# Graph Powers: Hardness Results, Good Characterizations and Efficient Algorithms. Dissertation

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1 Graph Powers: Hardness Results, Good Characterizations and Efficient Algorithms Dissertation zur Erlangung des akademischen Grades Doktor-Ingenieur (Dr.-Ing.) der Fakultät für Informatik und Elektrotechnik der Universität Rostock Vorgelegt von MSc. Ngoc Tuy Nguyen Rostock, November 009

2 Gutachter: Gutachter: Gutachter: Prof. Dr. Van Bang Le Universität Rostock. Institut fu r Informatik Prof. Dr. Egon Wanke Heinrich- Heine- Universität Düsseldorf. Institut fu r Informatik Prof. Dr. Ekkehard Köhler Brandenburgische Technische Universität Cottbus. Institut fu r Mathematik Tag der öffentlichen Verteidigung: 30. Oktober 009 ii

3 Abstract Given a graph H = (V H, E H ) and a positive integer k, the k-th power of H, written H k, is the graph obtained from H by adding new edges between any pair of vertices at distance at most k in H; formally, H k = (V H, {xy 1 d H (x, y) k}). A graph G is the k-th power of a graph H if G = H k, and in this case, H is a k-th root of G. For the cases of k = and k = 3, we say that H and H 3 is the square, respectively, the cube of H and H is a square root of G = H, respectively, a cube root of G = H 3. In this thesis we study the computational complexity for recognizing k-th powers of general graphs as well as restricted graphs. This work provides new NPcompleteness results, good characterizations and efficient algorithms for graph powers. The main results are the following. There exist reductions proving the NP-completeness for recognizing k-th powers of general graphs for fixed k, recognizing k-th powers of bipartite graphs for fixed k 3, recognizing k-th powers of chordal graphs, and finding k-th roots of chordal graphs for all fixed k. The girth of G, girth(g), is the smallest length of a cycle in G, For all fixed k, recognizing of k-th powers of graphs with girth at most k + is NP-complete. There is a polynomial time algorithm to recognize if G = H for some graph H of girth at least 6. This algorithm also constructs a square root of girth at least 6 if one exists. There exists a good characterization of squares of a graph having girth at least 7. This characterization not only leads to a simple algorithm to compute a square root of girth at least 7 but also shows such a square root, if it exists, is unique up to isomorphism. There is a good characterization of cubes of a graph having girth at least 10 that gives a recognition algorithm in time O(nm ) for such graphs. Moreover, this algorithm constructs a cube root of girth at least 10 if it exists. iii

4 These results almost provide a dichotomy theorem for the complexity of the recognition problem in terms of girth of the square roots. There is a good characterization of squares of strongly chordal split graphs that gives a recognition algorithm in time O(min{n, m log n}) for such squares. Moreover, this algorithm also constructs a strongly chordal split graph square root if it exists. There exists a good characterization and a linear-time recognition algorithm for squares of block graphs. This algorithm also constructs a block graph square root if one exists. Moreover, block graph square roots in which every endblock is an edge are unique up to isomorphism. The almost results in thesis have been published in the following papers of journal and proceedings of conferences. Computing Graph Roots Without Short Cycles, Proceedings of the 6 th International Symposium on Theoretical Aspects of Computer Science (STACS 009), pp Co-authors: Babak Farzad (Brock University, Canada), Lap Chi Lau (The Chinese University of Hong Kong), Van Bang Le (University of Rostock). Hardness Results and Efficient Algorithms for Graph Powers, to appear in: Proceedings of the 35 th International Workshop on Graph-Theoretic Concepts in Computer Science (WG 009). Co-author: Van Bang Le (University of Rostock). The square of a block graph, to appear in: Discrete Mathematics (009), doi: /j.disc Co-author: Van Bang Le (University of Rostock). iv

