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A065854 Smallest prime q such that (p^q-1)/(p-1) is a prime, where p = prime(n). 6
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 (list; graph; refs; listen; history; text; internal format)
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

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Last modified January 25 01:49 EST 2020. Contains 331229 sequences. (Running on oeis4.)