OFFSET
0,3
COMMENTS
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}}.
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..449
Eric Weisstein's World of Mathematics, Acyclic Digraph.
FORMULA
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.
a(n) = A038154(n)+1.
E.g.f.: exp(x)*(x^2-x+1)/(1-x). - Alois P. Heinz, Aug 21 2017
PROG
(MATLAB) @(n)(factorial(n)*sum(1./(factorial(0:n-2)))+1)
CROSSREFS
KEYWORD
nonn
AUTHOR
Shuaib Ahmed S, May 29 2014
EXTENSIONS
a(0)=1 prepended, one term corrected, more terms added by Alois P. Heinz, Aug 21 2017
STATUS
approved