Hamiltonicity of maximal planar graphs and planar triangulations [Elektronische Ressource] / vorgelegt von Guido Helden

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Hamiltonicity of maximal planar graphs andplanar triangulationsVon der Fakult at fur Mathematik, Informatik und Naturwissenschaftender Rheinisch-Westf alischen Technischen Hochschule Aachenzur Erlangung des akademischen Grades eines Doktors derNaturwissenschaften genehmigte Dissertationvorgelegt vonDiplom-Mathematiker Guido Heldenaus AachenBerichter: Professor Dr. Yubao GuoUniversit atsprofessor Dr. Dr. h.c. Hubertus Th. JongenTag der mundlic hen Prufung: 26.06.2007Diese Dissertation ist auf den Internetseiten der Hochschulbibliothekonline verfugbar.PrefaceOne of the typical topics in graph theory is the study of planar graphs. Planar graphsarise quite naturally in real-world applications, such as road or railway maps and chemicalmolecules. The cartographers of the past were aware of the fact that any map on the planecould be colored with four or fewer colors so that no adjacent countries were colored alike.It was the Four Color Problem which stimulated interest in hamiltonian planar graphs. Agraph is said to be hamiltonian if it has a cycle that contains all vertices exactly once. Ifa planar graph is hamiltonian, then it is easy to color its faces with four or fewer colors sothat no two adjacent faces are colored alike. Moreover, planar graphs play an importantrole as some practical problems can be e cien tly solved for planar graphs even if they areintractable for general graphs.The study of planar graphs was initiated by L.
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01 janvier 2007

