Enumeration of Remarkable Families of Polyominoes
Dominique Gouyou-Beauchamps
LRI, Université de Paris Sud (Orsay)
Algorithms Seminar
Mars 2, 1998
[summary by
Cyril Banderier]
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1 Introduction
Polyominoes are objects sprung from recreative mathematics and from different
domains in physics (such as Ising's model;
its generalisation, Pott's model; directed percolation and branched
polymer problems) [14, 20, 21, 25].
Two great classes of problems relative to polyominoes are
-
tiling problems;
- enumeration problems.
David Klarner began to study polyomino tilings in 1965.
There are still open questions in this
field [12, 15, 16, 17], however
several (un)decidability results are known [1, 2].
What is more, aperiodic tilings are today a new spring of inspiration in
noncommutative geometry [8].
In the remainder, we only consider enumeration problems.
Exact asymptotics of polyominoes on a square lattice is still unknown.
Accurate results are then limited to special
families of polyominoes, for which we know a generative grammar. We are
therefore brought back to the study of a functional equation which
defines the generating function. Nevertheless, obtaining of a closed
form (i.e., an explicit solution) or even any form of solution
often remains difficult. We will show several methods to obtain them.
2 Definitions
A polyomino is a connected set on a lattice.
A polyomino is said to be convex if it is both column-convex and row-convex.
A polyomino is said to be directed if, for each couple of points of the
polyomino, there exists
a path only made of North and West steps which links this two points.
One can find in previous summaries [4, 13] how to obtain
functional equations satisfied by the generating functions
(most of the methods are tricky decompositions [5] of polyominoes
into very regular smaller pieces, such as ``strata'' or ``wasp-waist''
decompositions). For results in dimension greater than 2,
see [3, 6].
3 Differential Equation Method
Enumeration of convex polyominoes with perimeter 2n on the honeycomb
(or ``hexagonal'') lattice can be solved with this method.
Let Pn the number of such polyominoes with perimeter 2n+6,
Enting [10] gives the following result.
The generating function P(x)=ån=0¥pnxn satisfies
the differential equation
P''(x) (x2-7x4-2x5+12x6+8x7)+P'(x)(-11x-4x2+53x3+22x4-40x5-16x6) |
+P(x)(20+22x-52x2-20x3-16x4-32x5)=20+22x-52x2+8x3+4x4+8x5 |
which leads to
P(x)= |
1-2x+x2-x4-x2 (1-4x2)1/2 |
|
(1+x)2 (1-2x)2 |
|
. |
Let us mention that the package GFUN in Maple is able to make
such translations (recurrences, differential equations, algebraic equations,
closed forms), see [23].
4 Temperley's Method
We are going to illustrate Temperley's method [24] with the
enumeration of column convex polyominoes (on a square lattice)
with respect to perimeter [7].
The generating function can be rewritten as
where the gr satisfy a recurrence
gr+4-2(1+y2)gr+3+(1+3y2+3y4-y6)gr+2-2y2(1+y2)gr+1+y4gr=0
and g1, g2, g3, g4, the ``initial conditions'', are known.
If we ``guess'' that gr has the shape lr (or is a linear combination of such
monomials),
we can obtain l by solving the fourth degree equation associated
to the recurrence formula, and we find, as the equation easily splits:
(l2-l(1+y+y2-y3)+y4)(l2-l (1-y+y2+y3)+y2)=0.
So solving the two second degree equations gives four values (closed
forms)
l1, l2, l3, l4, two of which are O(1) at 0.
We then have to find the Aj such that gr=åj=14 Aj ljr.
But gr=O(y2r+2) at 0, so Ai=0 if li=O(1).
There are still two coefficients to determine, say A2 and A4.
They can be found by solving a system involving
g1, g2, l2, l4, A2, A4 and one finally obtains a closed form
A very similar method is applied for
unidirectional-convex polygons on the honeycomb lattice in [19].
5 Kernel Method
In his talk, Dominique Gouyou-Beauchamps has also presented an
exploitation of the ``kernel method'' for the enumeration of
parallelogram polyominoes
with respect to horizontal and vertical half-perimeter, area and first column
height, respectively marked by x, y, q, s.
