nLab: Ordinal
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Reference article from the nLab research wiki giving the definition of ordinal numbers as well-ordered sets, their arithmetic, and constructive and structural variants. Suited to readers who know basic set theory and want precise statements, alternative formulations, and literature references.
More resources on Ordinals
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.
Set Theory
Learn ordinal numbers with William Hamilton's Set Theory course! Explore fundamental concepts and deepen your understanding.
Stanford Encyclopedia of Philosophy - Set Theory
Joan Bagaria's peer-reviewed Stanford Encyclopedia of Philosophy entry surveying modern set theory: the ZFC axioms, ordinals and cardinals, the continuum hypothesis, forcing and independence results, large cardinals, and determinacy. Readers gain a map of the field's central questions and results.
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.