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A296953
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Number of bisymmetric, quasitrivial, and order-preserving binary operations on the n-element set {1,...,n}.
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1
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0, 1, 4, 10, 22, 46, 94, 190, 382, 766, 1534, 3070, 6142, 12286, 24574, 49150, 98302, 196606, 393214, 786430, 1572862, 3145726, 6291454, 12582910, 25165822, 50331646, 100663294, 201326590, 402653182, 805306366, 1610612734, 3221225470, 6442450942, 12884901886
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OFFSET
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0,3
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COMMENTS
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LINKS
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FORMULA
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a(0)=0, a(1)=1, a(n+1)-2*a(n) = 2.
G.f.: x*(1 + x) / ((1 - x)*(1 - 2*x)).
a(n) = 3*2^(n-1) - 2 for n>0.
a(n) = 3*a(n-1) - 2*a(n-2) for n>2.
(End)
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MATHEMATICA
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Nest[Append[#, 2 Last@ # + 2] &, {0, 1}, 32] (* or *)
Array[3*2^(# - 1) - 2 + Boole[# == 0]/2 &, 34, 0] (* or *)
CoefficientList[Series[x (1 + x)/((1 - x) (1 - 2 x)), {x, 0, 33}], x] (* Michael De Vlieger, Dec 22 2017 *)
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PROG
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(PARI) concat(0, Vec(x*(1 + x) / ((1 - x)*(1 - 2*x)) + O(x^40))) \\ Colin Barker, Dec 22 2017
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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