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 A131481 #13 May 15 2021 13:20:36 %S A131481 1,1,1,3,4,11,23,62,149,409,1066,2931,7981,22166,61508,172267,483088, %T A131481 1361475,3845139,10894630 %N A131481 a(n) is the number of n-cell polyiamonds (triangular polyominoes) with perimeter n+2. %C A131481 n+2 is the maximal perimeter for an n-celled polyiamond. a(n) is the number of n-celled polyiamonds that have a tree as their connectedness graph (vertices of this graph correspond to cells and two vertices are connected if the corresponding cells have a common edge) %H A131481 Andrew Clarke, <a href="http://www.recmath.com/PolyPages/PolyPages/index.htm?Isopoly1s.html">Isoperimetrical Polyiamonds</a> %Y A131481 a(n) <= A000577(n), a(n) <= A057729(n+2). %K A131481 nonn,hard,more %O A131481 1,4 %A A131481 _Tanya Khovanova_, Jul 27 2007 %E A131481 Offset corrected and terms a(17)-a(20) added by _John Mason_, May 15 2021