%I

%S 3,5,7,4703,30113

%N Numbers n such that (9^n - 7^n)/2 is prime.

%C All terms are prime.

%C The corresponding primes: 193, 21121, 1979713, ...

%t Select[Range[1, 10000], PrimeQ[(9^# - 7^#)/2] &]

%o (PARI) for(n=1,10000, if(isprime((9^n - 7^n)/2), print1(n,", ")))

%Y Cf. A081202, A173718, A128346, A059803.

%K nonn,more,hard

%O 1,1

%A _Tim Johannes Ohrtmann_, May 13 2016