A128337 Numbers k such that (7^k + 5^k)/12 is prime.

11, 31, 173, 271, 547, 1823, 2111, 5519, 7793, 22963, 41077, 49739

Offset: 1

Author

Alexander Adamchuk, Feb 27 2007

Keywords

• hard
• more
• nonn

Comments

All terms are primes.

a(13) > 10^5. - Robert Price, Nov 20 2012

Crossrefs

Cf. A057171, A082387, A122853, A128335, A128336, A128338, 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=7; Do[p=Prime[n]; f=(k^p+5^p)/(k+5); If[ PrimeQ[f], Print[p] ], {n,1,100}]

PARI

is(n)=isprime((7^n+5^n)/12) \\ Charles R Greathouse IV, Feb 17 2017

Extensions

a(6)-a(12) from Robert Price, Nov 20 2012