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Hamiltonicity of maximal planar graphs and
planar triangulations
Von der Fakult at fur Mathematik, Informatik und Naturwissenschaften
der Rheinisch-Westf alischen Technischen Hochschule Aachen
zur Erlangung des akademischen Grades eines Doktors der
Naturwissenschaften genehmigte Dissertation
vorgelegt von
Diplom-Mathematiker Guido Helden
aus Aachen
Berichter: Professor Dr. Yubao Guo
Universit atsprofessor Dr. Dr. h.c. Hubertus Th. Jongen
Tag der mundlic hen Prufung: 26.06.2007
Diese Dissertation ist auf den Internetseiten der Hochschulbibliothek
online verfugbar.Preface
One of the typical topics in graph theory is the study of planar graphs. Planar graphs
arise quite naturally in real-world applications, such as road or railway maps and chemical
molecules. The cartographers of the past were aware of the fact that any map on the plane
could be colored with four or fewer colors so that no adjacent countries were colored alike.
It was the Four Color Problem which stimulated interest in hamiltonian planar graphs. A
graph is said to be hamiltonian if it has a cycle that contains all vertices exactly once. If
a planar graph is hamiltonian, then it is easy to color its faces with four or fewer colors so
that no two adjacent faces are colored alike. Moreover, planar graphs play an important
role as some practical problems can be e cien tly solved for planar graphs even if they are
intractable for general graphs.
The study of planar graphs was initiated by L. Euler in the 18th century. It was sub-
sequently neglected for 180 years and only came to life again with Kuratowski’s theorem,
which provides a criterion for a graph to be planar. At the same time, one of the most
outstanding results for maximal planar graphs was proved by H. Whitney. A maximal
planar graph is a planar graph having the property that no additional edges can be added
so that the resulting graph is still planar. In 1931 H. Whitney [55] proved that each
maximal planar graph without separating triangles is hamiltonian; a separating triangle
is a triangle whose removal separates the graph. In particular, all 4{connected maximal
planar graphs are hamiltonian.
Another connection of maximal planar graphs and the Four Color Problem is the
following. In 1884 P.G. Tait [49] conjectured that every cubic, 3{connected and planar
graph is hamiltonian. Note that the dual graph of a cubic, 3{connected and planar graph
is a maximal planar graph. The Conjecture of Tait became more important, in the sense
that its correctness would imply the positive correctness of the Four Color Problem. Un-
fortunately, the latter conjecture is wrong, as shown by W.T. Tutte in 1946. Therefore,
the Four Color Problem is true if and only if every hamiltonian planar graph is 4{colorable.
In 1956 W.T. Tutte [53] generalized Whitney’s result and proved that each 4{connected
planar graph is hamiltonian. In 1990 M.B. Dillencourt [24] generalized Whitney’s result
in a di eren t direction, namely to graphs called NST{triangulations. M.B. Dillencourt
iii
de ned NST{triangulations to be those planar graphs that are triangulations in the sense
that every face, perhaps except for the exterior face, is a triangle but that there is no
separating triangle. In the class of the 3{connected maximal planar graphs numerous ex-
amples which are non-hamiltonian were found. In 2003 C. Chen [17] obtained a stronger
version of Whitney’s result and he proved that any maximal planar graph with only one
separating triangle is hamiltonian. An interesting problem with respect to the class of
the 3{connected maximal planar graphs is the following: What is the maximal number
, so that every planar graph with at most separating triangles is hamiltonian?
In this thesis we will extend the results of C. Chen and M.B. Dillencourt in various
ways. We will mainly study the existence of hamiltonian cycles and hamiltonian paths
in maximal planar graphs and planar triangulations. We will prove that each maximal
planar graph with at most v e separating triangles is hamiltonian. In the case of more
separating triangles, we will introduce a special structure of the position of the separating
triangles to each other. The latter structure will also generate hamiltonicity.
Chapter 1 will be devoted to an introduction of the terminology and the basic concepts
of planar graphs and maximal planar graphs. We will introduce one of the most powerful
tools in the theory of planar graphs, namely Euler’s formula. This concept will be the
corner-stone of many results of us.
In Chapter 2 we will study some fundamentals of maximal planar graphs. The rst
section will be concerned with the generation of some classes of maximal planar graphs.
There are two main reasons why this will be of interest: On the one hand,
this will provide a basis for inductive proof; and on the other hand, this can be used to
develop e cien t algorithms for the constructive enumeration of the structures. In the
second section we will focus on the structures of some classes of maximal planar graphs.
Moreover, we will study the subgraphs of a maximal planar graph G induced by vertices
of G with the same degree. In the last section we will turn towards the connectivity of
maximal planar graphs.
The most interesting problem regarding hamiltonian maximal planar graphs will be
investigated in Chapter 3. In the rst section we will extend the result of Chen [17].
We will prove that each maximal planar graph with at most v e separating triangles is
hamiltonian. We will give a counterexample for the case of six triangles. In
the case of more separating triangles, we will establish a special structure of the position
of the separating triangles to each other, which will also generate hamiltonicity. In the
second section we will deal with hamiltonian paths. Since maximal planar graphs with
more than v e separating triangles need not be hamiltonian, we will show that maximal
planar graphs with exactly six separating triangles have at least one hamiltonian path.
It would be nice to have a theorem that says: \A graph G is non-hamiltonian if G hasiii
property ‘Q’, where ‘Q’ can be checked in polynomial time". This motivates the con-
cept of 1{toughness, which is introduced in the fourth section. In the fth section we
will construct speci c maximal planar graphs. We will show that for all n 13 and
k 6, there exist a hamiltonian as well as a non-hamiltonian maximal planar graph with
n vertices and k separating triangles. In the sixth section we will study the number of
cycles of certain length that a maximal planar graph G on n vertices may have. In the
seventh section we will consider pancyclicity rather than hamiltonicity. In fact, we will
determine whether a maximal planar graph G contains a cycle of each possible length l
with 3 l n. In the eighth section we will deal with the question: How many vertices
of a hamiltonian maximal planar graph can be deleted, so that the remaining graph is
still hamiltonian?
In Chapter 4 we will turn to general planar triangulations. Clearly, each maximal pla-
nar graph is a planar triangulation. In the rst section we will consider some properties
of planar triangulations. The second section of this chapter deals with general results
on 2{connected planar In the third section we will see that the deletion
of vertices of a hamiltonian maximal planar graph leads to some interesting results in
the class of 3{connected planar triangulations. In the last section we will concentrate on
4{connected planar triangulation.
In Chapter 5 we will consider some applications of hamiltonian maximal planar graphs
and planar triangulations. In the rst section of this chapter we will turn towards algo-
rithms, which will be useful for applications. In the second section we will show that the
dual graph of a maximal planar graph is of interest for applications. In the third section
we will deal with some applications of hamiltonian maximal planar graphs and planar
triangulations in computer graphics. In the last section we will focus on applications in
chemistry.
I am particularly indebted to my advisor, Prof. Dr. Y. Guo, for introducing me to
graph theory and for giving me the opportunity to conduct my doctorial studies under his
supervision. I also would like to express my deep gratitude to Prof. Dr. Dr. h.c. H. Th.
Jongen for acting as co-referee and for his support during the last few years. Many thanks
also to my colleague Jinfeng Feng for stimulating discussions and excellent teamwork.
Finally, I would like to express my appreciation for my wife and two children, who
allowed me take the time to write this thesis.Contents
1 Introduction 1
1.1 Terminology and notation . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Subject . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Fundamentals of maximal planar graphs 5
2.1 Generating planar graphs . . . . . . . . . . . . . . . . . . . . . . 5
2.2 Properties of . . . . . . . . . . . . . . . . . . . . . 8
2.2.1 Properties of MPG-3 graphs . . . . . . . . . . . . . . . . . . . . . . 10
2.2.2 of MPG-4 . . . . . . . . . . . . . . . . . . . . . . 12
2.2.3 Properties of MPG-5 graphs . . . . . . . . . . . . . . . . . . . . . . 16
2.3 Connectivity of maximal planar graphs . . . . . . . . . . . . . . . . . . . . 17
3 Hamiltonicity of maximal planar graphs 21
3.1 Hamiltonian c

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