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A328159
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Number of n X n matrices (up to permutation of their rows and columns) with entries {0,1,2} with all row and column sums equal to 3.
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1
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0, 1, 3, 7, 19, 72, 280, 1348, 7542, 48290, 348902, 2805666, 24797845, 238678891, 2482575930, 27731295522, 330955304428, 4201424981020, 56521276891669, 803122164193328, 12018137424023094, 188906053776728602, 3111632374404898304, 53597059526977558291
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OFFSET
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1,3
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COMMENTS
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Also, bipartite cubic multigraphs up to color-preserving isomorphism, with no edges of multiplicity 3.
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LINKS
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EXAMPLE
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For n=3 the a(3)=3 solutions are [2,1,0; 1,0,2; 0,2,1], [1,1,1; 2,0,1; 0,2,1] and [1,1,1; 1,1,1; 1,1,1].
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CROSSREFS
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KEYWORD
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nonn,hard
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AUTHOR
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EXTENSIONS
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a(13) corrected and terms a(14) and beyond from Andrew Howroyd, Apr 11 2020
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STATUS
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approved
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