mathworld.wolfram.com
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Wolfram MathWorld is a comprehensive online mathematics encyclopedia offering detailed, browsable articles on a wide range of math topics. For covering spaces, it provides definitions, key theorems, example constructions, and related topics in algebraic topology with references.
More resources on Covering Spaces
Topology Without Tears
Free online book with topology sections including coverings
Hatcher's Algebraic Topology (free PDF)
Comprehensive free textbook with detailed covering spaces chapter
Algebraic Topology
Learn about covering spaces in algebraic topology with this course by N J Wildberger. Explore topological concepts and their applications.
ocw.mit.edu
MIT OpenCourseWare (OCW) offers free, openly available MIT course materials across a broad range of subjects. You’ll find lectures, notes, assignments, and exams for math and topology courses (including topics related to covering spaces) along with supporting multimedia.
ncatlab.org
The nLab is a collaborative wiki of higher mathematics, physics, and philosophy with detailed articles and interlinked definitions on topics like category theory, homotopy theory, and algebraic topology. It offers pages on covering spaces, fundamental groups, fiber bundles, and other related concepts.