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 A352720 #36 Jun 21 2023 08:21:05 %S A352720 1,1,4,5,12,18,37,60,117,200,379,669,1250,2247,4168,7570,13987,25549, %T A352720 47108,86319,158978,291806,537105,986786,1815699,3337560,6140047, %U A352720 11289571,20767180,38189927,70246680,129191148,237627757,437042337,803861244,1478488577,2719392160,5001663330,9199544069 %N A352720 a(n) is the number of free polyominoes of width 2 and size n. %H A352720 John Mason, <a href="/A352720/b352720.txt">Table of n, a(n) for n = 2..1000</a> %H A352720 John Mason, <a href="/A352720/a352720.txt">Java program</a> %H A352720 John Mason, <a href="/A352720/a352720.pdf">Explanation of formulas</a> %H A352720 R. J. Mathar, <a href="https://vixra.org/abs/1905.0474">Corrigendum to "polyomino enumeration results" (Parkin et al)</a>, vixra:1905.0474 (2019) Table 1 column 2. %F A352720 For a set of recursive formulas to generate a(n), see the link for the Java program extract. %e A352720 There is one polyomino, the domino, of width 2 and size 2. So a(2) = 1. %e A352720 There is one tromino, L-shaped, of width 2. So a(3) = 1. %Y A352720 Cf. A000105, A335711, A353067. %K A352720 nonn %O A352720 2,3 %A A352720 _John Mason_, Mar 31 2022