53113233473

This number is a prime.

                                                                5 3113233473

E. Rowland proved that if you set a(1) = 7, and for n ≥ 2, a(n) = a(n-1) + gcd(n,a(n-1)); then a(n) - a(n-1) (the "first differences") are all either 1 or prime. The list of prime generated by this formula begins 5, 3, 11, 3, 23, 3, 47, 3... (but it generates mostly 1s). [Caldwell]