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Number of n X n symmetric binary matrices with each 1 adjacent to no more than 2 horizontally or vertically neighboring 1s
1

%I #6 Dec 18 2015 18:17:39

%S 2,8,52,646,14570,599702,45191945,6225542127,1567856872728,

%T 722008623925586,607932060252871024,935917819388424802645,

%U 2634499346573878930718088,13559220442178025486399949918

%N Number of n X n symmetric binary matrices with each 1 adjacent to no more than 2 horizontally or vertically neighboring 1s

%H R. H. Hardin, <a href="/A191507/b191507.txt">Table of n, a(n) for n = 1..21</a>

%e Some solutions for n=4

%e ..1..0..0..1....0..1..0..1....1..1..0..0....0..0..1..0....1..0..1..0

%e ..0..1..1..1....1..1..0..0....1..1..0..1....0..0..0..0....0..0..1..1

%e ..0..1..0..0....0..0..0..0....0..0..0..0....1..0..0..1....1..1..0..1

%e ..1..1..0..0....1..0..0..0....0..1..0..1....0..0..1..0....0..1..1..0

%K nonn

%O 1,1

%A _R. H. Hardin_ Jun 04 2011