73

This number is a prime.

The largest known prime that starts a chain (generated by the quadratic 73 - x - x², for x = 0, 1, 2, ... , 7) of smaller, increasingly distant primes with successive gap 2n, (n = 1, 2, 3, ..., 7): 73, 71, 67, 61, 53, 43, 31, 17. [Beedassy]