%I #6 Dec 18 2015 18:17:39
%S 1,2,4,13,53,278,1915,17298,206161,3270381,68710560,1908681423,
%T 70045542183,3395385492725,217427456813709,18398892775456083,
%U 2057817979125478159,304212919428910114290,59440186835670090471326
%N Number of n X n symmetric binary matrices with each 1 adjacent to exactly 2 horizontally or vertically neighboring 1s
%H R. H. Hardin, <a href="/A191293/b191293.txt">Table of n, a(n) for n = 1..22</a>
%e Some solutions for n=4
%e ..1..1..1..1....1..1..0..0....0..0..0..0....1..1..1..0....1..1..1..0
%e ..1..0..0..1....1..1..0..0....0..0..0..0....1..0..1..0....1..0..1..1
%e ..1..0..1..1....0..0..1..1....0..0..1..1....1..1..1..0....1..1..0..1
%e ..1..1..1..0....0..0..1..1....0..0..1..1....0..0..0..0....0..1..1..1
%K nonn
%O 1,2
%A _R. H. Hardin_ May 29 2011
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