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.

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

Author

Brendan McKay, Oct 05 2019

Keywords

• nonn
• hard

Comments

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

Examples

			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.

Extensions

a(13) corrected and terms a(14) and beyond from Andrew Howroyd, Apr 11 2020