Discrete Mathematics: An Open Introduction
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Oscar Levin's free, openly licensed textbook for a first discrete mathematics course, with interactive exercises and solutions. Its opening chapter treats sets and functions, including injections, surjections and bijections, before counting, sequences, logic, proofs and graph theory; relations get little coverage.
More resources on Relations & Functions
Discrete Mathematics and Its Applications
Standard undergraduate discrete mathematics textbook covering logic, proof, sets, functions, algorithms, number theory, counting, relations, graphs and trees, with thousands of exercises. Its relations chapter develops equivalence relations, partial orders, closures and matrix representations to a level suited to computer science study.
Wolfram MathWorld
MathWorld is an online mathematics encyclopedia from Wolfram Research offering detailed, browsable articles on topics across the math spectrum, including algebra, geometry, calculus, and number theory. Each entry includes definitions, theorems, formulas, diagrams, worked examples, and links to further reading.
Khan Academy Functions
Algebra I unit of video lessons and graded practice on function notation, domain and range, graph interpretation, and inverse functions. Progress is tracked per skill, so learners can identify which specific ideas still fail.
Mathematics for Computer Science (MIT 6.042J)
Discrete mathematics for computer science with an emphasis on definitions and proofs: logic, induction, sets and relations, graph theory, modular arithmetic, asymptotics, counting and discrete probability. 25 lecture videos, problem sets and exams with solutions build fluency in writing proofs.
Discrete Mathematics
Dominik Scheder's proof-based course on sets, functions and relations, enumerative combinatorics, graph theory, and network flows and matchings, each concept paired with a fully proved non-trivial result. Learners read formal statements and write rigorous proofs of their own.