ncatlab.org
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nLab is a collaborative wiki for advanced mathematics, with rigorous articles on topics like category theory, algebra, and topology. It includes representation theory and character theory articles, offering definitions, explanations, and literature references.
More resources on Character Theory
Stony Brook Rep Theory Notes
Free notes on finite group representations and characters
nLab: Character
Comprehensive notes on character theory
mathworld.wolfram.com
MathWorld is a comprehensive online mathematics encyclopedia by Wolfram Research, featuring in-depth, well-structured articles on topics across math, including character theory within group theory. Each article includes definitions, theorems, formulas, examples, and references to aid study and research.
ocw.mit.edu
MIT OpenCourseWare (OCW) is a free, openly accessible repository of MIT course materials. It offers lecture notes, assignments, exams, and occasionally video lectures across a wide range of subjects, including mathematics and abstract algebra, for self-paced study of topics like group theory relevant to character theory.