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

Author

Alexander Adamchuk, Feb 06 2007

Keywords

• hard
• more
• nonn

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

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.

Programs

Mathematica

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

PARI

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

Extensions

More terms from Ryan Propper, Mar 23 2007