A215487 Numbers k such that (7^k - 2^k)/5 is prime.

3, 7, 19, 79, 431, 1373, 1801, 2897, 46997, 341063, 508867, 720497, 846913

Offset: 1

Author

Robert Price, Aug 12 2012

Keywords

• hard
• more
• nonn

Comments

All terms are prime.

a(14) > 10^6.

Links

• Jon Grantham and Andrew Granville, Fibonacci primes, primes of the form 2^n-k and beyond, arXiv:2307.07894 [math.NT], 2023.

Crossrefs

Cf. A004063, A028491, A057468, A059801, A121877, A128024, A128025, A128026, A128027, A128028, A128029, A128030, A128031, A128032.

Programs

Mathematica

Select[ Prime[ Range[1, 300] ], PrimeQ[ (7^# - 2^#)/5 ]& ]

PARI

is(n)=ispseudoprime((7^n-2^n)/5) \\ Charles R Greathouse IV, Jun 13 2017

Extensions

a(10)-a(13) from Jon Grantham, Jul 29 2023