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%I #23 Jul 25 2021 18:32:07
%S 5,7,15,47,81,115,267,285,7641,19089,25831,32115,59811,70155,178715
%N Numbers k such that 5^k - 4 is prime.
%C a(15) > 10^5. - _Robert Price_, Feb 03 2014
%C a(16) > 2*10^5. - _Tyler NeSmith_, Jul 25 2021
%e 81 is present because 5^81 - 4 is prime.
%t Select[Range[10000], PrimeQ[5^# - 4] &] (* _Vincenzo Librandi_, Oct 03 2012 *)
%o (PARI) is(n)=ispseudoprime(5^n-4) \\ _Charles R Greathouse IV_, Feb 20 2017
%Y Cf. A164785.
%K nonn,hard,more
%O 1,1
%A _Andrey V. Kulsha_, Feb 07 2001
%E a(10)-a(14) from _Robert Price_, Feb 03 2014
%E a(15) from _Tyler NeSmith_, Jul 25 2021