Elementary Number Theory (Raji)
by Wissam Raji · LibreTexts
Open-access undergraduate textbook by Wissam Raji of the American University of Beirut. Opens with the division algorithm, greatest common divisors, the Euclidean algorithm and Bezout's identity, then builds to primes, unique factorization, congruences and multiplicative functions.
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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.
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Community-maintained Art of Problem Solving wiki page defining divisibility of integers and collecting the standard divisibility rules for small divisors, with links to related number theory articles and competition problems useful for MATHCOUNTS and AMC preparation.
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