Start here for math related topics.
- Enumerative Combinatorics by Richard Stanley (Twelvefold way on p. 79)
- Twelvefold Way A summary of the twelve ways of counting via enumerative combinatorics
- Essence of Linear Algebra by Grant Sanderson
- Linear Algebra by Gil Strang
- Differential Equations by Arthur Mattuck
- Calculus by Robert Grhist at Penn State
- Abstract Algebra by Benedict Gross at Havard