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%I #8 May 02 2023 15:03:24
%S 2,8,56,792,21984,1188160,125522112,25932064896,10472077118464,
%T 8266074697979136,12754433913314441216,38469498012053658897408,
%U 226810241439860068417249280,2613967215944945112514234085376
%N Number of n X n symmetric binary matrices with each 1 adjacent to no more than 3 horizontally or vertically neighboring 1's.
%H R. H. Hardin, <a href="/A191508/b191508.txt">Table of n, a(n) for n = 1..25</a>
%e Some solutions for n=4
%e ..0..1..0..0....0..0..1..1....0..0..0..1....0..0..0..1....1..0..0..0
%e ..1..0..1..0....0..0..0..1....0..0..0..1....0..1..1..1....0..0..1..0
%e ..0..1..0..1....1..0..0..0....0..0..0..1....0..1..1..0....0..1..0..0
%e ..0..0..1..1....1..1..0..1....1..1..1..0....1..1..0..1....0..0..0..0
%K nonn
%O 1,1
%A _R. H. Hardin_, Jun 04 2011
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