

A089920


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


0




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^(p1)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^p2), 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



