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
Parity of Power-Sum Divisors
1685
15
A Fraction Between Fractions
1635
19
Counting Values of a Sum of Floor Functions
1667
13
Evaluating a Sum of Floor Logarithms
1468
25
Counting Fractions with a Specific Numerator
1392
52
Sum of Units Digits of Perfect Squares
1205
31
Counting Integers Satisfying a Floor Inequality
1650
24
Counting n Making an Expression a Perfect Square
1660
17
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