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 A057409 #14 Jan 29 2025 12:23:13 %S A057409 1,2,6,19,63,216,756,2685,9650,35018,128084,471623,1746492,6499356, %T A057409 24290272,91123171,342984175,1294829776,4901319978,18597856445, %U A057409 70723784744,269486503694,1028736811230,3933715966653 %N A057409 Number of self-avoiding polygons of area n with any number of (self-avoiding polygon) holes on square lattice (not allowing rotations). %H A057409 I. Jensen, <a href="/A057409/b057409.txt">Table of n, a(n) for n = 1..40</a> (from link below) %H A057409 Anthony J. Guttmann, Iwan Jensen and Aleksander L. Owczarek, <a href="https://doi.org/10.1088/0305-4470/34/18/302">Polygonal polyominoes on the square lattice</a>, J. Phys. A: Math. Gen., 34 (2001), 3721-3733. %H A057409 I. Jensen, <a href="https://web.archive.org/web/20160102114343/http://www.ms.unimelb.edu.au/~iwan/polygons/series/square.area.allholes.ser">More terms</a> %Y A057409 Cf. A006724 (no holes, first differs at n=8), A057406, A057407, A057408 (resp. 1, 2, 3 holes); A001168 (any holes, first differs at n=7); A088702 (by perimeter); A000104 (no holes, rotations and reflections allowed). %K A057409 nonn %O A057409 1,2 %A A057409 _N. J. A. Sloane_, Aug 30 2000