Remember that the generating function P with respect to
horizontal and vertical half-perimeter is easy to obtain:
The wasp-waist decomposition directly leads to P=xy+xP+yP+P2 so
P(x, y)= |
1-x-y-(1-2x-2y+x2+y2-2xy)1/2 |
|
2 |
|
. |
The full generating function P(x, y, q, s) satisfies a more
intricate equation (obtained by a strata decomposition), namely
P(x, y, q, s)= |
|
+ |
|
P(x, y, q, 1)- |
|
P(x, y, q, sq). |
When q=1, this can be rewritten
(1-(1-x-y)s+y2) P(x, y, 1, s) =xsP(x, y, 1, 1)+xys(1-s).
It is typically the type of equation on which the kernel method applies.
This method belongs to mathematical folklore (see [18],
exercise 2.2.1.4 for an early example).
It works as follows: If one cancels the kernel (1-(1-x-y)s+y2),
i.e., one finds s0 such that (1-(1-x-y)s0+y2)=0,
then one gets 0=xs0P(x, y, 1, 1)+xys0(1-s0), from which follows
a closed form for P(x, y, 1, 1)
and finally one obtains a closed form for P(x, y, 1, s), viz.,
P(x, y, 1, s) = |
xs |
æ ç ç è |
1-x-y-(1-2x-2y-2xy+x2+y2)1/2 |
|
2 |
|
|
ö ÷ ÷ ø |
+xys(1-s) |
|
|
1-(1-x-y)s+y2 |
|
. |
6 Physicists' Guesses
We have already mentioned that polyominoes are present in physical
problems and in fact the first people who found interesting results
on this subject where physicists. They sometimes base their works
on empirical results. For example, in [9], the authors are doing as if
Nrs=Nr-1s-1+Nrs-1+Nr+1s-1
(Nrs is the number of
directed animals of size s with a ``compact source'' of size r)
was a recurrence formula satisfied by the Nrs
although it is only empirically verified for the first values.
Nevertheless, they go on and find that
Nrs= |
|
|
ó õ |
|
(1+eit) e-irt (1+2 cos t)s-1 dt
and in particular |
N1s=(s-1)! |
|
|
|
. |
Another example of a typical physicist's method is [14] (enumeration of
directed animals on a strip of width k);
they consider a transfer matrix as an operator acting on a spin space
and are drawing their inspiration from standard techniques on integrable systems.
When k tends to infinity, they obtain:
an= |
|
|
and thus |
|
|
an tn = |
|
|
æ ç ç è |
( |
|
)1/2 -1 |
ö ÷ ÷ ø |
. |
Analysis of singularities gives an ~ 3n n-1/2.
7 Matricial and Continued Fraction Method
We will show on a simple example (Dyck paths) how this method works.
Let
the ordinary generating function of Dyck paths which end at height h.
A path of length n which ends at height h is
either a path of length n-1 which ends at height h-1 followed by
a NE step,
or a path of length n-1 which ends at height h+1 followed by
a SE step. Thus one obtains the following infinite system
ì ï ï ï ï í ï ï ï ï î |
d0(x)=1+x d1(x) |
d1(x)=xd0(x)+xd2(x) |
d2(x)=xd1(x)+xd3(x) |
· · · |
dh(x)=xdh-1(x)+xdh+1(x) |
· · · |
|
which can be written as
|
æ ç ç ç ç ç ç è |
-1 |
x |
0 |
0 |
... |
x |
-1 |
x |
0 |
... |
0 |
x |
-1 |
x |
... |
0 |
0 |
x |
-1 |
... |
· · · |
· · · |
· · · |
· · · |
· · · |
|
ö ÷ ÷ ÷ ÷ ÷ ÷ ø |
|
|
æ ç ç ç ç ç ç è |
d0(x) |
d1(x) |
d2(x) |
d3(x) |
· · · |
|
ö ÷ ÷ ÷ ÷ ÷ ÷ ø |
|
=
|
|
.
