A225397 - OEIS

%I #9 Jun 13 2017 10:31:43

%S 31,109,373,409,619,823,1531,6637,70687

%N Numbers n such that (16^n + 5^n)/21 is prime.

%C All terms are primes.

%C a(10) > 10^5.

%t k=16; Do[p=Prime[n]; f=(k^p+5^p)/(k+5); If[ PrimeQ[f], Print[p] ], {n, 1, 9592}]

%o (PARI) is(n)=ispseudoprime((16^n+5^n)/21) \\ _Charles R Greathouse IV_, Jun 13 2017

%Y Cf. A128340, A128341, A128342, A128343, A227046, A128073.

%K hard,more,nonn

%O 1,1

%A _Robert Price_, Jul 25 2013