5 Acknowledgments Although a few words do not do justice to their contribution I would like to thank the following people for making this work possible. First of all I would like to express my sincere gratitude to my advisor Professor Van Bang Le, for his patience, invaluable advice and encouragement throughout. His lectures and advices have been of great value for my research. This work would not have been possible without his support, patience and encouragement. I wish to express my sincere thanks to Professor Andreas Brandstädt for a lot of personal help during the time. I thank Professor Egon Wanke and Professor Ekkehard Köhler for their reading and commenting on my thesis. I am grateful to my co-authors for the joint works and successful discussions, and for allowing me to include the results in this thesis. I am also grateful to colleagues of the Institute of theoretical informatics for their help and support during my study. Special thanks go to Roswitha Fengler, Katrin Erdmann, Christian Hundt and Ragnar Nevries for a lot of help. Furthermore, I also would like to express my gratitude to my university, HongDuc, who recommended and encouraged me to study well. Especially, I am truly deeply grateful to all my colleagues at the Department of Computer Science, HongDuc University for their encouragements. While working on this thesis, financial support by the Ministry of Education and Training, Vietnam (3 Project) by Grant No.3766/QD-BGD&DT is gratefully acknowledged. I am also grateful to DAAD for their support for me in learning German. Great thanks go to my buddies for their support and encouragement. I also want to thank to all friends in Rostock - Rostockers for everything we shared together over the years of living. Last but not least, I am indebted to my parents, my sisters and brothers who have given me the best opportunities. All my love and thanks go to my wife Hạnh and my children Quỳnh Anh and Minh Lộc. Without their love and gentle encouragement I could not complete this work. Thank you very much! Rostock, November 4, 009 Nguyêñ Ngọc Túy v

6 Contents Abstract Acknowledgments List of Figures iii v ix I Introduction 1 1 Overview 1.1 Introduction Contributions of the thesis Background 6.1 Definitions and notation Graph powers and related works II NP-completeness 13 3 Preliminaries Introduction Preliminaries Powers of Bipartite and Chordal Graphs Powers of bipartite graphs Powers of chordal graphs Finding k-th roots of chordal graphs vi

7 4.4 Concluding remarks Powers of Graphs with Girth Conditions Squares of graphs with girth at most four k-th powers of graphs with girth k Concluding remarks III Good Characterizations and Efficient Algorithms 60 6 Squares of Graphs with Girth At Least Six Introduction Squares of graphs with girth at least seven Basic facts Good characterizations of squares of graphs with girth at least seven Squares of trees revisited Further considerations Squares of graphs with girth at least six Square root with a specified neighborhood Square root with a specified edge Concluding remarks Cubes of Graphs with Girth At Least Ten Basic facts Good characterization Further considerations Concluding remarks Squares of Strongly Chordal Split Graphs Squares of strongly chordal split graphs Concluding remarks Squares of Block Graphs Introduction vii

8 9. Basic facts Good characterizations of squares of block graphs A linear time recognition for squares of block graphs Squares of trees revisited Concluding remarks Bibliography 103 Index 108 Thesen 110 Zusammenfassung 11 Summary 114 Erklärung 116 Curriculum Vitae 117 viii

9 List of Figures 3.1 Tail in H (left) and in G = H (right) Tail in k-th root H of G and in G The graph G for the example instance of set splitting and k = The bipartite 4-th root graph H of G to the solution S 1, S A bipartite k-th root H in Lemma to the example solution S 1, S 4.4 The graph G for the example instance of set splitting and k = Cube root graph H to the solution S 1, S A chordal k-th root H in Lemma 4..1 to the example solution S 1, S The chordal graph G for the example instance of set splitting and k = Fourth root graph H to the solution S 1, S A k-th root H in Lemma to the example solution S 1, S The graph G for the example instance of set splitting An example of root H with girth The graph G for the example instance of set splitting and k = The fourth root graph H with girth six of G to the solution S 1, S The graph G for the example instance of set splitting and k = The fifth root graph H with girth six of G to the solution S 1, S A k-th root H with girth k + for even k in Lemma A k-th root H with girth k + 1 for odd k in Lemma An input graph G (a), the subgraph F of G (b) and a square root H (c) constructed according to the proof of Theorem ix

