OFFSET
1,2
COMMENTS
Except for p=2, 2, 5^p - 2 cannot be prime. This immediately follows from the fact that a number N = (3k+2)^p - 2 cannot be prime for p > 2 because N = 3H + 2^p - 2 = 3H + 2(2^(p-1)-1) is divisible by 3.
a(5) > 8742, if it exists (cf. J.W.L. (Jan) Eerland's comment in A147782). - Amiram Eldar, Jul 07 2024
a(5) > 23967, if it exists (using A090669). - Michael S. Branicky, Aug 17 2024
FORMULA
MATHEMATICA
Select[Range[500], PrimeQ[7^Prime[#]-2]&] (* Harvey P. Dale, May 02 2011 *)
Position[Prime[Range[150]], _?(PrimeQ[7^#-2]&)]//Flatten (* Harvey P. Dale, May 11 2016 *)
PROG
(PARI) forprime(p=2, 1e4, if(ispseudoprime(7^p-2), print1(x", ")))
CROSSREFS
KEYWORD
nonn,more,changed
AUTHOR
Cino Hilliard, Jan 11 2004
EXTENSIONS
Definition clarified by Harvey P. Dale, May 02 2011.
STATUS
approved