A191601 - OEIS

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

%S 2,7,41,428,7588,235992,12750773,1196790145,195443817921,

%T 55484180929526,27388366928551712,23507551792998353446,

%U 35081808543792928475951,91032423727427789694636918,410722028606679785606473590135

%N Number of n X n symmetric binary matrices with each 1 adjacent to no more than 2 horizontally, vertically or diagonally neighboring 1s

%H R. H. Hardin, <a href="/A191601/b191601.txt">Table of n, a(n) for n = 1..17</a>

%e Some solutions for n=4

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

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

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

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

%K nonn

%O 1,1

%A _R. H. Hardin_ Jun 08 2011