Sadly the math I have forgotten the most is prepositional logic and proving things. It's probably the easiest to learn and pick up as well and is incredibly use

Sadly the math I have forgotten the most is prepositional logic and proving things. It's probably the easiest to learn and pick up as well and is incredibly useful for reading proofs, but alas I never exercised that bit of my brain. Probably the math I do remember is the applicative bits of number theory and cryptography.