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A308914
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Number of unordered pairs of non-intersecting non-selfintersecting paths with nodes that cover all vertices of a convex n-gon, n > 3.
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3
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2, 15, 75, 308, 1120, 3744, 11760, 35200, 101376, 282880, 768768, 2042880, 5324800, 13647872, 34467840, 85917696, 211681280, 516096000, 1246429184, 2984509440, 7090470912, 16724787200, 39190528000, 91276443648, 211392921600, 487025803264, 1116607610880
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OFFSET
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4,1
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COMMENTS
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Paths must have at least two nodes.
The number of non-selfintersecting paths that cover all vertices of a convex n-gon is given by A001792(n-2).
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LINKS
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FORMULA
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a(n) = (1/3)*n*(n-1)*(n-3)*(n+4)*2^(n-8).
O.g.f.: x^4*(-2 + 5*x - 5*x^2 + 2*x^3)/(-1 + 2*x)^5.
E.g.f.: x^2*(3 + exp(2*x)*(-3 + 6*x + 2*x^2))/96.
a(n) = 10*a(n-1) - 40*a(n-2) + 80*a(n-3) - 80*a(n-4) + 32*a(n-5) for n > 8.
(End)
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EXAMPLE
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a(5) = 15 since one of the non-selfintersecting paths has to be a segment connecting two adjacent vertices (5 choices) and the other path will connect the remaining vertices in one of three ways.
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MAPLE
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gf := x^2*(3 + exp(2*x)*(-3 + 6*x + 2*x^2))/96: ser := series(gf, x, 36):
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MATHEMATICA
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Array[(1/3) # (# - 1) (# - 3) (# + 4)*2^(# - 8) &, 27, 4] (* Michael De Vlieger, Feb 25 2020 *)
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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STATUS
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approved
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