604

This number is a composite.

604 is the only three-digit number whose sum of its single and double factorials plus one is prime. I.e., 604! + 604!₂ + 1 is prime. [Opao]

Nebula NGC 604 was discovered by William Herschel in 1784. It is 3 million light years away. Note that 604*1784+3 is prime. [Opao]

The least n such that n-2, n-1, n, n+1, and n+2 are each 3-almost primes: 602 = 2 * 7 * 43, 603 = 3^2 * 67, 604 = 2^2 * 151, 605 = 5 * 11^2, and 606 = 2 * 3 * 101. [Post]