%I #19 Dec 15 2017 17:36:44

%S 1,2,3,5,6,7,9,13,26,42,153,188,204,282,699,2886,11597

%N Numbers n such that 100^n + 10^n - 1 is prime.

%C a(1) through a(17) have been proved to be prime with WinPFGW or pfgw.

%C a(18) > 20000. - _Giovanni Resta_, Feb 19 2013

%C a(18) > 100000. - _James G. Merickel_ Jun 02 2014

%e a(3) = 3 because 100^3 + 10^3 - 1 = 1000999, a prime.

%t Select[Range[700], PrimeQ[100^# + 10^# - 1] & ] (* _Giovanni Resta_, Feb 19 2013 *)

%o (PARI) is(n)=ispseudoprime(100^n+10^n-1) \\ _Charles R Greathouse IV_, Feb 20 2017

%K more,nonn

%O 1,2

%A _Jason Earls_, Aug 13 2004

%E a(17) from _Giovanni Resta_, Feb 19 2013