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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 A358790 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 June 17 07:23 EDT 2024. Contains 373433 sequences. (Running on oeis4.)