10 6. An input graph G (a) with the specified neighborhood U = {8, c, d} N G (b); a (C 3, C 5 )-free square root H (b) constructed by Algorithm 6.3.1: c3c5-free The vertices u, v are maximal vertices in the given graph An input graph G (a), a clique C in G (b) and a square root H (c) constructed by Algorithm Two block graphs (a) and (b) with the same square (c) An input graph G (a), the subgraph F of G (b) and a square root H (c) constructed by Algorithm 9.3: BlockGraphRoot x

11 Part I Introduction 1

12 Chapter 1 Overview 1.1 Introduction In graph theory, graph H is a root of graph G = (V, E) if there exists a positive integer k such that x and y are adjacent in G if and only if their distance in H is at most k. If H is a k-th root of G, then we write G = H k and call G the k-th power of H. Graph powers and roots are fundamental graph-theoretic concepts and have been extensively studied in the literature, both in theoretic and algorithmic senses; see e.g., [17, 9, 4, 45, 46, 49] for recent results and the numerous references listed there. The first motivation comes directly from the fact that root and root computing are concepts familiar to most branches of mathematics. Any result on graph roots and powers would be a very valuable contribution in studying graph theory. Graph powers are also very useful in designing efficient algorithms for certain combinatorial optimization problems. For instance, the papers [, 1] used a (in linear time constructible) Hamiltonian cycle in the cube T 3 of the nontrivial tree T in their approximation algorithm for the Traveling Salesman Problem (TSP) with a parameterized triangle inequality. For the same problem, the approximation proposed in [9] used a Hamiltonian cycle in the square G of a -connected graph G. Note that these graph power-based algorithms are the best known approximations for this variant of the TSP problem. Note also that the square of any -connected graph is Hamiltonian is a fundamental and deep theorem in graph theory due to Fleischner [30], and the PhD thesis [44] provided an algorithm that constructs a Hamiltonian cycle in such a square in polynomial time. Moreover, graph roots and powers are of great interest and have a number of possible applications in different disciplines. For example, in distributed computing (cf. [55, 51]), the k-th power of a graph G represents the possible flow of information during k rounds of communication of a distributed network of processors organized according to G.

13 1.1. INTRODUCTION 3 In radio frequency planning, certain situations are closely related to the coloring of the power of the associated radio network; in such applications, the associated radio network is the graph where vertices represent the transmitters and adjacencies between vertices indicate possible interferences. To avoid interference, transmitters which are close (at distance at most k in the graph) receive different frequencies. This problem is exactly the coloring problem on k-th powers of the graph. There are many studies on this problem. For example (cf. [8, 10, 1,, 69]), radio frequency planning without direct and hidden collisions in the radio network G (also known as L(1, 1)-labeling problem or distance- coloring problem) is equivalent to the coloring of the square G of G, and has been well-studied. In computational biology, graph powers and roots, in particular tree powers and roots, are useful in the reconstruction of phylogeny (cf. [0, 50, 57, 15]). Graph powers and roots are fundamental graph-theoretic concepts and have been extensively studied in literature, both in theoretic and algorithmic senses. These investigations considered both characterization and recognition problems, see [13, Sec. 10.6] for a survey and [46, 45] for the most recent papers. For the characterization problem, in 1960, Ross and Harary [63], first studied the concept of a square of graphs. They characterized squares of trees and showed that tree square roots, when they exist, are unique up to isomorphism. Next, in 1967, Mukhopadhyay [56] provided a characterization of general graphs which have a square root. In 1974, Escalante et al. [7] characterized graphs and digraphs with a k-th root. However, these characterizations are not good in the sense that they do not lead to a polynomial time recognition algorithms for such as graphs. In fact, such a good characterization may not exist as Motwani and Sudan proved that it is NP-complete to determine if a given graph has a square root [55]. The computational complexity of recognizing of k-th powers of graphs was unresolved until 1994 when Motwani and Sudan [55] proved that recognizing of square of graphs is NP-complete. Very recently, in 006, Lau [45] proved the NP-completeness for recognizing of cubes of graphs. He conjectured that recognizing k-th powers of some graph is NP-complete for all fixed k (cf. Conjecture..1, p. 11) and recognizing k-th powers of bipartite graphs is NP-complete for all fixed k 3 (cf. Conjecture.., p. 1). By the above-mentioned negative results of Motwani and Sudan, and of Lau, a natural way (in theory and in applications) in considering k-th powers of graphs is to restrict the root graphs H. In this thesis, we study the computational complexity for recognizing k-th powers of general graphs as well as restricted graphs. In particular, the aim of this work is as follows. Reductions proving the NP-completeness for recognizing k-th powers of general graphs for fixed k, recognizing k-th powers of bipartite graphs for fixed

