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A linear Diophantine equation ax + by = c has integer solutions if and only if gcd(a, b) divides c.
Euler's Totient Function φ(n) counts how many integers from 1 to n are coprime with n.
Modular arithmetic is arithmetic with wrap-around at a fixed modulus m, like numbers on a clock.
A modular inverse of a modulo m is a number a_inv such that a × a_inv ≡ 1 (mod m).
The Chinese Remainder Theorem (CRT) reconstructs an integer from its remainders modulo pairwise coprime moduli and guarantees a unique answer modulo the product.
Prime factorization expresses any integer greater than 1 as a product of primes raised to powers, uniquely up to ordering.
The Extended Euclidean Algorithm finds integers x and y such that ax + by = gcd(a, b) while also computing gcd(a, b).
The greatest common divisor (gcd) of two integers is the largest integer that divides both without a remainder.