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Number of n X n symmetric 0..1 arrays with each element equal to at least two horizontal or vertical neighbors, containing at least one 1 and with new values 0..1 introduced in lower triangular row major order
1

%I #7 Dec 18 2015 18:17:40

%S 0,0,0,4,23,183,2051,34480,852316,31436447,1719054682,139749375351,

%T 16887682211104,3032513808121100,809442991768500675,

%U 321179427812462958356,189437827654558885755437

%N Number of n X n symmetric 0..1 arrays with each element equal to at least two horizontal or vertical neighbors, containing at least one 1 and with new values 0..1 introduced in lower triangular row major order

%H R. H. Hardin, <a href="/A192840/b192840.txt">Table of n, a(n) for n = 1..19</a>

%e Some solutions for 6X6

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

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

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

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

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

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

%K nonn

%O 1,4

%A _R. H. Hardin_ Jul 11 2011