A191713 - OEIS

%I #6 Dec 18 2015 18:17:40

%S 2,8,56,760,19792,978304,92183968,16556792256,5666183103872,

%T 3695209097658560,4592223914052994688,10875252973619159084864,

%U 49078195656242127591078144,422056710044472651098920886272

%N Number of n X n symmetric binary matrices with each 1 adjacent to no more than 2 E, S or NW neighboring 1s

%H R. H. Hardin, <a href="/A191713/b191713.txt">Table of n, a(n) for n = 1..20</a>

%e Some solutions for n=5

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

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

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

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

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

%K nonn

%O 1,1

%A _R. H. Hardin_ Jun 12 2011