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
Counting Solutions to a Fraction Equation
1483
14
A Power Modulo Thirty-Seven
1167
11
Divisible by Twelve, Not by Eight
1167
6
Divisor Power Sum
1083
4
Largest Prime Factor of a Power Minus One
1000
4
Last two digits of a large power
1167
5
Last Two Digits of a Power
1167
5
Modular Congruence II
1083
4
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