A228558 Numbers k such that (17^k + 4^k)/21 is prime.

13, 61, 67, 107, 383, 647, 3571, 37967

Offset: 1

Author

Robert Price, Aug 25 2013

Keywords

• hard
• nonn
• more

Comments

Conjecture: All terms are prime.

a(9) > 10^5.

Crossrefs

Cf. A227049, A228225, A227046.

Programs

Mathematica

Select[ Prime[ Range[1, 100000] ], PrimeQ[ (17^# + 4^#)/21 ]& ]

PARI

is(n)=ispseudoprime((17^n+4^n)/21) \\ Charles R Greathouse IV, May 22 2017