Ask HN: Math Skills for ML?

I have a BS and MS in CS and at the time I did my MS focus in ML/AI.

Fast forward so many years, my career has been focused on distributed systems stuff. I’m interested in ML for fun, but pretty far removed. In MS, it was a lot of theory, calculus, proofs, etc. I haven’t done deep graduate school math for years.

My question is - how much deep math knowledge do I need to build some cool ML stuff these days?