%I #22 Mar 07 2021 02:12:59
%S 2,7,11,31,47,139,263,769,20551,21491,112247
%N Numbers k such that (19^k - 4^k)/15 is prime.
%C All terms are prime.
%C a(12) > 2*10^5.
%t Select[Prime[Range[1, 100000]], PrimeQ[(19^# - 4^#)/15]&]
%o (PARI) is(n)=ispseudoprime((19^n-4^n)/15) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y Cf. A224691, A128354, A128032.
%K nonn,hard,more
%O 1,1
%A _Robert Price_, Aug 09 2013
%E a(11) from _Robert Price_, Mar 06 2021