7

This number is a prime.

A Mersenne prime (2^p - 1) with exponent p = 3. The key prime factor of the perfect number 28. There are twenty-eight 3-digit numbers with digits summing to 7 (e.g., 601; 412) among which exactly 7 (e.g., 241; 313) are prime and also among which one (511) is of the form 2^q - 1 where q = 3 * 3. The seven 3-digit primes sum to 2281, a prime with an embedded "28" and with first and last digits summing to 3. Note that the number 2281 is also the exponent of the 17th Mersenne prime.