ncatlab.org
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nLab is a collaborative online wiki for higher category theory and related areas of mathematics, offering informal expositions, precise definitions, and references. It covers topics such as characteristic classes, higher category theory, topology, topos theory, and homotopy theory, with interconnected pages for deep, reference-style exploration.
More resources on Characteristic Classes
nLab: Characteristic Class
Comprehensive overview and references
Characteristic Classes
Characteristic Classes - A book resource
mathworld.wolfram.com
MathWorld is an online mathematics encyclopedia from Wolfram Research that provides concise, well-structured articles on a wide range of mathematical topics. It offers definitions, explanations, formulas, and illustrations, with entries on topics like characteristic classes alongside algebra, topology, geometry, and beyond.
ocw.mit.edu
MIT OpenCourseWare is a free, open educational resource publishing full MIT course materials, including lecture notes, assignments, exams, and videos. It covers mathematics and related fields, providing access to courses on algebraic topology, differential geometry, and other topics useful for learning characteristic classes.