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A326237
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Number of non-nesting digraphs with vertices {1..n}, where two edges (a,b), (c,d) are nesting if a < c and b > d or a > c and b < d.
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12
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OFFSET
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0,2
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COMMENTS
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These are digraphs with the property that, if the edges are listed in lexicographic order, the sequence of targets is weakly increasing. For example, the digraph with lexicographically ordered edge set {(1,2),(2,1),(3,1),(3,2)} is nesting because the targets are (2,1,1,2), a sequence that is not weakly increasing.
Also the number of non-semicrossing digraphs with vertices {1..n}, where two edges (a,b), (c,d) are semicrossing if a < c and b < d or a > c and b > d. For example, the a(2) = 4 non-semicrossing digraph edge-sets are:
{}
{11}
{12}
{21}
{22}
{11,12}
{11,21}
{12,21}
{12,22}
{21,22}
{11,12,21}
{12,21,22}
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LINKS
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FORMULA
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EXAMPLE
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The a(2) = 12 non-nesting digraph edge-sets:
{}
{11}
{12}
{21}
{22}
{11,12}
{11,21}
{11,22}
{12,22}
{21,22}
{11,12,22}
{11,21,22}
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MATHEMATICA
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Table[Length[Select[Subsets[Tuples[Range[n], 2]], OrderedQ[Last/@#]&]], {n, 4}]
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CROSSREFS
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Non-nesting set partitions are A000108.
Non-capturing set partitions are A326254.
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KEYWORD
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nonn,more
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AUTHOR
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STATUS
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approved
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