An Introductory Course in Elementary Number Theory

Concise lecture notes on number theory covering divisibility, primes, modular arithmetic, and classic theorems by Fermat, Euler, and Wilson. A clear, exercise-driven entry point.

Vahagn Aslanyan

A widely used open introduction to discrete mathematics, with counting, graph theory, logic, and proofs. Inquiry-based exercises make it ideal for a first course in the subject.

Oscar Levin

An open textbook on combinatorics and discrete structures, from counting techniques and generating functions to graphs, posets, and network flows. Strong on applied problem solving.

Mitchel T. Keller, William T. Trotter