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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.
a(5) > 8742, if it exists (cf. J.W.L. (Jan) Eerland's comment in A147782). - Amiram Eldar, Jul 07 2024
LINKS
FORMULA
a(n) = A000720(A147782(n)). - Amiram Eldar, Jul 07 2024
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), A000720.
Sequence in context: A018242 A006248 A119571 * A303881 A305277 A118197
KEYWORD
nonn,more
AUTHOR
Cino Hilliard, Jan 11 2004
EXTENSIONS
Definition clarified by Harvey P. Dale, May 02 2011.
STATUS
approved

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Last modified July 24 10:37 EDT 2024. Contains 374583 sequences. (Running on oeis4.)