Owlet puzzle #5

Released Friday, July 10, 2026 UTC

561

the answer for puzzle #5

The five clues

A three-digit odd number.
It is not prime, yet passes Fermat's compositeness test for every base coprime to it.
It is a squarefree product of three primes.
561 = 3 · 11 · 17.
The smallest Carmichael number.

First Carmichael number. Alford, Granville, and Pomerance proved infinitely many exist (1994).