%I

%S 1,2,3,4,5,6,12,17,43,46,50,88,110,187,472,832,834,1034,1311,1700,

%T 2756,5470,6124,7404,7891,12010

%N Numbers n such that 609 * 10^n - 1 is prime.

%e a(4)=4 because 609*10^4-1 = 6089999, a prime.

%o (PARI) is(n)=ispseudoprime(609*10^n-1) \\ _Charles R Greathouse IV_, Jun 13 2017

%Y Cf. A053580.

%K more,nonn

%O 1,2

%A _Jason Earls_, Jun 30 2005