ocw.mit.edu
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MIT OpenCourseWare offers free, openly licensed MIT course materials across math and other subjects, including lecture notes, assignments, and exams. It’s a rich self-study resource for learning topics like cohomology and other areas of mathematics.
More resources on Cohomology
mathworld.wolfram.com
MathWorld is an extensive online mathematics encyclopedia from Wolfram that provides concise, well-structured articles on a broad range of topics. Each article includes definitions, key theorems, formulas, diagrams, and references, covering subjects like cohomology and beyond.
ncatlab.org
ncatlab.org is a collaborative online wiki for advanced mathematics, with a focus on category theory, algebraic topology, and related fields. It offers detailed definitions, expositions, and references on cohomology, including cohomology theories, derived categories, spectral sequences, and higher category theory.