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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.
1

%I #12 Apr 11 2020 22:41:00

%S 0,1,3,7,19,72,280,1348,7542,48290,348902,2805666,24797845,238678891,

%T 2482575930,27731295522,330955304428,4201424981020,56521276891669,

%U 803122164193328,12018137424023094,188906053776728602,3111632374404898304,53597059526977558291

%N 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.

%C Also, bipartite cubic multigraphs up to color-preserving isomorphism, with no edges of multiplicity 3.

%e 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].

%Y a(n) = A232215(n) - A232215(n-1) for n >= 1.

%K nonn,hard

%O 1,3

%A _Brendan McKay_, Oct 05 2019

%E a(13) corrected and terms a(14) and beyond from _Andrew Howroyd_, Apr 11 2020