Art of Problem Solving Wiki
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Comprehensive wiki with problems and solutions
More resources on Inclusion-Exclusion
Inclusion exclusion principle
Detailed proof and applications by Michael Penn
Mathematics for Computer Science
Learn inclusion-exclusion & other essential math concepts for computer science in this course by Leighton & van Dijk. [MIT OCW]
mathworld.wolfram.com
MathWorld is a comprehensive online mathematics encyclopedia from Wolfram that provides detailed, peer‑reviewed articles on math topics. Each article includes definitions, theorems, proofs, formulas, examples, and diagrams, covering areas from algebra and calculus to probability and combinatorics.
ocw.mit.edu
MIT OpenCourseWare offers free, openly accessible MIT course materials across many subjects, including mathematics. It provides lecture notes, problem sets, exams, and multimedia for self-paced study, useful for topics like inclusion-exclusion in combinatorics.
nptel.ac.in
NPTEL is a MOOC platform run by IITs and IISc that provides free online courses in engineering and sciences. It offers video lectures, downloadable notes, assignments, quizzes, and certificates.