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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
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
Sequence in context: A112755 A239278 A284721 * A246917 A102294 A358790
KEYWORD
hard,more,nonn
AUTHOR
Alexander Adamchuk, Feb 09 2007, Feb 10 2007
STATUS
approved

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Last modified June 17 07:23 EDT 2024. Contains 373433 sequences. (Running on oeis4.)