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 A326219 Number of labeled n-vertex Hamiltonian digraphs (without loops). 7
 0, 1, 1, 15, 1194 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,4 COMMENTS A digraph is Hamiltonian if it contains a directed cycle passing through every vertex exactly once. LINKS Wikipedia, Hamiltonian path FORMULA A053763(n) = a(n) + A326218(n). EXAMPLE The a(3) = 15 edge-sets:   {12,23,31}  {12,13,21,32}  {12,13,21,23,31}  {12,13,21,23,31,32}   {13,21,32}  {12,13,23,31}  {12,13,21,23,32}               {12,21,23,31}  {12,13,21,31,32}               {12,23,31,32}  {12,13,23,31,32}               {13,21,23,32}  {12,21,23,31,32}               {13,21,31,32}  {13,21,23,31,32} MATHEMATICA Table[Length[Select[Subsets[Select[Tuples[Range[n], 2], UnsameQ@@#&]], FindHamiltonianCycle[Graph[Range[n], DirectedEdge@@@#]]!={}&]], {n, 0, 4}] (* Mathematica 8.0+. Warning: Using HamiltonianGraphQ instead of FindHamiltonianCycle returns a(4) = 1200 which is incorrect *) CROSSREFS The unlabeled case is A326225. The undirected case is A326208 (without loops) or A326240 (with loops). The case with loops is A326204. Digraphs (without loops) not containing a Hamiltonian cycle are A326218. Digraphs (without loops) containing a Hamiltonian path are A326217. Cf. A000595, A002416, A003024, A003216, A053763, A246446, A326220, A326226. Sequence in context: A230669 A027492 A001728 * A211901 A059383 A206394 Adjacent sequences:  A326216 A326217 A326218 * A326220 A326221 A326222 KEYWORD nonn,more AUTHOR Gus Wiseman, Jun 15 2019 STATUS approved

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Last modified January 28 16:43 EST 2022. Contains 350656 sequences. (Running on oeis4.)