A147782 -id:A147782 - cp's OEIS frontend

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A089920 Indices of primes p such that 7^p - 2 is prime.

Original entry on oeis.org

1, 4, 11, 149

Offset: 1

Cino Hilliard, Jan 11 2004

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

Cf. A147782 (primes p such that 7^p - 2 is prime), A000720, A090669.

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 *)

forprime(p=2,1e4,if(ispseudoprime(7^p-2),print1(primepi(p),",")))

a(n) = A000720(A147782(n)). - Amiram Eldar, Jul 07 2024

Definition clarified by Harvey P. Dale, May 02 2011

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