concepts from real analysis and functional analysis. I'd say GPT4 does pretty good sometimes (this is mostly me testing to see how well it works, I don't have a particular use case). I use Github copilot (I believe it uses GPT-3?) to write my latex documents and it works really well at predicting the equations I am writing and saves a lot of time!