### Svjetlan Feretić and Anthony J. Guttmann

Faculty of Civil Engineering, University of Rijeka, Viktora Cara Emina 5, 51000 Rijeka, Croatia

ARC Centre of Excellence for Mathematics and Statistics of Complex Systems, Department of Mathematics and Statistics, The University of Melbourne, Parkville, Victoria 3010, Australia
e-mail: tonyg@ms.unimelb.edu.au

Abstract.   Column-convex polyominoes were introduced in 1950's by Temperley, a mathematical physicist working on ``lattice gases''. By now, column-convex polyominoes are a popular and well-understood model. There exist several generalizations of column-convex polyominoes. However, the enumeration by area has been done for only one of the said generalizations, namely for multi-directed animals. In this paper, we introduce a new sequence of supersets of column-convex polyominoes. Our model (we call it level m column-subconvex polyominoes) is defined in a simple way: every column has at most two connected components and, if there are two connected components, the gap between them consists of at most m cells. We focus on the case when cells are hexagons and we compute the area generating functions for the levels one and two. Both of those generating functions are q-series, whereas the area generating function of column-convex polyominoes is a rational function. The growth constants of level one and level two level two column-subconvex polyominoes are 4.319139 and 4.509480, respectively. For comparison, the growth constants of column-convex polyominoes, multi-directed animals and all polyominoes are 3.863131, 4.587894 and 5.183148, respectively.

2000 Mathematics Subject Classification.   05B50, 05A15.

Key words and phrases.   Polyomino, hexagonal cell, nearly convex column, area generating function, growth constant.

Full text (PDF) (free access)

DOI: 10.3336/gm.45.2.03

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14. http://www.gradri.hr/adminmax/files/staff/A_2.mw (This file is a Maple 9.5 worksheet. The file can also be obtained from Svjetlan Feretić via e-mail.)