OFFSET
1,1
COMMENTS
a(15) > 10^5. - Robert Price, Feb 03 2014
a(16) > 2*10^5. - Tyler NeSmith, Jul 25 2021
EXAMPLE
81 is present because 5^81 - 4 is prime.
MATHEMATICA
Select[Range[10000], PrimeQ[5^# - 4] &] (* Vincenzo Librandi, Oct 03 2012 *)
PROG
(PARI) is(n)=ispseudoprime(5^n-4) \\ Charles R Greathouse IV, Feb 20 2017
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Andrey V. Kulsha, Feb 07 2001
EXTENSIONS
a(10)-a(14) from Robert Price, Feb 03 2014
a(15) from Tyler NeSmith, Jul 25 2021
STATUS
approved