A128338 Numbers k such that (8^k + 5^k)/13 is prime.

7, 19, 167, 173, 223, 281, 21647

Offset: 1

Author

Alexander Adamchuk, Feb 27 2007

Keywords

• hard
• more
• nonn

Comments

All terms are primes.

a(8) > 10^5. - Robert Price, Jan 21 2013

Crossrefs

Cf. A057171, A082387, A122853, A128335, A128336, A128337, A128339, A128340, A128341, A128342, A128343. Cf. A004061, A082182, A121877, A059802. Cf. A062572, A128344, A128345, A128346, A128347, A128348, A128349, A128350, A128351, A128352, A128353, A128354.

Programs

Mathematica

k=8; Do[p=Prime[n]; f=(k^p+5^p)/(k+5); If[ PrimeQ[f], Print[p] ], {n,1,100}]

PARI

is(n)=isprime((8^n+5^n)/13) \\ Charles R Greathouse IV, Feb 17 2017

Extensions

a(7) from Robert Price, Jan 21 2013