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Number of triangle-free acyclic digraphs (or DAGs) up to isomorphism with n vertices, maximum indegree 2 and unique maximal element.
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%I #11 Jan 08 2022 22:01:47

%S 1,1,2,5,18,81,475,3394,29140,293198

%N Number of triangle-free acyclic digraphs (or DAGs) up to isomorphism with n vertices, maximum indegree 2 and unique maximal element.

%e a(1) = 1, a(2) = 1 and a(3) = 2 coincide with the first three terms of A001190, as all connected triangle-free DAGs with at most three vertices are necessarily trees.

%e For n = 4, the a(4) = 5 solutions are the following three trees:

%e (((())))

%e ((), (()))

%e (((), ()))

%e together with two 4-cycles.

%Y Lower-bounded by A001190.

%K nonn,more

%O 1,3

%A _Adam P. Goucher_, Jun 12 2019