OFFSET
0,3
COMMENTS
Number of n X n symmetric matrices with nonnegative integer entries and all row and column sums 4. - Andrew Howroyd, Apr 07 2020
In A005816 matrix elements on the diagonal are counted with a factor 2. This sequence here counts labeled multigraphs with n nodes (may be disconnected, undirected edges) without loops and degree at each node <=4. - R. J. Mathar, Jun 05 2022
LINKS
Andrew Howroyd, Table of n, a(n) for n = 0..50
EXAMPLE
All solutions for 8X2
..1..1....1..1....1..1....1..0....1..1
..1..0....1..1....1..1....1..0....1..1
..1..0....1..1....1..0....1..0....1..1
..1..0....1..0....1..0....1..0....1..1
..0..1....0..1....0..1....0..1....0..0
..0..1....0..0....0..1....0..1....0..0
..0..1....0..0....0..0....0..1....0..0
..0..0....0..0....0..0....0..1....0..0
CROSSREFS
KEYWORD
nonn
AUTHOR
R. H. Hardin, Mar 30 2011
EXTENSIONS
a(0)=1 prepended and terms a(12) and beyond from Andrew Howroyd, Apr 07 2020
STATUS
approved