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 A126742 #9 Jan 23 2019 20:01:07 %S A126742 0,2,13,284,13375,660690,51941832 %N A126742 Number of n-indecomposable polyominoes with at least 2n cells. %C A126742 A polyomino is called n-indecomposable if it cannot be partitioned (along cell boundaries) into two or more polyominoes each with at least n cells. %C A126742 For full lists of drawings of these polyominoes for n <= 6, see the links in A125759. %H A126742 N. MacKinnon, <a href="http://www.jstor.org/stable/3618845">Some thoughts on polyomino tilings</a>, Math. Gaz., 74 (1990), 31-33. %H A126742 Simone Rinaldi and D. G. Rogers, <a href="http://www.jstor.org/stable/27821767">Indecomposability: polyominoes and polyomino tilings</a>, The Mathematical Gazette 92.524 (2008): 193-204. %e A126742 The five 2-indecomposable polyominoes: %e A126742 ...................X. %e A126742 XX..XXX..XX..XXX..XXX %e A126742 ..........X...X....X. %e A126742 Only the last two have >= 4 cells, so a(2) = 2. %Y A126742 Row sums of A126743. Cf. A000105, A125759, A125761, A125709, A125753. %K A126742 nonn,more %O A126742 1,2 %A A126742 _David Applegate_ and _N. J. A. Sloane_, Feb 01 2007 %E A126742 a(4) and a(5) from Peter Pleasants, Feb 13 2007 %E A126742 a(6) and a(7) from _David Applegate_, Feb 16 2007