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 A075220 #11 Dec 05 2014 02:43:36 %S A075220 1,1,1,3,4,12,21,56,111,261,462,1431,1875,4696,9410,20099,24034,77884, %T A075220 79925,266786,358096,658594,796571,3227163,2980476,6268017,10167660, %U A075220 24321735,21477012,85964698 %N A075220 Number of polyiamonds with n cells that tile the plane by 180-degree rotation (Conway criterion). %D A075220 Branko Grünbaum and G. C. Shephard, Tilings and Patterns. W. H. Freeman, New York, 1987, Section 9.4. %H A075220 Joseph Myers, <a href="http://www.polyomino.org.uk/mathematics/polyform-tiling/">Polyiamond tiling</a> %Y A075220 Cf. A071332, A075216, A075217, A075218, A075219, A075221, A075222, A075223, A075224, A075202, A075211. %K A075220 hard,nonn %O A075220 1,4 %A A075220 _Joseph Myers_, Sep 08 2002 %E A075220 More terms from _Joseph Myers_, Nov 11 2003 %E A075220 a(29) and a(30) from _Joseph Myers_, Nov 21 2010