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1/4 the number of n X n 0..3 symmetric matrices with every element equal to zero or two horizontal and vertical neighbors
0

%I #5 Dec 18 2015 18:18:08

%S 1,10,240,13872,1924416,641053824,512493408768,983311673066496,

%T 4527565995515535360,50027087182874567811072,

%U 1326495212666507820379570176

%N 1/4 the number of n X n 0..3 symmetric matrices with every element equal to zero or two horizontal and vertical neighbors

%e Some solutions for n=4

%e ..3..1..3..1....2..1..0..3....1..2..3..2....1..1..3..0....2..1..0..3

%e ..1..2..1..3....1..0..3..1....2..1..2..0....1..1..1..3....1..2..3..0

%e ..3..1..0..1....0..3..1..2....3..2..1..3....3..1..1..2....0..3..0..1

%e ..1..3..1..0....3..1..2..0....2..0..3..1....0..3..2..0....3..0..1..3

%K nonn

%O 1,2

%A _R. H. Hardin_ Mar 31 2012