653

653 is a prime number. It is a Sophie Germain prime because 2 x 653 + 1 = 1307 is a prime.

653 is the first three-digit prime number to occur in the decimal expansion of pi.

653 has a representation as a sum of two squares: 653 = 13² + 22².

653 is the hypotenuse of a primitive Pythagorean triple: 653² = 315² + 572².

653 is the sum of the squares of five consecutive primes: 653 = 5² + 7² + 11² + 13² + 17².

ARINC 653 is a software specification for space and time partitioning in safety-critical avionics real-time operating systems.

Source: Number Gossip