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 A277565 #13 Oct 20 2016 12:37:27 %S A277565 1,2,7,40,281,2538 %N A277565 Number of flattenable free polyominoids. %C A277565 A polyominoid is flattenable if, by a process of unfolding, it may be transformed into a polyomino with the same number of squares. Tearing is not allowed - if two squares are adjacent in the polyominoid, they must be adjacent in the polyomino. Overlapping squares are not allowed - the polyomino must be exactly "one square thick". %C A277565 To avoid ambiguity, the squares are infinitely flexible during the unfolding process; this is important for large polyominoids that thread through themselves. On the other hand, a polyominoid containing two intersecting rings is obviously not flattenable. %C A277565 It is interesting that flattening is not a reversible process. In many cases, the resulting polyomino may not be folded to produce the original polyominoid without tearing. %C A277565 See the link for drawings of the polyominoes of sizes 1 through 5, and all polyominoids that will flatten to those shapes. At the end of the file are all polyominoids of sizes 1 through 5 that are not flattenable. %H A277565 John Mason, <a href="/A277565/a277565.pdf">Drawings of flattenable and unflattenable polyominoids</a> %Y A277565 Cf. A075679. %K A277565 nonn,more %O A277565 1,2 %A A277565 _John Mason_, Oct 20 2016