ncatlab.org
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nLab is a collaborative wiki focused on category theory, higher categories, and related areas of mathematics and physics. It provides formal definitions, expositions, and bibliographic references across topics such as category theory, topology, representation theory (including unitary representations), and quantum algebra.
More resources on Unitary Representations
mathworld.wolfram.com
MathWorld is a comprehensive online mathematics encyclopedia from Wolfram, offering in-depth, encyclopedia-style articles on topics across math. It includes content on unitary representations, group theory, linear algebra, and related concepts, with definitions, formulas, examples, and references.
ocw.mit.edu
MIT OpenCourseWare (OCW) provides free, openly accessible MIT course materials—lecture notes, assignments, exams, and often video lectures. It spans numerous subjects, including mathematics and physics topics related to unitary representations, for self-paced learning.