Category
Number Theory
Primes, divisibility, and modular arithmetic worth the detour.
Number theory is the study of the integers, and it is where elementary tools go furthest. Divisibility, primes and modular arithmetic settle questions that look far out of reach.
Problems here range from digit puzzles and divisibility arguments up to congruences, Diophantine equations, and orders modulo n. Checking small cases is not cheating — it is usually how the pattern gets found.
Latest problems
137 problems
Common Elements of Two Arithmetic Sequences
1250
9
Coprime Compositions of Twenty
1495
15
Counting divisors of a factorial
1188
27
Counting n from a Floor-Ceiling Sum
1202
22
Divisible Binomial Coefficients
1115
20
Divisor Probability II
962
63
Divisors of a Central Binomial
1667
14
Divisors with Balanced Residues Mod 3
1667
11
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