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 A383731 #13 May 16 2025 17:12:51 %S A383731 2,3,17,41,140,389,1182,3369,9817,27903,79936,226784,645730,1831574, %T A383731 5204271,14766828,41938778,119061270 %N A383731 Number of hexagonal n-element polyominoes whose graph is a nonextensible path. %C A383731 A 2-sided strip polyhex is a shape formed by a set of connected cells on a hexagonal grid, all cells having exactly two neighbors except for the first and last cell, which have only one neighbor. The shape is considered identical to its mirror image, as well as any translation or rotation of itself. This sequence counts the number of 2-sided strip polyhexes with n cells to which no additional cell can be added without introducing a branch or cycle. See also illustration. %H A383731 Bert Dobbelaere, <a href="/A383731/a383731.png">Illustration of initial terms colored by symmetry</a> %H A383731 Ed Pegg, Jr., <a href="http://demonstrations.wolfram.com/PolyformExplorer/">Illustrations of polyforms</a> %H A383731 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/Polyhex.html">Polyhex</a> %Y A383731 Cf. A003104. %K A383731 nonn,more %O A383731 13,1 %A A383731 _Bert Dobbelaere_, May 07 2025