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
Exclusive Divisibility Count
1138
20
Floor and Fractional Part Equation
1083
20
Harmonic Sum Numerator Modulo a Prime
1570
21
Large Power Remainder II
1178
18
Largest Number Not a Sum of Special Numbers
1500
6
Last Three Digits of a Large Power
1430
17
Last Three Non-Zero Digits of a Factorial
1667
10
Last Two Non-Zero Digits of a Factorial
1583
7
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