Number Theory

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

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