J. Algebra Comb. Discrete Appl. 1(1) • 19-27 Received: 18 May 2014; Accepted: 24 June 2014 DOI 10.13069/jacodesmath.09554
Journal of Algebra Combinatorics Discrete Structures and Applications
Horizontal runs in domino tilings Research Article
Kamilla Oliver1∗ , Helmut Prodinger2∗∗ 1. 91502 Erlangen, Germany 2. Department of Mathematics, University of Stellenbosch 7602, Stellenbosch, South Africa
Abstract: We discuss tilings of a grid (of size n × 2) with dominoes of size 2 × 1. Parameters that might be called “longest run” are investigated, in terms of generating functions and also asymptotically. Extensions are also mentioned. 2010 MSC: 05A15, 68R05 Keywords: Tiling, Generating function, Asymptotic, Bootstrapping, Mellin transform, Fibonacci numbers
1.
Introduction
Donald Knuth [3] included tilings of an n × 2-rectangle using 2 × 1-sized tiles (“dominoes”) as an introductory example of the use of generating functions. If Tn is the number of these tilings, then Tn = Tn−1 + Tn−2 , since we have two choices to start: one vertical domino, leaving an (n − 1) × 2rectangle, or two horizontal dominos, leaving an (n − 2) × 2-rectangle. Since T0 = 1 and T1 = 1, this leads to Tn = Fn+1 (a Fibonacci number ). Here is one particular tiling of a 20 × 2-rectangle:
Denoting T the family (=set) of all tiled n × 2-rectangles, for n ≥ 0, then we can write a symbolic equation: T =
+
T +
T
∗
E-mail:
[email protected], This author did most of her research while visiting the University of Stellenbosch in September 2013. She thanks the department of mathematics for its hospitality. ∗∗ E-mail:
[email protected], This author was supported by an incentive grant of the NRF of South Africa.
JACODESMATH / ISSN 2148-838X
19
Horizontal runs in domino tilings
With the generating function T (z) :=
X
Tn z n ,
n≥0
the symbolic equation translates directly into T (z) = 1 + zT (z) + z 2 T (z) =
1 . 1 − z − z2
This equation is simple enough that, with partial fraction decomposition, one finds an explicit form √ n+1 √ n+1 1 1+ 5 1− 5 Tn = √ − . 2 2 5 The number √ 1+ 5 α := 2 is called the golden ratio; it is one of the most important constants in mathematics. In terms of asympotics it is dominating: √ n+1 1 1+ 5 Tn ∼ √ . 2 5 If one cannot resort to an explicit expression as above, one looks at the dominating singularity and writes T (z) ∼
C 1 − αz
as z →
1 . α
The constant C may be computed as C = lim1
z→ α
1 − αz α =√ . 1 − z − z2 5
In this paper, we are interested in the (consecutive) sequence of horizontal dominoes in a tiling. We are looking for the longest substructure of the type in the figure.
We will indicate in Section 2 how the expected value of this parameter may be computed. Then, in Section 3, we change our setting to tilings of n × 3 rectangles. Things become more involved, but it is highly instructive to see how one has to deal with the difficulties. For more material of a similar type we refer to [8].
2.
The longest horizontal run in tilings of n × 2-rectangles
As the first step of our analysis, we decompose a tiled rectangle according to (maximal) runs of horizontal dominoes. We indicate this for the example from the introduction: We see here runs of 3, 1, and 2 (stacked) horizontal dominoes. So our parameter is 3 for this example. Various runs of length 0 are
20
K. Oliver, H. Prodinger
not indicated. Based on this (unique!) decomposition, we introduce T
