ncatlab.org
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nLab (nCatLab) is a collaborative online knowledge base for higher mathematics, with emphasis on category theory, homotopy theory, higher categories, and related areas. It offers extensive, interlinked pages of definitions, concepts, and expositions—providing background, references, and connections for learning homotopy theory and beyond.
More resources on Homotopy
nLab: Homotopy Theory
Comprehensive wiki on homotopy theory in algebraic topology
Hatcher's Algebraic Topology
Free online textbook covering homotopy and fundamental group
Algebraic Topology
Explore algebraic topology with N J Wildberger's homotopy course! Learn fundamental concepts and build a strong foundation in this fascinating area.
Algebraic Topology
Learn homotopy theory with Ryan Budney's Algebraic Topology course! Explore fundamental concepts and build a strong foundation.
Algebraic Topology
Algebraic Topology - A book resource
mathworld.wolfram.com
Wolfram MathWorld is a comprehensive online mathematics encyclopedia offering clear definitions, explanations, formulas, and diagrams for a wide range of topics. Its Homotopy section provides formal definitions, essential theorems, examples, and cross-references to related concepts.