ocw.mit.edu
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MIT OpenCourseWare (ocw.mit.edu) is a free, open online repository of MIT course materials across disciplines, including mathematics. It offers lecture notes, assignments, exams, and sometimes video lectures for many courses, enabling self-study in topics like Number Theory.
More resources on Number Theory
Discrete Mathematics and its Applications - Kenneth Rosen
Comprehensive introduction to discrete structures, logic, and combinatorics.
mathworld.wolfram.com
MathWorld is a comprehensive online mathematics encyclopedia from Wolfram Research, offering detailed articles across all branches of math—from number theory to calculus—with definitions, theorems, formulas, proofs, and illustrative diagrams plus extensive cross-references.
khanacademy.org
Khan Academy is a free online learning platform offering video tutorials, interactive exercises, and practice problems across a wide range of math topics, including number theory. It covers fundamentals such as primes, divisibility, modular arithmetic, and related problem-solving techniques, with self-paced lessons and progress tracking.