OFFSET
1,2
COMMENTS
Each 1 <= k <= n appears k times in the multiset. Terms include rotations/reflections. Related to Latin squares. Inspired by related question in "Chess Set Latin Square" link.
LINKS
Mathematics Stack Exchange, Chess Set Latin Square
EXAMPLE
The a(3) = 5450400 (= 6!*7570) 6 X 6 matrices include
1 2 2 3 3 3
2 1 2 3 3 3
2 2 1 3 3 3
3 3 3 1 2 2
3 3 3 2 1 2
3 3 3 2 2 1.
Using terminology such as that suggested by Rebecca J. Stones in a "Chess Set Latin Square" comment, this matrix is a (3, 2, 1)-frequency square corresponding to a decomposition of K_{6,6} into one 3-regular spanning subgraph, one 2-regular spanning subgraph, and one 1-regular spanning subgraph.
CROSSREFS
KEYWORD
nonn,bref,hard,more
AUTHOR
Rick L. Shepherd, Feb 28 2016
STATUS
approved