Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #7 Dec 27 2023 00:14:39
%S 0,1,0,6,8,118,880,14400,318220,10446298,531060152,39577654480,
%T 4465022457276,757302604610482,191605833481050984
%N 1/3 the number of n X n 0..2 symmetric matrices with every element equal to two or three horizontal and vertical neighbors.
%e Some solutions for n=4
%e ..0..0..1..1....0..0..1..1....2..2..0..0....2..2..1..1....1..1..1..1
%e ..0..0..1..1....0..0..1..1....2..2..0..0....2..2..1..1....1..2..2..1
%e ..1..1..0..0....1..1..2..2....0..0..1..1....1..1..2..2....1..2..2..1
%e ..1..1..0..0....1..1..2..2....0..0..1..1....1..1..2..2....1..1..1..1
%K nonn
%O 1,4
%A _R. H. Hardin_, Mar 30 2012