nLab Cohomology
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nLab reference entry defining cohomology in the general framework of homotopy theory and higher category theory, as maps into classifying objects. Relates singular, sheaf, group and generalized cohomology, letting readers see how the classical theories fit one abstract definition.
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A collaborative, open-source reference on algebraic geometry run from Columbia University, with thousands of pages building from commutative algebra to schemes and algebraic stacks. Every result has a stable tag, so readers can look up precise statements and complete proofs.
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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.
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