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A331609
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Number of compositions of n with the multiplicity of the first part even.
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3
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0, 1, 0, 4, 4, 14, 20, 56, 98, 224, 420, 902, 1764, 3664, 7258, 14824, 29596, 59942, 120012, 241944, 484946, 975216, 1955244, 3926078, 7870980, 15790272, 31650090, 63456208, 127162580, 254845446, 510582236, 1022940392, 2049048890, 4104264424, 8219808108
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OFFSET
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1,4
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LINKS
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FORMULA
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G.f.: (1-x)/(1-2*x) - Sum_{i>=1} ((x-1)*x^i*(-x^(i+1)+x^i-2*x+1)) / ((2*x-1) * (-2*x^(i+1)+2*x^i-2*x+1)).
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EXAMPLE
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For n=4, a(4)=4 and counts 2+2, 1+2+1, 1+1+2 and 1+1+1+1.
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MAPLE
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b:= proc(n, p, t) option remember; `if`(n=0, t,
add(b(n-j, p, `if`(p=j, 1-t, t)), j=1..n))
end:
a:= n-> add(b(n-j, j, 0), j=1..n):
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MATHEMATICA
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gf[x_] := (1 - x)/(1 - 2 x) - Sum[ ((x - 1) x^i (-x^(i + 1) + x^i - 2 x + 1)) / ((2 x - 1) (-2 x^(i + 1) + 2 x^i - 2 x + 1)), {i, 1, 40}];
CL := CoefficientList[Series[gf[x], {x, 0, 35}], x]; Drop[CL, 1] (* Peter Luschny, Jan 23 2020 *)
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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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