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 A060379 #13 Mar 28 2021 22:54:37 %S A060379 1,2,7,28,124,588,2938,15266,81770,448698,2510813,14277838,82286365, %T A060379 479610362,2822332127,16745262798 %N A060379 Number of self-avoiding polygons on the 2-dimensional square lattice with perimeter 2n with at most 4 horizontal edges in each vertical cross-section. %H A060379 Doron Zeilberger, <a href="http://www.math.rutgers.edu/~zeilberg/mamarim/mamarimhtml/umbIV.html">The Umbral Transfer-Matrix Method. IV. Counting Self-AvoidingPolygons and walks</a>; <a href="/A060379/a060379.pdf">Local copy</a> [Pdf file only, no active links] %F A060379 See Appendix 2 of the reference (a 7-page system of linear functional equations for 5 unknown generating functions, one of which is the desired generating function). %e A060379 a(3) = 2 because there are 2 self-avoiding polygons of perimeter 2*3 with at most 4 horizontal edges per vertical cross-section. %Y A060379 Cf. A005435, A002931. %K A060379 hard,more,nonn %O A060379 2,2 %A A060379 _Doron Zeilberger_, Apr 03 2001