Stanford Encyclopedia of Philosophy - Set Theory
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Comprehensive entry on Zermelo-Fraenkel set theory, frequently cited on Reddit.
More resources on Zermelo-Fraenkel Axioms
Logic Matters - forall x (Set Theory Chapter)
Free online textbook excerpt on ZF axioms, rec'd in practitioner threads.
mathworld.wolfram.com
MathWorld is Wolfram’s comprehensive online mathematics encyclopedia, featuring detailed articles on a wide range of math topics, including definitions, theorems, formulas, and illustrative diagrams. It serves as a reference resource for students and researchers to explore concepts and related topics with links to further readings.
ocw.mit.edu
MIT OpenCourseWare provides free, openly available course materials from MIT across many disciplines. On the site you can access lecture notes, assignments, exams, syllabi, and often video lectures from individual courses, including mathematics topics related to Zermelo–Fraenkel axioms.
Introduction to Mathematical Thinking
Learn how to think the way mathematicians do – a powerful cognitive process developed over thousands of years. Mathematical thinking is not the same as doing mathematics – at least not as mathematics is typically presented in our school system. School math typically focuses on learning procedures to solve highly stereotyped problems. Professional mathematicians think a certain way to solve real problems, problems that can arise from the everyday world, or from science, or from within mathematics itself. The key to success in school math is to learn to think inside-the-box. In contrast, a key feature of mathematical thinking is thinking outside-the-box – a valuable ability in today’s world. This course helps to develop that crucial way of thinking.
nptel.ac.in
NPTEL is a government-backed online learning platform offering free video lectures and course materials from Indian institutes across engineering, science, and mathematics. It provides structured courses with readings, assignments, and certificates, including topics in logic and set theory.