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 A056786 #50 Jul 02 2025 16:02:00 %S A056786 1,1,4,26,255,2874,35520,454491,5954914,79238402,1067193518 %N A056786 Number of inequivalent connected planar figures that can be formed from n non-overlapping 1 X 2 rectangles (or dominoes). %C A056786 "Connected" means "connected by edges", rotations and reflections are not considered different, but the internal arrangement of the dominoes does matter. %C A056786 I have verified the first three entries by hand. The terms 255 and 2874 were taken from the Vicher web page. - _N. J. A. Sloane_. %H A056786 Gordon Hamilton, <a href="http://youtu.be/7efCz2FvUDI">Three integer sequences from recreational mathematics</a>, Video (2013?). %H A056786 R. J. Mathar, <a href="/A056786/a056786_1.pdf">Illustration of the 255 figures for the 4th term</a> %H A056786 N. J. A. Sloane, <a href="/A056786/a056786.jpg">Illustration of initial terms of A056786, A216598, A216583, A216595, A216492, A216581</a> %H A056786 N. J. A. Sloane, <a href="/A056786/a056786.pdf">Illustration of third term of A056786, A216598, A216583, A216595, A216492, A216581</a> (a better drawing for the third term) %H A056786 M. Vicher, <a href="http://www.vicher.cz/puzzle/polyforms.htm">Polyforms</a> %H A056786 <a href="/index/Do#domino">Index entries for sequences related to dominoes</a> %Y A056786 Cf. A121194, A216598, A216583, A216595, A216492, A216581. %K A056786 nonn,nice,more %O A056786 0,3 %A A056786 _James Sellers_, Aug 28 2000 %E A056786 Edited by _N. J. A. Sloane_, Aug 17 2006, May 15 2010, Sep 09 2012 %E A056786 a(6) and a(7) from _Owen Whitby_, Nov 18 2009 %E A056786 a(8) from Anton Betten, Jan 18 2013, added by _N. J. A. Sloane_, Jan 18 2013. Anton Betten also verified that a(0)-a(7) are correct. %E A056786 a(9) from Anton Betten, Jan 25 2013, added by _N. J. A. Sloane_, Jan 26 2013. Anton Betten comments that he used 8 processors, each for about 1 and a half day (roughly 300 hours CPU time). %E A056786 a(10) from _Aaron N. Siegel_, May 18 2022. [It took just 30 minutes to verify a(9) and 7.2 hours to compute a(10), on a single CPU core!]