%I #28 Sep 06 2024 08:06:29

%S 6,8,9,11,15,31,48,60,314,701,940,942,2164,2981,3810,6971,12068,13641,

%T 16502,68800

%N Numbers k such that (5^p - 3^p)/2 is prime, where p = prime(k).

%C The corresponding primes p = prime(a(n)) are listed in A121877.

%C The corresponding primes of the form (5^p - 3^p)/2 are listed in A123705.

%C a(13) is greater than 1000. - _Farideh Firoozbakht_, Oct 11 2006

%F a(n) = primepi(A121877(n)).

%t Do[If[PrimeQ[(5^Prime[n] - 3^Prime[n])/2], Print[n]], {n, 1000}] (* _Robert G. Wilson v_, Jan 12 2007 *)

%t PrimePi[#]&/@Select[Prime[Range[1000]],PrimeQ[(5^#-3^#)/2]&] (* _Harvey P. Dale_, Sep 23 2018 *)

%Y Cf. A005059, A121877, A123705.

%K nonn,more

%O 1,1

%A _Alexander Adamchuk_, Oct 08 2006

%E More terms from _Farideh Firoozbakht_, Oct 11 2006

%E a(13)-a(16) computed from A121877 by _Jinyuan Wang_, Mar 23 2020

%E a(17)-a(19) from _Kellen Shenton_, May 18 2022

%E a(20) from _Amiram Eldar_, Sep 06 2024