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 A210997 #17 Feb 16 2025 08:33:17 %S A210997 1,2,12,108,1285,17073,238591,3426576,50107909,742624232,11123060678, %T A210997 168047007728,2557227044764,39153010938487,602621953061978, %U A210997 9317706529987950,144648268175306702,2253491528465905342,35218318816847951974 %N A210997 Number of free polyominoes with 2n-1 cells. %H A210997 John Mason, <a href="/A210997/b210997.txt">Table of n, a(n) for n = 1..25</a> %H A210997 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/Polyomino.html">Polyomino</a> %H A210997 Wikipedia, <a href="http://en.wikipedia.org/wiki/Polyomino">Polyomino</a> %Y A210997 Bisection of A000105. %Y A210997 Cf. A000988, A210989, A210996. %K A210997 nonn %O A210997 1,2 %A A210997 _Omar E. Pol_, Sep 15 2012