Brilliant.org - Modular Arithmetic
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Brilliant's community wiki page on modular arithmetic, explaining congruence notation, addition and multiplication rules, and modular exponentiation through worked examples. Readers practice reducing large expressions mod n and solving short competition-style problems on remainders and divisibility.
More resources on Modular Arithmetic
Wolfram MathWorld
MathWorld is an online mathematics encyclopedia from Wolfram Research offering detailed, browsable articles on topics across the math spectrum, including algebra, geometry, calculus, and number theory. Each entry includes definitions, theorems, formulas, diagrams, worked examples, and links to further reading.
Wikipedia - Modular Arithmetic
Encyclopedia article covering congruence relations, residue classes, modular inverses, and the ring of integers modulo n, with links to Fermat's little theorem and the Chinese remainder theorem. Useful as a reference for checking definitions, notation, and applications in cryptography and computing.
Art of Problem Solving Wiki - Modular Arithmetic
Art of Problem Solving's wiki entry on modular arithmetic, written for math-competition students. It defines congruences, shows how to compute with them, and works through AMC and AIME-style problems, so learners can use remainders to solve divisibility and digit problems.
Number Theory
Four-week UC San Diego course building from division with remainder and modular arithmetic to Euclid's algorithm and RSA. Finishing it, you can encrypt and decrypt messages and break RSA implementations that reuse or mishandle keys.