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
Smallest Product of Composite Numbers
833
1
Square Ending in Itself
833
1
Sum of Divisors
833
1
Trailing Zeros
750
2
Trailing Zeros of a Product
750
2
Counting Divisors
667
7
Counting Zeros in a Power of Ten
667
4
Denominator of a Factorial Ratio
667
4
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