A228558 - OEIS

%I #10 Oct 01 2021 17:17:26

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

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

%C Conjecture: All terms are prime.

%C a(9) > 10^5.

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

%o (PARI) is(n)=ispseudoprime((17^n+4^n)/21) \\ _Charles R Greathouse IV_, May 22 2017

%Y Cf. A227049, A228225, A227046.

%K hard,nonn,more

%O 1,1

%A _Robert Price_, Aug 25 2013