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 Multiples II
1000
3
Counting Reduced Fractions
1000
3
Counting Relatively Prime Numbers
1000
2
Day of the Week Calculation
1000
3
Divisor Probability III
1000
3
Even Numbers in Two Families
1000
3
Exponent System Value
1000
2
Largest Index Before a Sequence Becomes Non-Integer
1583
6
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