painintheazz

Like the others said you want to know basic linear algebra and probability. Another thing very helpful in general is mathematical analysis. From elementary real analysis up through measure theory and Lebesgue integral, at least. From here a lot of advanced math will open up to you: for example, big-boi probability theory and statistics.

Here below is an intro to how math people think (FREE):

* Book of Proof by Hammack: https://www.people.vcu.edu/~rhammack/BookOfProof/

For intro linal and probability, try

* Introduction to Probability by Bertsekas, Tsitsiklis: https://www.amazon.com/Introduction-Probability-2nd-Dimitri-...

* Linear Algebra by Kuldeep Singh: https://www.amazon.com/Linear-Algebra-Step-Kuldeep-Singh-ebo...

Both (clear and easy to read) books above are on l#ibgen, I believe. Also, there's a free online course based on Bertsekas'/Tsitsiklis' book on edx if I recall correctly.

I prefer textbooks to online stuff when it comes to math (especially analysis). The many elementary analysis books that I know (and like) are print-only.

Although, you could probably do the following one after the other:

For pre-analysis, try

* From Calculus to Analysis by Schinazi: https://www.google.com/books/edition/From_Calculus_to_Analys...

For modern analysis, try

* Measure, Integration and Real Analysis by Axler: https://link.springer.com/content/pdf/10.1007%2F978-3-030-33...

The one by Schinazi is on l#ibgen and the one by Axler is available free.