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%I #7 Dec 18 2015 18:17:39
%S 1,2,6,24,125,1055,11754,187498,4372206,142427556,6649078485,
%T 443418423639,42015106781746,5681500677861845,1094937801983952063,
%U 300642401916076713227,117675702292067958725570
%N Number of n X n symmetric binary matrices with each 1 adjacent to exactly 1 horizontally, vertically or diagonally neighboring 1
%H R. H. Hardin, <a href="/A191343/b191343.txt">Table of n, a(n) for n = 1..20</a>
%e Some solutions for n=4
%e ..0..0..0..0....0..0..0..0....0..0..1..0....0..0..1..0....1..0..0..0
%e ..0..0..0..1....0..0..0..0....0..0..0..1....0..1..0..1....0..1..0..1
%e ..0..0..0..1....0..0..0..0....1..0..0..0....1..0..1..0....0..0..0..1
%e ..0..1..1..0....0..0..0..0....0..1..0..0....0..1..0..0....0..1..1..0
%K nonn
%O 1,2
%A _R. H. Hardin_ May 30 2011
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