ocw.mit.edu
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MIT OpenCourseWare (OCW) is a free, open-access repository of MIT course materials, including lecture notes, assignments, exams, and videos. It offers math and science courses with resources that cover topics like fiber bundles, topology, and differential geometry.
More resources on Fiber Bundles
Hatcher's Algebraic Topology (Ch. 15 on Fibrations)
Free online textbook covering fibrations and bundles
nLab: Fiber Bundle
Comprehensive reference with definitions and theory
Geometrical Anatomy of Theoretical Physics
Learn fiber-bundles with Frederic Schuller's "Geometrical Anatomy of Theoretical Physics" course! Explore the math behind modern physics.
mathworld.wolfram.com
MathWorld is a comprehensive online mathematics encyclopedia from Wolfram Research, offering detailed, encyclopedia‑style articles on a wide range of topics. Each entry provides definitions, theorems, formulas, diagrams, and examples with cross-referenced links—covering subjects from algebra and geometry to topology (including fiber bundles) and beyond.
ncatlab.org
nLab is a collaborative online wiki for higher category theory, homotopy theory, and related areas of mathematics, offering extensive definitions, expositions, and references on topics like fiber bundles, infinity-categories, stacks, and more.