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A126659 Least number k > 0 such that ((2n-1)^k + 1)/(2n) is prime, or 0 if no such prime exists. 17
3, 5, 3, 3, 5, 3, 3, 7, 17, 3, 11, 3, 0, 7, 109, 5, 11, 5, 3, 17, 5, 103, 5, 7, 3, 21943, 3, 53, 17, 7, 3, 19, 3, 11, 5, 7, 5, 37, 3, 3, 19, 167, 7, 13, 3, 89, 43 (list; graph; refs; listen; history; text; internal format)
OFFSET

2,1

COMMENTS

All terms are odd primes.

a(49) > 143800. - Robert Price, Mar 29 2019

LINKS

Table of n, a(n) for n=2..48.

H. Dubner and T. Granlund, Primes of the Form (b^n+1)/(b+1), J. Integer Sequences, 3 (2000), #P00.2.7.

Eric Weisstein's World of Mathematics, Repunit.

FORMULA

a(n) = A084742(2n-1).

MATHEMATICA

A126659[n_] := Module[{k = 1}, If[n == 14, Return[0]]; While[! PrimeQ[((2 n - 1)^k + 1)/(2 n)], k++]; k]; Table[A126659[n], {n, 2, 48}] (* Robert Price, Oct 29 2018 *)

PROG

(PARI) a(n) = {if (n==14, return(0)); my(k=3); while (! isprime(((2*n-1)^k + 1)/(2*n)), k = nextprime(k+1)); k; } \\ Michel Marcus, Nov 23 2018

CROSSREFS

Cf. A084741, A084742, A125954, A125958.

Sequence in context: A112755 A239278 A284721 * A246917 A102294 A021287

Adjacent sequences:  A126656 A126657 A126658 * A126660 A126661 A126662

KEYWORD

hard,more,nonn

AUTHOR

Alexander Adamchuk, Feb 09 2007, Feb 10 2007

STATUS

approved

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Last modified January 23 19:45 EST 2020. Contains 331175 sequences. (Running on oeis4.)