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A306827
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Isomorphism classes of connected 2-in 2-out digraphs, allowing parallel edges but not loops.
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4
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1, 2, 5, 13, 59, 285, 1987, 16057, 149430, 1551863, 17747299, 221015026
(list;
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refs;
listen;
history;
text;
internal format)
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OFFSET
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2,2
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COMMENTS
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The multiset transform (graphs with n>=2 nodes and k>=1 components) is
1
2
5 1
13 2
59 8 1
285 23 2
1987 100 8 1
16057 468 27 2
149430 2943 115 8 1
The row sums (Euler transform) are A307180. (End)
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LINKS
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EXAMPLE
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For n=3, the a(3)=2 solutions consist of two superimposed directed triangles, one with the triangles oriented consistently and the other with the triangles oriented oppositely.
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CROSSREFS
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Cf. A219889 (no multiedges, not necessarily connected), A306892 (loops permitted), A307180 (Euler transf.)
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KEYWORD
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nonn,hard
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AUTHOR
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STATUS
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approved
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