mathworld.wolfram.com
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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.
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
ocw.mit.edu
MIT OpenCourseWare offers free, openly licensed MIT course materials for self-guided learning across disciplines, including mathematics. You can download lecture notes, assignments, exams, and often video lectures from MIT courses on topics like topology and homotopy.