14 4 CHAPTER 1. OVERVIEW k 3, recognizing k-th powers of chordal graphs, finding k-th roots of chordal graphs for all fixed k, and recognizing k-th powers of graphs with girth at most k + for all fixed k. Good characterizations of squares of a graph having girth at least seven and cubes of a graph having girth at least ten. Squares of graphs with girth at least six can be recognized in polynomial time. Good characterization of squares of strongly chordal split graphs that leads to a recognition algorithm in time O(min{n, m log n}) for such squares. Good characterization and a linear-time recognition algorithm for squares of block graphs. 1. Contributions of the thesis There are three parts in this thesis. In Part I after a brief introduction of motivations and overview of thesis (Chapter 1), Chapter provides basic notion and facts used throughout the thesis. The main results of this thesis appear in Part II and Part III. Part II includes the NP-completeness results for recognizing k-th powers of graphs. Chapter 3 recalls the NP-completeness results on graph power recognition problems in the literature. This chapter presents also some tools that are used in reductions in Chapter 4 and Chapter 5. Chapter 4 gives reductions proving the Conjecture..1 and Conjecture.. are indeed true. This chapter also shows that recognizing k-th powers of chordal graphs and k-th roots of chordal graphs are NPcomplete for all fixed k. Chapter 5 considers the complexity for recognizing k-th powers of graphs with girth conditions. It shows that recognizing of k-th powers of graphs with girth at most k + is NP-complete. Part III includes four chapters presenting efficient algorithms for recognizing squares and cubes of some restricted graphs. Chapter 6 provides a good characterization for graphs that are squares of some graph of girth at least seven. This characterization not only leads to a simple algorithm to compute a square root of girth at least 7 but also shows that such a square root, if it exists, is unique up to isomorphism. This chapter also shows that squares of graphs with girth at least six can be recognized in polynomial time. Chapter 7 gives a good characterization of graphs that are cubes of a graph having girth at least 10. This characterization leads to the immediate consequence that recognizing cubes of (C 4, C 6, C 8 )-free bipartite graphs is polynomial time, whereas recognizing cubes of general bipartite is NP-complete. Chapter 8 shows that there exists a good characterization of squares of strongly chordal split graphs that gives a recognition algorithm in time O(min{n, m log n})

15 1.. CONTRIBUTIONS OF THE THESIS 5 for such squares. Part III is closed by Chapter 9. This chapter provides good characterizations for squares of block graphs and a linear-time recognition algorithm for such squares. This algorithm also constructs a square block graph root if one exists. Moreover, block graph square roots in which every endblock is an edge are unique up to isomorphism.

