Brilliant.org Sets
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Community-written wiki page introducing sets, with notation, subsets, union, intersection, complement and cardinality, each followed by worked examples and short practice problems. Readers come away able to read set-builder notation and compute basic set operations and Venn diagram counts.
More resources on Sets & Operations
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.
Wikipedia: Set Theory
Encyclopedia article surveying set theory as a branch of mathematical logic, from Cantor's work and the paradoxes to ZFC axioms, ordinals, cardinals and independence results. Readers get a map of the field's history, vocabulary and main research areas, with references.
Introduction to Mathematical Thinking
Stanford course by Keith Devlin on how mathematicians reason, starting with the logic of language (and, or, not, implication, quantifiers) and moving to the structure of proofs, including contradiction and induction. Learners become able to read, construct and critique simple mathematical proofs.