A225397 Numbers n such that (16^n + 5^n)/21 is prime.

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

Offset: 1

Author

Robert Price, Jul 25 2013

Keywords

• hard
• more
• nonn

Comments

All terms are primes.

a(10) > 10^5.

Crossrefs

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

Programs

Mathematica

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

PARI

is(n)=ispseudoprime((16^n+5^n)/21) \\ Charles R Greathouse IV, Jun 13 2017