ncatlab.org
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nLab is a collaborative wiki-style reference for higher mathematics, with a focus on category theory and related areas, offering formal definitions, explanations, and discussions of universal properties, adjunctions, and other advanced concepts.
More resources on Universal Properties
Category Theory for Programmers (Blog)
Free online companion to Milewski's course with universal property examples
A Gentle Introduction to Category Theory
Learn category theory basics with Paolo Perrone! This course gently explores universal properties and more. Start your journey now!
Category Theory for Programmers
Learn universal properties in category theory with Bartosz Milewski's engaging course for programmers. Master this powerful abstraction!
mathworld.wolfram.com
MathWorld is Wolfram's comprehensive online mathematics encyclopedia with rigorously written articles, definitions, theorems, proofs, and worked examples across all areas of math. Each entry includes cross-references and visuals, making it a solid resource for exploring topics like universal properties and related concepts.
ocw.mit.edu
MIT OpenCourseWare (OCW) is MIT's free, public repository of course materials. It offers lecture notes, assignments, exams, and multimedia from thousands of MIT courses across science, engineering, humanities, and more, enabling self-paced learning without registration.