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A328159
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.
1
0, 1, 3, 7, 19, 72, 280, 1348, 7542, 48290, 348902, 2805666, 24797845, 238678891, 2482575930, 27731295522, 330955304428, 4201424981020, 56521276891669, 803122164193328, 12018137424023094, 188906053776728602, 3111632374404898304, 53597059526977558291
OFFSET
1,3
COMMENTS
Also, bipartite cubic multigraphs up to color-preserving isomorphism, with no edges of multiplicity 3.
EXAMPLE
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].
CROSSREFS
a(n) = A232215(n) - A232215(n-1) for n >= 1.
Sequence in context: A362137 A341323 A135741 * A163571 A217611 A322852
KEYWORD
nonn,hard
AUTHOR
Brendan McKay, Oct 05 2019
EXTENSIONS
a(13) corrected and terms a(14) and beyond from Andrew Howroyd, Apr 11 2020
STATUS
approved