666

This number is a composite.

If we define S(n) as the sum of the digits of n, then we can write that S((prime(6+6+6))⁶) = S(prime(6+6+6)) + S(prime(666)) + S(prime(6*6*6)), where all the terms are prime. Note that 666 = (6+6+6) * S((prime(6+6+6))⁶). [Capelle]