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A132310
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a(n) = 3^n*Sum_{ k=0..n } binomial(2*k,k)/3^k.
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13
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1, 5, 21, 83, 319, 1209, 4551, 17085, 64125, 240995, 907741, 3428655, 12990121, 49370963, 188229489, 719805987, 2760498351, 10615101273, 40920439119, 158106581157, 612166272291, 2374756691313, 9228369037659, 35918537840577
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OFFSET
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0,2
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COMMENTS
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Simpler definition from N. J. A. Sloane, Jan 21 2009. Colin Mallows and I studied this sequence on Feb 21 1981 in connection with integration over a regular (solid) hexagon.
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LINKS
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FORMULA
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a(n) = C(2n,n) * sum_{k=0..2n} trinomial(n,k)/C(2n,k) where trinomial(n,k) = [x^k] (1 + x + x^2)^n, where [x^k] denotes "coefficient of x^k in ...".
G.f.: A(x) = 1/sqrt(1 - 10*x + 33*x^2 - 36*x^3).
a(n) = sum_{k=0..2n} trinomial(n,k) * k!*(2*n-k)! / (n!)^2.
D-finite with recurrence: n*a(n) = (7*n-2)*a(n-1) - 6*(2*n-1)*a(n-2) . - Vaclav Kotesovec, Oct 20 2012
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EXAMPLE
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a(1) = C(2,1)*(1/1 + 1/2 + 1/1) = 2*(5/2) = 5;
a(2) = C(4,2)*(1/1 + 2/4 + 3/6 + 2/4 + 1/1) = 6*(7/2) = 21;
a(3) = C(6,3)*(1/1 + 3/6 + 6/15 + 7/20 + 6/15 + 3/6 + 1/1) = 20*(83/20) = 83.
2*a(6) = sum(A182411(7,i), i=0..6) = 3432+858+572+572+728+1092+1848 = 9102 = 2*4551. - Bruno Berselli, May 02 2012
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MATHEMATICA
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CoefficientList[Series[1/Sqrt[1-10*x+33*x^2-36*x^3], {x, 0, 20}], x] (* Vaclav Kotesovec, Oct 20 2012 *)
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PROG
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(PARI) a(n)=binomial(2*n, n)*sum(k=0, 2*n, polcoeff((1+x+x^2)^n, k)/binomial(2*n, k) )
(PARI) a(n)=sum(k=0, 2*n, polcoeff((1+x+x^2)^n, k) * k!*(2*n-k)! / (n!)^2 )
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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