Márton Bognár – Probability & Statistics Notes
Classic probability/STAT handouts; concise and rigorous.
Mathematics Resources
MIT OCW – 18.06 Linear Algebra (Strang)
Full set of lectures, notes, problem sets.
Real Analysis (Abbott) – Companion notes
Helpful summaries aligned to Abbott's Understanding Analysis.
Category Theory for Programmers (Bartosz)
Gentle but deep introduction connecting math and FP.
Measure Theory notes (Tao)
Discussion posts and explanations by Terence Tao.