ncatlab.org
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nLab is a collaborative wiki for advanced mathematics, logic, and related disciplines, known for its dense, formal entries and cross-linked notation. It offers in-depth coverage of category theory topicsāfunctors, natural transformations, adjunctions, higher categoriesāand their applications, with precise definitions and connections to related ideas.
More resources on Functors
Category Theory for Programmers
Free online book with functor chapters.
nLab: Functor
Comprehensive reference on functors in category theory.
Category Theory for Programmers
Learn functors and other category theory concepts applied to programming! This course by Bartosz Milewski makes abstract math practical.
mathworld.wolfram.com
MathWorld is Wolfram's comprehensive online mathematics encyclopedia, offering clear definitions, explanations, formulas, and diagrams across a wide range of topics. It includes entries on category theory concepts such as functors, with examples and references.
ocw.mit.edu
MIT OpenCourseWare offers free, self-paced access to lecture notes, videos, assignments, and exams from MIT courses across disciplines, including math and computer science, with materials on topics like category theory and functors.