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 A177735 a(0)=1, a(n)=A002445(n)/6 for n>=1. 3
 1, 1, 5, 7, 5, 11, 455, 1, 85, 133, 55, 23, 455, 1, 145, 2387, 85, 1, 319865, 1, 2255, 301, 115, 47, 7735, 11, 265, 133, 145, 59, 9464455, 1, 85, 10787, 5, 781, 23350145, 1, 5, 553, 38335, 83, 567385, 1, 10235, 45353, 235, 1, 750295, 1, 5555, 721, 265, 107 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS For n>=1: denominators of the Bernoulli numbers (A002445) divided by 6. All entries are odd. a(n)= A002445(n) / A020793(n). 5 divides a(2*n) for n>=1. These numbers also equal to the lengths of the repeating patterns for the excluded integer values of c/6, when both p^n + c and p^n - c are prime, for an infinite number of primes p>2, and a given integer n>0, arising from the union of one or more prime-based modulo cycles, determined by the divisors of n. See A005097 for details and connection to the von Staudt-Clausen Theorem below. - Richard R. Forberg, Jul 19 2016 LINKS Harvey P. Dale, Table of n, a(n) for n = 0..1000 C. M. Bender and K. A. Milton, Continued fraction as a discrete nonlinear transform, arXiv:hep-th/9304062, 1993. Eric Weisstein's World of Mathematics, von Staudt-Clausen Theorem FORMULA a(n) = denominator(BernoulliB(2*n, 1/2))/(3*2^(2*n)). - Jean-François Alcover, Apr 16 2013 A simple direct calculation of the denominators, for n>=1, is based on the von Staudt-Clausen Theorem:  Product{d|n}(2d+1), for d>1 and 2d+1 prime. See in the Mathematica section below. - Richard R. Forberg, Jul 19 2016 MAPLE A002445 := proc(n) bernoulli(2*n) ; denom(%) ; end proc: A177735 := proc(n) if n = 0 then 1; else A002445(n)/6 ; end if; end proc: seq(A177735(n), n=0..60) ; # R. J. Mathar, Aug 15 2010 MATHEMATICA Join[{1}, Denominator[BernoulliB[Range[2, 120, 2]]]/6] (* Harvey P. Dale, Oct 19 2012 *) result = {}; Do[prod = 1; Do[If[PrimeQ[2*Divisors[n][[i]] + 1], prod *= (2*Divisors[n][[i]] + 1)], {i, 2, Length[Divisors[n]]}]; AppendTo[result, prod] , {n, 1, 100}]  ; result (* Richard R. Forberg, Jul 19 2016 *) PROG (PARI) a(n)= {     my(bd=1);     forprime (p=5, 2*n+1, if( (2*n)%(p-1)==0, bd*=p ) );     bd; } /* Joerg Arndt, May 06 2012 */ (PARI) a(n)=if(n<2, return(1)); my(s=1); fordiv(n, d, if(isprime(2*d+1) && d>1, s *= 2*d+1)); s \\ Charles R Greathouse IV, Jul 20 2016 (Sage) def A177735(n):     if n == 0: return 1     M = map(lambda i: i+1, divisors(2*n))     return mul(filter(lambda s: is_prime(s), M))//6 print([A177735(n) for n in (0..53)]) # Peter Luschny, Feb 20 2016 CROSSREFS Cf. A002445, A165949. Sequence in context: A121595 A226660 A125294 * A139428 A303574 A063005 Adjacent sequences:  A177732 A177733 A177734 * A177736 A177737 A177738 KEYWORD nonn AUTHOR Paul Curtz, May 12 2010 STATUS approved

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Last modified October 21 23:34 EDT 2021. Contains 348160 sequences. (Running on oeis4.)