mathworld.wolfram.com
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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.
More resources on Cohomology
ocw.mit.edu
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.
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.