A065854 Smallest prime q such that (p^q-1)/(p-1) is a prime, where p = prime(n).

2, 3, 3, 5, 17, 5, 3, 19, 5, 5, 7, 13, 3, 5, 127, 11, 3, 7, 19, 3, 5, 5, 5, 3, 17, 3, 19, 17, 17, 23, 5, 3, 11, 163, 7, 13, 17, 7, 3, 3, 19, 17, 17, 5, 31, 577, 41, 239, 5, 11, 113, 5, 17, 7, 23, 5

Offset

1,1

Comments

a(n) = 2*A065813(n) + 1, n > 1.

Links

Table of n, a(n) for n=1..56.

Andy Steward, Titanic Prime Generalized Repunits

Mathematica

Do[p = Prime[n]; k = 1; While[ !PrimeQ[ (p^Prime[k] - 1)/(p - 1)], k++ ]; Print[ Prime[k]], {n, 1, 56} ]

Prog

(PARI) { allocatemem(932245000); for (n=1, 100, p=prime(n); q=2; while (!isprime((p^q - 1)/(p - 1)), q=nextprime(q + 1)); write("b065854.txt", n, " ", q) ) } \\ Harry J. Smith, Nov 01 2009

Crossrefs

Cf. A084740 (least k such that (n^k-1)/(n-1) is prime).

Sequence in context: A275914 A154923 A154693 * A263769 A064776 A270592

Adjacent sequences: A065851 A065852 A065853 * A065855 A065856 A065857

Keyword

hard,nonn

Author

Vladeta Jovovic, Nov 26 2001

Status

approved