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A121936
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Number of digraphs with labeled vertices and labeled arcs, with n arcs and with no vertex of indegree 0 or outdegree 0.
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1
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1, 0, 2, 18, 518, 23610, 1600982, 150451098, 18694217558, 2966151496410, 584994048653462, 140357794553191578, 40253455215544778198, 13598018000464234802010, 5343837921922909297592342
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listen;
history;
text;
internal format)
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OFFSET
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0,3
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LINKS
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FORMULA
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E.g.f.: Sum(Sum((-1)^(n-k)*binomial(n,k)*(exp((k-1)*x)-1)^k*(exp(k*x)-1)^(n-k),k=0..n),n=0..infinity).
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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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EXTENSIONS
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STATUS
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approved
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