This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A228277 #27 Dec 07 2016 23:31:43 %S A228277 1,1,13,133,3631,172082,16566199,3057290265,1105411581741, %T A228277 776531523355217,1063228770141145384,2834013489992345694498, %U A228277 14712337761578682394367473,148727865257442275211424889367 %N A228277 Number of n X n binary arrays with top left value 1 and no two ones adjacent horizontally, vertically or nw-se diagonally. %C A228277 Main diagonal of A228285. %H A228277 R. H. Hardin, <a href="/A228277/b228277.txt">Table of n, a(n) for n = 1..25</a> %F A228277 No known recurrence. %e A228277 The thirteen solutions for n=3 correspond to the thirteen possible values of 5-bit numbers with no two adjacent bits equal to 1, namely, the matrices %e A228277 ( 1 0 a ) %e A228277 ( 0 0 b ) %e A228277 ( e d c ) ; with abcde = A014417(0,...,12) = 0, 1, 10, 100, 101, 1000, 1001, 1010, 10000, 10001, 10010, 10100, 10101 (leading zeros omitted). - _M. F. Hasler_, Apr 27 2014 %e A228277 Some solutions for n=4: %e A228277 .1..0..0..1. .1..0..0..0. .1..0..0..0. .1..0..0..0. .1..0..0..0 %e A228277 .0..0..0..0. .0..0..0..1. .0..0..0..0. .0..0..1..0. .0..0..0..0 %e A228277 .0..0..0..0. .0..0..0..0. .0..0..0..0. .0..0..0..0. .0..0..0..1 %e A228277 .1..0..0..1. .0..1..0..1. .0..1..0..1. .0..1..0..0. .0..0..1..0 %e A228277 The last example shows that sw-ne (= anti)diagonally adjacent "1"s are allowed. See A228476, A228506 and A228390 for other variants. %Y A228277 Cf. A228277-A228285. %Y A228277 See also the variants A228390, A228476, A228506, etc. %K A228277 nonn %O A228277 1,3 %A A228277 _R. H. Hardin_, Aug 19 2013