7933

This number is a prime.

Consider f(n) = (3^n + 7^n)/10, which is an integer when n is odd. For which primes n is f(n) also prime? This is a hard question since f(n) grows rapidly, so only the first 9 values of n are known, and 7933 is one of them. The full list is: 3, 13, 31, 313, 3709, 7933, 14797, 39689, and 39333. The prime f(7933) has 6704 digits. [Keith]