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
Estimating Square Roots
667
5
The Largest Prime Factor in a Factorial Sum
1542
6
Three Times Its Own Digits
750
1
Units Digit of a Sum
667
4
Integer sequence and perfect squares II
1750
8
Integer Solution
823
29
Twenty-Seven from Negative Seven
1275
20
Triples satisfy modulo primes condition
1693
14
Browse every Number Theory problem →