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%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
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