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A100534
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Number of partitions of 2*n into parts of two kinds.
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1
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1, 5, 20, 65, 185, 481, 1165, 2665, 5822, 12230, 24842, 49010, 94235, 177087, 326015, 589128, 1046705, 1831065, 3157789, 5374390, 9035539, 15018300, 24697480, 40210481, 64854575, 103679156, 164363280, 258508230, 403531208, 625425005
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OFFSET
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0,2
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LINKS
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FORMULA
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Expansion of q^(1/24) * eta(q^4)^5 / (eta(q)^5 * eta(q^8)^2) in powers of q. - Michael Somos, Sep 24 2011
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EXAMPLE
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G.f.: 1 + 5*x + 20*x^2 + 65*x^3 + 185*x^4 + 481*x^5 + 1165*x^6 + 2665*x^7 + ...
G.f.: 1/q + 5*q^23 + 20*q^47 + 65*q^71 + 185*q^95 + 481*q^119 + 1165*q^143 + ...
a(1)=5 because we have 2, 2', 11, 1'1 and 1'1'.
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MAPLE
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MATHEMATICA
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a[n_] := Sum[PartitionsP[k] PartitionsP[2 n - k], {k, 0, 2 n}]; Table[a[n], {n, 0, 30}] (* Jean-François Alcover, Nov 30 2015, adapted from Maple *)
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PROG
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(PARI) {a(n) = local(A); if( n<0, 0, A = x * O(x^n); polcoeff( eta(x^4 + A)^5 / (eta(x + A)^5 * eta(x^8 + A)^2), n))} /* Michael Somos, Sep 24 2011 */
(PARI) {a(n) = local(A); if( n<0, 0, n = 2*n; A = x * O(x^n); polcoeff( 1 / eta(x + A)^2, n))} /* Michael Somos, Sep 24 2011 */
(Magma)
m:=40;
f:= func< x | (&*[ (1-x^(4*n))^5/((1-x^n)^5*(1-x^(8*n))^2) : n in [1..m+2]]) >;
R<x>:=PowerSeriesRing(Rationals(), m);
(SageMath)
m=40
def f(x): return product( (1-x^(4*n))^5/((1-x^n)^5*(1-x^(8*n))^2) for n in range(1, m+2) )
P.<x> = PowerSeriesRing(QQ, prec)
return P( f(x) ).list()
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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