A191602 - OEIS

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

%S 2,8,52,686,16484,740480,62224406,9743390402,2847964953844,

%T 1553270286186274,1580697070071238528,3001650015211411938494,

%U 10635826342196669611219640,70320933672952732438772180006

%N Number of n X n symmetric binary matrices with each 1 adjacent to no more than 3 horizontally, vertically or diagonally neighboring 1s

%H R. H. Hardin, <a href="/A191602/b191602.txt">Table of n, a(n) for n = 1..16</a>

%e Some solutions for n=4

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

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

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

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

%K nonn

%O 1,1

%A _R. H. Hardin_ Jun 08 2011