A125955 Numbers k such that (2^k + 7^k)/9 is prime.

5, 23, 73, 101, 401, 419, 457, 811, 1163, 1511, 8011

Offset

1,1

Comments

All terms are primes. Corresponding primes of the form (2^k + 7^k)/9 are {1871, 3040971926676589439, 5469081705798319217773539465593130845206220817280793349743311, ...}.

a(12) > 10^5. - Robert Price, Aug 28 2012

Links

Table of n, a(n) for n=1..11.

Mathematica

Do[p=Prime[n]; f=(2^p+7^p)/9; If[PrimeQ[f], Print[{p, f}]], {n, 1, 1000}]

Prog

(PARI) is(n)=ispseudoprime((2^n+7^n)/9) \\ Charles R Greathouse IV, Jun 13 2017

Crossrefs

Cf. A000978 = numbers n such that (2^n + 1)/3 is prime. Cf. A057469 = numbers n such that (2^n + 3^n)/5 is prime. Cf. A082387 = numbers n such that (2^n + 5^n)/7 is prime.

Sequence in context: A064395 A230497 A138905 * A103478 A327976 A121868

Adjacent sequences: A125952 A125953 A125954 * A125956 A125957 A125958

Keyword

hard,more,nonn

Author

Alexander Adamchuk, Feb 06 2007

Extensions

More terms from Ryan Propper, Mar 23 2007

Status

approved