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
Time Until Buzzers Sync Again
1313
22
Divisible by 63
1302
38
Heads, Tails, and a Row of Coins
1203
22
Linear congruence in jellybeans
1010
47
Dilating a Lattice Polygon
1410
22
Avoiding Multiples of Twenty-Seven
1488
30
A Pair of Mutually Dividing Primes
1713
12
Coins in a Line
1152
56
Browse every Number Theory problem →