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 A139007 #8 Mar 09 2015 04:28:03 %S A139007 1,2,5,25,205,2849,66677,2633485,175143651,19638725775,3710411382331, %T A139007 1181447019186469,633957805831439213,573281808520220290885, %U A139007 873645764409736229092874,2243689725607702437916265557 %N A139007 Number of n X n symmetric binary matrices containing no more than two 1s in any 2 X 2 sub-block. %H A139007 R. H. Hardin, May 31 2008, <a href="/A139007/b139007.txt">Table of n, a(n) for n = 0..20</a> %K A139007 nonn %O A139007 0,2 %A A139007 _R. H. Hardin_, May 31 2008