561

This number is a composite.

561 is the smallest Carmichael number.

561² ± 2 are two consecutive primes. [Luhn]

The smallest Euler-Jacobi pseudoprime base 2 is 561. [Pinch]

561 is a sphenic number, i.e., a positive integer that is the product of three distinct prime numbers (3*11*17). [Spriggs]