

A126659


Least number k > 0 such that ((2n1)^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
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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(2n1).


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*n1)^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



