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 #6 Mar 07 2022 13:22:37
%S 0,47,18769,15675953,40094432569,349773663772607,10762101043733093559,
%T 1178656962552052684421464,460402167947340968868291699047,
%U 641654577152679219502203022480885713
%N Number of (n+1) X (n+1) 0..3 symmetric matrices containing all values 0..3 with every 2 X 2 subblock having two or three distinct values, and new values 0..3 introduced in lower triangle row major order.
%e Some solutions for n=3
%e ..0..1..2..1....0..1..2..0....0..1..2..0....0..1..0..3....0..1..2..0
%e ..1..0..0..0....1..2..2..2....1..1..2..3....1..2..0..1....1..2..0..1
%e ..2..0..2..3....2..2..1..3....2..2..1..2....0..0..0..0....2..0..0..1
%e ..1..0..3..1....0..2..3..0....0..3..2..0....3..1..0..1....0..1..1..3
%K nonn
%O 1,2
%A _R. H. Hardin_, Mar 27 2012