The OEIS is supported by the many generous donors to the OEIS Foundation.

 Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 59th year, we have over 358,000 sequences, and we’ve crossed 10,300 citations (which often say “discovered thanks to the OEIS”). Other ways to Give
 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A156769 a(n) = denominator(2^(2*n-2)/factorial(2*n-1)). 13
 1, 3, 15, 315, 2835, 155925, 6081075, 638512875, 10854718875, 1856156927625, 194896477400625, 49308808782358125, 3698160658676859375, 1298054391195577640625, 263505041412702261046875 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Resembles A036279, the denominators in the Taylor series for tan(x). The first difference occurs at a(12). The numerators of the two formulas for this sequence lead to A001316, Gould's sequence. Stephen Crowley indicated on Aug 25 2008 that a(n) = denominator(Zeta(2*n)/Zeta(1-2*n)) and here numerator((Zeta(2*n)/Zeta(1-2*n))/(2*(-1)^(n)*(Pi)^(2*n))) leads to Gould's sequence. This sequence appears in the Eta and Zeta triangles A160464 and A160474. Its resemblance to the sequence of the denominators of the Taylor series for tan(x) led to the conjecture A156769(n) = A036279(n)*A089170(n-1). - Johannes W. Meijer, May 24 2009 LINKS G. C. Greubel, Table of n, a(n) for n = 1..250 FORMULA a(n) = denominator( Product_{k=1..n-1} (2/(k*(2*k+1)) ). G.f.: (1/2)*z^(1/2)*sinh(2*z^(1/2)). From Johannes W. Meijer, May 24 2009: (Start) a(n) = abs(A117972(n))/A000265(n). a(n) = A036279(n)*A089170(n-1). (End) a(n) = A049606(2*n-1). - Zhujun Zhang, May 29 2019 MAPLE a := n ->(2*n-1)!*2^(add(i, i=convert(n-1, base, 2))-2*n+2); # Peter Luschny, May 02 2009 MATHEMATICA a[n_] := Denominator[4^(n-1)/(2n-1)!]; Array[a, 15] (* Jean-François Alcover, Jun 20 2018 *) PROG (Magma) [Denominator(4^(n-1)/Factorial(2*n-1)): n in [1..25]]; // G. C. Greubel, Jun 19 2021 (Sage) [denominator(4^(n-1)/factorial(2*n-1)) for n in (1..25)] # G. C. Greubel, Jun 19 2021 CROSSREFS Cf. A036279 Denominators in Taylor series for tan(x). Cf. A001316 Gould's sequence appears in the numerators. Cf. A000265, A036279, A089170, A117972, A160464, A160469 (which resembles the numerators of the Taylor series for tan(x)), A160474. - Johannes W. Meijer, May 24 2009 Sequence in context: A090627 A070234 A036279 * A333691 A029758 A103031 Adjacent sequences: A156766 A156767 A156768 * A156770 A156771 A156772 KEYWORD easy,nonn AUTHOR Johannes W. Meijer, Feb 15 2009 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified December 8 07:38 EST 2022. Contains 358691 sequences. (Running on oeis4.)