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A156769 a(n) = denominator(2^(2*n-2)/factorial(2*n-1)). 13

%I #31 Sep 08 2022 08:45:41

%S 1,3,15,315,2835,155925,6081075,638512875,10854718875,1856156927625,

%T 194896477400625,49308808782358125,3698160658676859375,

%U 1298054391195577640625,263505041412702261046875

%N a(n) = denominator(2^(2*n-2)/factorial(2*n-1)).

%C Resembles A036279, the denominators in the Taylor series for tan(x). The first difference occurs at a(12).

%C The numerators of the two formulas for this sequence lead to A001316, Gould's sequence.

%C 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.

%C 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

%H G. C. Greubel, <a href="/A156769/b156769.txt">Table of n, a(n) for n = 1..250</a>

%F a(n) = denominator( Product_{k=1..n-1} (2/(k*(2*k+1)) ).

%F G.f.: (1/2)*z^(1/2)*sinh(2*z^(1/2)).

%F From _Johannes W. Meijer_, May 24 2009: (Start)

%F a(n) = abs(A117972(n))/A000265(n).

%F a(n) = A036279(n)*A089170(n-1). (End)

%F a(n) = A049606(2*n-1). - _Zhujun Zhang_, May 29 2019

%p a := n ->(2*n-1)!*2^(add(i,i=convert(n-1,base,2))-2*n+2); # _Peter Luschny_, May 02 2009

%t a[n_] := Denominator[4^(n-1)/(2n-1)!];

%t Array[a, 15] (* _Jean-François Alcover_, Jun 20 2018 *)

%o (Magma) [Denominator(4^(n-1)/Factorial(2*n-1)): n in [1..25]]; // _G. C. Greubel_, Jun 19 2021

%o (Sage) [denominator(4^(n-1)/factorial(2*n-1)) for n in (1..25)] # _G. C. Greubel_, Jun 19 2021

%Y Cf. A036279 Denominators in Taylor series for tan(x).

%Y Cf. A001316 Gould's sequence appears in the numerators.

%Y 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

%K easy,nonn

%O 1,2

%A _Johannes W. Meijer_, Feb 15 2009

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Last modified March 28 10:31 EDT 2024. Contains 371240 sequences. (Running on oeis4.)