%I #43 Oct 14 2024 23:57:35
%S 1,4,8,140,32060
%N Numbers k such that 7^k - 5^k + 1 is prime.
%C No further terms k <= 5000. - _Harvey P. Dale_, Jan 25 2011
%C No further terms k <= 10000. - _Tyler NeSmith_, May 06 2021
%C No further terms k <= 20000. - _Jon E. Schoenfield_, May 07 2021
%C No further terms k <= 100000. - _Lucas A. Brown_, Oct 14 2024
%t Select[Range[5000],PrimeQ[7^#-5^#+1]&] (* _Harvey P. Dale_, Jan 25 2011 *)
%o (Magma) [n: n in [0..1000]|IsPrime(7^n-5^n+1)];
%o (PARI) is(n)=ispseudoprime(7^n-5^n+1) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y Cf. A155640.
%K nonn,hard,more
%O 1,2
%A _Vincenzo Librandi_, Jan 22 2011
%E a(5) from _Michael S. Branicky_, Mar 31 2023