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
Marble Remainder Problem
1000
2
Smallest Addition to Reach a Perfect Square
1000
2
Smallest k for Square and Cube
1000
2
Sum of Divisors II
1000
2
Sum of Divisors III
1000
2
Sum of Floor Square Roots
1000
7
Triangular Perfect Square
1000
2
A Factorial Sum Modulo One Hundred
917
11
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