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A023916
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Theta series of A*_4 lattice.
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4
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1, 0, 10, 20, 0, 20, 0, 60, 50, 0, 30, 0, 60, 120, 0, 60, 0, 160, 70, 0, 60, 0, 120, 220, 0, 120, 0, 200, 180, 0, 40, 0, 210, 240, 0, 180, 0, 360, 200, 0, 150, 0, 120, 420, 0, 140, 0, 460, 220, 0, 130, 0, 360, 520, 0, 240, 0, 400, 300, 0, 180, 0, 320, 420, 0, 360, 0, 660, 480, 0, 120
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OFFSET
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0,3
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REFERENCES
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J. H. Conway and N. J. A. Sloane, "Sphere Packings, Lattices and Groups", Springer-Verlag, p. 114.
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LINKS
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FORMULA
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Expansion of f(-x)^5 / f(-x^5) + 5 * x * f(-x^5)^5 / f(-x) in powers of x where f() is a Ramanujan theta function. - Michael Somos, Jan 29 2011
G.f. is a period 1 Fourier series which satisfies f(-1 / (5 t)) = 5^(1/2) (t/i)^2 g(t) where q = exp(2*Pi*i*t) and g() is g.f. for A008444. - Michael Somos, Jan 29 2011
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EXAMPLE
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1 + 10*x^2 + 20*x^3 + 20*x^5 + 60*x^7 + 50*x^8 + 30*x^10 + 60*x^12 + ...
1 + 10*q^4 + 20*q^6 + 20*q^10 + 60*q^14 + 50*q^16 + 30*q^20 + 60*q^24 + 120*q^26 + 60*q^30 + 160*q^34 + 70*q^36 + 60*q^40 + 120*q^44 + 220*q^46 + 120*q^50 + 200*q^54 + 180*q^56 + 40*q^60 + O(q^62).
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MATHEMATICA
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a[n_] := Module[{A = x*O[x]^n}, SeriesCoefficient[QPochhammer[x+A]^5 / QPochhammer[x^5+A] + 5*x*(QPochhammer[x^5+A]^5 / QPochhammer[x+A]), {x, 0, n}]]; Table[a[n], {n, 0, 80}] (* Jean-François Alcover, Nov 05 2015, adapted from PARI *)
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PROG
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(Magma) L:=Lattice("A", 4); D:=Dual(L); T1<q> := ThetaSeries(D, 60);
(PARI) {a(n) = local(A); if( n<0, 0, A = x * O(x^n); polcoeff( eta(x + A)^5 / eta(x^5 + A) + 5 * x * eta(x^5 + A)^5 / eta(x + A), n))}; /* Michael Somos, Jan 29 2011 */
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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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