Counting unlabeled acyclic digraphs
The previously known ways to count acyclic digraphs, both labeled and unlabeled, are reviewed. Then a new method of enumerating unlabeled acyclic digraphs is developed. It involves computing the sum of the cyclic indices of the automorphism groups of the acyclic digraphs, achieving a considerable gain in efficiency through an application of the inclusion-exclusion principle. Numerical results are reported on, and a table of the numbers of unlabeled acyclic digraphs on up to 18 points is included.
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