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A069725
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Number of nonisomorphic unrooted unicursal planar maps with n edges and two vertices of valency 1 (unicursal means that exactly two vertices are of odd valency; there is an Eulerian path).
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2
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1, 1, 3, 11, 62, 342, 2152, 13768, 91800, 622616, 4301792, 30100448, 213019072, 1521473984, 10954616064, 79420280064, 579300888960, 4248201302400, 31302536066560, 231638727063040
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OFFSET
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1,3
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COMMENTS
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There is an easy formula.
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LINKS
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MATHEMATICA
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a[1] = a[2] = 1;
a[n_] := With[{m = Floor[(n+1)/2]}, 1/n 2^(n-3) Binomial[2n-2, n-1] + 2^(m-3) Binomial[2m-2, m-1]];
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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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