609

This number is a composite.

100^609 - 609 is prime. Note that 609 is the third number with this property. [Firoozbakht]

n = 609 = 5^2^2 - 2^2^2 is the (5^2)th natural number with the property p = n^2 + 2 is prime; additionally (p - 2^2, p) is a pair of prime cousins. Note that n^2 - (2*5) and n^2 + (2*5) are primes too! [Herault]

π(609) is strobogrammatic. The only known case greater than 1. [Resta]