%I #12 Aug 25 2016 17:34:38
%S 4,32,512,12288,393216,15728640,754974720,42278584320,2705829396480,
%T 194819716546560,15585577323724800,1371530804487782400,
%U 131666957230827110400,13693363552006019481600
%N a(n) = 4 * A051189(n).
%C A binomial sequence that produces Pi: 1/Pi= Binomial[2*n+1,n+1/2]/(2*n+1)!!
%H G. C. Greubel, <a href="/A153511/b153511.txt">Table of n, a(n) for n = 0..100</a>
%F a(n) = 4 * A051189(n).
%F From _Ilya Gutkovskiy_, Aug 22 2016: (Start)
%F E.g.f.: 4/(1 - 8*x).
%F a(n) ~ sqrt(Pi)*2^(3*n+5/2)*n^(n+1/2)/exp(n). (End)
%t Table[(2*n + 1)!!*Pi*Gamma[2*n + 2]/(Gamma[n + 3/2]^2), {n, 0, 20}]
%o (PARI) a(n) = 4*n!*8^n; \\ _Michel Marcus_, Aug 22 2016
%Y Cf. A051189.
%K nonn
%O 0,1
%A _Roger L. Bagula_, Dec 28 2008
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