997

997 is a prime number. It is the largest three-digit prime.

997 is the smallest prime of the form 10^p - p, where p is prime (3).

997 is the smallest prime whose sum of digits (25) is an odd composite number.

997 is the smallest prime p such that the sum of the digits of p (9 + 9 + 7 = 25) is not prime, but the sum of the squares of the digits is prime (9² + 9² + 7² = 211).

997 is the smallest prime p such that the sum of the digits of p (9 + 9 + 7 = 25) is not prime, but the sum of the cubes of the digits is prime (9³ + 9³ + 7³ = 1801).

997 has a representation as a sum of two squares: 997 = 6² + 31².

997 is the hypotenuse of a primitive Pythagorean triple: 997² = 372² + 925².

Sourcea: Number Gossip and Prime Curios!