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%I #13 Mar 28 2021 22:54:37
%S 1,2,7,28,124,588,2938,15266,81770,448698,2510813,14277838,82286365,
%T 479610362,2822332127,16745262798
%N 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 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 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 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 Cf. A005435, A002931.
%K hard,more,nonn
%O 2,2
%A _Doron Zeilberger_, Apr 03 2001
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