|
With an analog of Cramer's formula for infinite matrices, one has
d0(x)= |
det
|
æ ç ç ç ç ç ç è |
-1 |
x |
0 |
0 |
... |
0 |
-1 |
x |
0 |
... |
0 |
x |
-1 |
x |
... |
0 |
0 |
x |
-1 |
... |
· · · |
· · · |
· · · |
· · · |
· · · |
|
ö ÷ ÷ ÷ ÷ ÷ ÷ ø |
|
|
|
det
|
æ ç ç ç ç ç ç è |
-1 |
x |
0 |
0 |
... |
x |
-1 |
x |
0 |
... |
0 |
x |
-1 |
x |
... |
0 |
0 |
x |
-1 |
... |
· · · |
· · · |
· · · |
· · · |
· · · |
|
ö ÷ ÷ ÷ ÷ ÷ ÷ ø |
|
|
|
= |
|
|
|
= |
|
|
where ()k × k
stands for the k× k truncated associated matrices.
The special structure of these matrices gives the recurrence
ì í î |
Pk(x)=-Qk-1(x)=-Pk-1(x)-x2 Pk-2(x)
|
with P1(x)=-1, |
Qk(x)=-Qk-1(x)-x2Qk-2(x) |
with Q1(x)=-1 and Q2(x)=1-x2. |
|
from which follows
|
= |
|
=
|
-Qk-1(x) |
|
-Qk-1(x)-x2 Qk-2(x) |
|
=
|
|
=
|
|
and then
hence
In fact the continued fraction is a special case
of a much more general result that we will express in the next section.
8 Multicontinued Fractions Theorem
We will need the following notations. Let
(ll, k)0£ k £ l be a family of elements of a
commutative field and let (Pk)k³ 0 be a family of monic polynomials
which satisfy a recurrence relation:
Pk+1(x)=xPk(x)- |
|
lk, k-i Pk-i(x). |
One then defines a multicontinued fraction by
Let d be the operator defined by d(lk, l)=lk+1, l+1.
We note P* the reciprocal polynomial of P:
Theorem 1 [Roblet, Viennot]
If one sets li, j:=0 in L(l, t) for i³ k+1 and j£ i,
one gets a rational fraction Lk(t), it is the k-th convergent of the
multicontinued fraction L(l, t) and we have
and the following approximation near t=0 holds
L(l, t)=Lk(t)+O(tk+1).
For a deeper understanding of links between continued fractions
and combinatorics, see [11, 22].
The multicontinued fraction method allows to find the generating functions of
diagonally convex directed, diagonally convex, parallelogram,
vertically convex directed, vertically convex polyominoes
and remains to be exploited to obtain generating functions of other
classes of polyominoes or directed animals.
You are now ready to try the different kinds of methods presented here
on your favourite class of polyominoes or even on other classes
of combinatorial objects!
References
- [1]
-
Beauquier (Danièle), Nivat (Maurice), Remila (Éric), and Robson
(Mike). --
Tiling figures of the plane with two bars. Computational
Geometry. Theory and Applications, vol. 5, n°1, 1995,
pp. 1--25.
- [2]
-
Berger (Robert). --
The undecidability of the domino problem. Memoirs of the
American Mathematical Society, vol. 66, 1966, p. 72.
- [3]
-
Bousquet-Mélou (M.) and Guttmann (A. J.). --
Three-dimensional self-avoiding convex polygons. Physical Review
E. Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary
Topics. Third Series, vol. 55, n°6, part A, 1997,
pp. R6323--R6326.
- [4]
-
Bousquet-Mélou (Mireille). --
Counting convex polyominoes according to their area. In Algorithms seminar 1995-1996. --
INRIA, 1996. Available at http://pauillac.inria.fr/algo/seminars/sem95-96/bousquet1.ps.
- [5]
-
Bousquet-Mélou (Mireille). --
A method for the enumeration of various classes of column-convex
polygons. Discrete Mathematics, vol. 154, n°1-3,
1996, pp. 1--25.
- [6]
-
Bousquet-Mélou (Mireille) and Guttmann (Anthony J.). --
Enumeration of three-dimensional convex polygons. Annals of
Combinatorics, vol. 1, n°1, 1997, pp. 27--53.
- [7]
-
Brak (R.), Guttmann (A. J.), and Enting (I. G.). --
Exact solution of the row-convex polygon perimeter generating
function. Journal of Physics. A. Mathematical and General, vol. 23,
n°12, 1990, pp. 2319--2326.
