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%I #7 Dec 29 2023 23:25:31
%S 1,2,55,5892,2081181,2353015360,8533282805517,99227545384370981,
%T 3699909251614783425693,442375363534050667939662012,
%U 169602369911112134429648742314656
%N Number of n X n 0..3 symmetric matrices with every element equal to zero or one horizontal and vertical neighbors, and new values 0..3 introduced in lower triangle row major order.
%e Some solutions for n=4
%e ..0..1..2..0....0..1..3..0....0..1..1..2....0..1..2..1....0..1..3..3
%e ..1..2..1..3....1..2..0..2....1..2..3..1....1..0..3..3....1..2..0..0
%e ..2..1..2..3....3..0..1..2....1..3..1..2....2..3..2..1....3..0..3..2
%e ..0..3..3..2....0..2..2..0....2..1..2..3....1..3..1..0....3..0..2..0
%K nonn
%O 1,2
%A _R. H. Hardin_, Mar 29 2012