A269544 Numbers n such that 7*8^n + 1 is prime.

2, 40, 58, 60, 130, 144, 752, 7462, 18162, 69028, 187272, 268178, 270410, 497284, 713304, 722600, 1005254

Offset: 1

Author

Robert Price, Feb 29 2016

Keywords

• nonn
• hard
• more

Comments

a(n) is even because 7*8^(2*k+1) + 1 is divisible by 3.

Crossrefs

Cf. A056799, A245241, A268061.

Programs

Mathematica

Select[Range[0, 200000,2], PrimeQ[7*8^# + 1] &]

PARI

is(n)=ispseudoprime(7*8^n+1) \\ Charles R Greathouse IV, Jun 13 2017

Formula

a(n) = A032353(n)/3 where this is an integer.