- [8]
-
Connes (Alain). --
Noncommutative geometry. --
Academic Press Inc., San Diego, CA, 1994, xiv+661p.
- [9]
-
Dhar (Deepak), Phani (Mohan K.), and Barma (Mustansir). --
Enumeration of directed site animals on two-dimensional lattices.
Journal of Physics. A. Mathematical and General, vol. 15, n°6,
1982, pp. L279--L284.
- [10]
-
Enting (I. G.) and Guttmann (A. J.). --
Polygons on the honeycomb lattice. Journal of Physics. A.
Mathematical and General, vol. 22, n°9, 1989,
pp. 1371--1384.
- [11]
-
Flajolet (P.). --
Combinatorial aspects of continued fractions. Discrete
Mathematics, n°2, 1980, pp. 125--161.
- [12]
-
Grünbaum (Branko) and Shephard (G. C.). --
Tilings and patterns. --
W. H. Freeman and Company, New York, 1989, A
Series of Books in the Mathematical Sciences, xii+446p. An introduction.
- [13]
-
Guttmann (Tony). --
Staircase polygons, elliptic integrals and Heun functions. In Algorithms seminar 1996-1997. --
1997. Available at http://pauillac.inria.fr/algo/seminars/sem96-97/guttmann.ps.
- [14]
-
Hakim (V.) and Nadal (J. P.). --
Exact results for 2D directed animals on a strip of finite width.
Journal of Physics. A. Mathematical and General, vol. 16, n°7,
1983, pp. L213--L218.
- [15]
-
Klarner (D.). --
My life among the polyominoes. Nieuw Archief voor Wiskunde.
Derde Serie, vol. 29, n°2, 1981, pp. 156--177.
- [16]
-
Klarner (David A.). --
Some results concerning polyominoes. The Fibonacci Quarterly,
vol. 3, 1965, pp. 9--20.
- [17]
-
Klarner (David A.). --
Packing a rectangle with congruent n-ominoes. Journal of
Combinatorial Theory, vol. 7, 1969, pp. 107--115.
- [18]
-
Knuth (Donald E.). --
The art of computer programming. --
Addison-Wesley Publishing Co., Reading, Mass.-London-Amsterdam,
1973, second edition, xxii+634p. Volume 1: Fundamental
algorithms, Addison-Wesley Series in Computer Science and Information
Processing.
- [19]
-
Lin (K. Y.) and Wu (F. Y.). --
Unidirectional-convex polygons on the honeycomb lattice. Journal
of Physics. A. Mathematical and General, vol. 23, n°21,
1990, pp. 5003--5010.
- [20]
-
Privman (V.) and Svrakic (N. M.). --
Difference equations in statistical mechanics. I. Cluster
statistics models. Journal of Statistical Physics, vol. 51, n°5-6,
1988, pp. 1091--1110. --
New directions in statistical mechanics (Santa Barbara, CA, 1987).
- [21]
-
Privman (V.) and Svrakic (N. M.). --
Directed models of polymers, interfaces, and clusters: scaling
and finite-size properties. --
Springer-Verlag, Berlin, 1989, Lecture Notes in
Physics, vol. 338, vi+120p.
- [22]
-
Roblet (Emmanuel) and Viennot (Xavier Gérard). --
Théorie combinatoire des T-fractions et approximants de Padé
en deux points. In Proceedings of the 5th Conference on Formal Power
Series and Algebraic Combinatorics (Florence, 1993), vol. 153,
pp. 271--288. --
1996.
- [23]
-
Salvy (Bruno) and Zimmermann (Paul). --
Gfun: a Maple package for the manipulation of generating and
holonomic functions in one variable. ACM Transactions on Mathematical
Software, vol. 20, n°2, 1994, pp. 163--177. --
ftp://ftp.inria.fr/INRIA/publication/publi-ps-gz/RT/RT-0143.ps.gz.
- [24]
-
Temperley (H. N. V.). --
Combinatorial problems suggested by the statistical mechanics of
domains and of rubber-like molecules. Physical Review (2), vol. 103,
1956, pp. 1--16.
- [25]
-
Viennot (Gérard). --
Problèmes combinatoires posés par la physique statistique. Astérisque, n°121-122, 1985, pp. 225--246. --
Seminar Bourbaki, Vol. 1983/84.
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