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 II
1343
22
Finding a Number from a Digit-Pair Condition
1440
26
Numbers with a Specific Prime Factorization Pattern
1382
18
Values Making Two Expressions Perfect Squares
1378
12
Counting Numbers Whose Double Is Also a Palindrome
1333
12
Positive Divisors and Möbius Function
1500
9
Primes Dividing the Fermat-like Form $x^4 + 4$
1667
13
Sum of Repeating Decimal Digits
1260
28
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