A180745 Numbers k such that 7^k - 5^k + 1 is prime.

1, 4, 8, 140, 32060

Offset: 1

Author

Vincenzo Librandi, Jan 22 2011

Keywords

• nonn
• hard
• more

Comments

No further terms k <= 5000. - Harvey P. Dale, Jan 25 2011

No further terms k <= 10000. - Tyler NeSmith, May 06 2021

No further terms k <= 20000. - Jon E. Schoenfield, May 07 2021

No further terms k <= 100000. - Lucas A. Brown, Oct 14 2024

Crossrefs

Cf. A155640.

Programs

Magma

[n: n in [0..1000]|IsPrime(7^n-5^n+1)];

Mathematica

Select[Range[5000],PrimeQ[7^#-5^#+1]&]  (* Harvey P. Dale, Jan 25 2011 *)

PARI

is(n)=ispseudoprime(7^n-5^n+1) \\ Charles R Greathouse IV, Jun 13 2017

Extensions

a(5) from Michael S. Branicky, Mar 31 2023