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 A151521 #9 Feb 16 2025 08:33:09 %S A151521 1,3,4,13,25,72,166,450,1124,2973,7603,19865,51199,132982,343290, %T A151521 888309,2292325,5918742,15258937,39329418 %N A151521 Number of 2-sided strip polytans with n cells. %H A151521 Ed Pegg, Jr., <a href="http://demonstrations.wolfram.com/PolyformExplorer/">Illustrations of polyforms</a> %H A151521 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/Polytan.html">Polytan</a> %Y A151521 Cf. A006074, A151520. %K A151521 nonn,hard,more %O A151521 1,2 %A A151521 _Ed Pegg Jr_, May 13 2009 %E A151521 a(6)-a(20) from _John Mason_, Jan 07 2022