The area generating function for simplex-duplex polyominoes

Svjetlan Feretic

Published 2009-10-26Version 1

Back in the early days of polyomino enumeration, a model called column-convex polyominoes was introduced and its area generating function was found. That generating function is rational: the numerator has degree four and the denominator has degree three. Let a column-duplex polyomino be a polyomino whose columns can have either one or two connected components. A simplex-duplex polyomino is a column-duplex polyomino in which there is no occurrence of two adjacent columns each having two connected components. Simplex-duplex polyominoes are not easy to deal with, but their area generating function can still be found. To find this generating function, we use an upgraded version of the Temperley method. Though that technique is widely used in these times, our application presents two interesting features. Firstly, we add one or two columns at a time, thus bypassing those simplex-duplex polyominoes which end with a two-component column. (It is somewhat more usual to add just one column at a time.) Secondly, we obtain a functional equation that involves both the first and the second derivatives of the sought-for generating function. (Such equations usually involve the first derivative only. In some cases, no derivative is involved at all.) Right because of this latter interesting feature, the Temperley method produces a very complicated formula for the generating function. Anyway, from that formula it is easy to compute Taylor polynomials. Thus we get plenty of evidence that the number of n-celled simplex-duplex polyominoes behaves asymptotically as 0.119443*3.522020^n. For comparison, the number of n-celled column-convex polyominoes behaves asymptotically as 0.180916*3.205569^n.