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A243014
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Number of acyclic digraphs (DAGS) on n labeled nodes, where the indegree and outdegree of each node is at most 1.
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2
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1, 1, 3, 13, 61, 321, 1951, 13693, 109593, 986401, 9864091, 108505101, 1302061333, 16926797473, 236975164791, 3554627472061, 56874039553201, 966858672404673, 17403456103284403, 330665665962403981, 6613313319248079981, 138879579704209680001
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OFFSET
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0,3
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COMMENTS
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a(n) is the number of acyclic digraphs (DAGS) on n labeled nodes, where the indegree and outdegree of each node is at most 1. For example, with vertex set {A,B,C} the possible ways are: one 3-component graph {A,B,C}, six 2-component graph {{A->B,C},{B->A,C},{A->C,B},{C->A,B},{C->B,A},{B->C,A}}, and six 1-component graph {{A->B->C},{B->A->C},{A->C->B},{C->A->B},{C->B->A},{B->C->A}}.
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LINKS
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FORMULA
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a(n) = (n!*Sum(1/k!))+1, k=0..n-2.
a(n) = (n*(a(n-1)+n-2))+1, for n>1, a(1)=1.
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PROG
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(MATLAB) @(n)(factorial(n)*sum(1./(factorial(0:n-2)))+1)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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a(0)=1 prepended, one term corrected, more terms added by Alois P. Heinz, Aug 21 2017
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STATUS
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approved
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