Discrete Mathematics. An Open Introduction

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 comprehensive introduction to number theory covering divisibility, congruences, multiplicative functions, primitive roots, and quadratic residues. Clear proofs and exercises make it suitable for self-study.

Wissam Raji

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