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A089920 Indices of primes p such that 7^p - 2 is prime. 0
1, 4, 11, 149 (list; graph; refs; listen; history; text; internal format)
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.

LINKS

Table of n, a(n) for n=1..4.

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

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

Sequence in context: A018242 A006248 A119571 * A303881 A305277 A118197

Adjacent sequences:  A089917 A089918 A089919 * A089921 A089922 A089923

KEYWORD

nonn

AUTHOR

Cino Hilliard, Jan 11 2004

EXTENSIONS

Definition clarified by Harvey P. Dale, May 02 2011.

STATUS

approved

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Last modified February 24 04:36 EST 2020. Contains 332197 sequences. (Running on oeis4.)