16 Chapter Background Section.1 provides basic notions and definitions which are necessary for understanding the remaining part of this thesis. In Section. we recall two fundamental problems on graph powers, and collect several facts that are related to this work..1 Definitions and notation Notions and definitions not given here can be found in any standard textbook on graph theory or graph algorithms, e.g. [11, 13, 68, 33, 35]. In the following we always consider finite, undirected and simple graphs G = (V, E) where V is the vertex set of G and E the edge set of G. To avoid ambiguities we sometimes use V (G) or V G to refer to V and E(G) or E G to refer to E. The cardinality of the vertex set V is denoted by n, and the cardinality of the edge set E is denoted by m. A set of graphs is generally called a graph class. The complement G of a graph G is the graph with vertex set V G defined by uv E G if and only if uv E G. A graph H = (V H, E H ) is a subgraph of G = (V, E) if V H V and E H E. A graph H = (V H, E H ) is an induced subgraph of G, if it is a subgraph of G and it contains all the edges uv such that u, v V H and uv E G. We say that H is induced by V H and write G[V H ] for H. Given a set of vertices X V, if X = {a, b, c,...}, we write G[a, b, c,...] for G[X]. Also, we often identify a subset of vertices with the subgraph induced by that subset, and vice versa. Let F denote a set of graphs. A graph G is F free if none of its induced subgraphs is isomorphic to a graph in F. G is connected if for all u, v V, u v, there is a path u, v-path in G connecting u and v; otherwise, G is disconnected. A maximal connected subgraph of G is a subgraph that is connected and is not contained in any other connected subgraph. The connected components of a graph are its maximal connected subgraphs. Let G = (V, E) be a connected graph. A subset S V is a cutset (or separator) 6

17 .1. DEFINITIONS AND NOTATION 7 of G if G[V \ S] is disconnected. For a positive integer k, a k-connected component in a graph G is a maximal (induced) k-connected subgraph of G; the 1-connected components of G are the usual connected components, and the -connected components of G are also called blocks of G. A k-cut in a graph is a cutset with k vertices; a 1-cut is also called a cut-vertex. An endblock in a graph is a block that contains at most one cut-vertex of the graph. An isomorphism from G to H is a bijection f : V G V H such that uv E G if and only if f(u)f(v) E H for all u, v V G. We say G is isomorphic to H. If G has a u, v-path, then the distance from u to v, written d G (u, v) is the length, i.e., number of edges, of a shortest path in G between u and v. The diameter of G, written diam(g), is the maximum distance between two vertices (and if G is not connected). For k 1, let P k denote a chordless path with k vertices and k 1 edges, and for k 3, let C k denote a chordless cycle with k vertices and k edges. The length of the cycle is the number k of its edges. An even (odd) path (cycle) is a path (cycle) of even (odd) length. Chordless cycles C k, k 5, are holes. Chordless cycles C k+1, k, are odd holes. Complements C k of chordless cycles C k, k 5 are antiholes. Complements C k+1 of odd holes C k+1, are odd antiholes. Let G = (V G, E G ) be a graph. We often write xy E G for {x, y} E G. We sometimes also write x y for the adjacency of x and y in the graph in question; this is particularly the case when we describe reductions in NP-completeness proofs. For disjoint sets of vertices X and Y, we write X Y, meaning each vertex in X is adjacent to each vertex in Y ; if X = {x}, we simply write x Y. The girth of G, girth(g), is the smallest length of a cycle in G; in case G has no cycles, we set girth(g) =. In other words, G has girth k if and only if G contains a cycle of length k but does not contain any (induced) cycle of length l = 3,..., k 1. It is quite usual to measure the sparseness of a graph in terms of its girth; a graph is sparse if its girth is large enough. A girth(g) = if and only if G is a tree. Note that the girth of a graph can be computed in time O(nm) [41]. Definition.1.1 (Powers of Graphs) Let H = (V H, E H ) be a graph. Given a positive integer k, the k-th power of H, written H k, is the graph obtained from H by adding new edges between any pair of vertices at distance at most k in H; formally, H k = (V H, {xy 1 d H (x, y) k}). A graph G is the k-th power of a graph H if G = H k, and in this case, H is a k-th root of G. For the cases of k = and k = 3, we say that H and H 3 is the square, respectively, the cube of H and H is a square root of G = H, respectively, a cube root of G = H 3.

18 8 CHAPTER. BACKGROUND Definition.1. (Neighborhood) Let G = (V, E) be a graph and v V. N(v) = {u u V, u v and uv E} denotes the (open) neighborhood of v, N[v] = N(v) {v} denotes the closed neighborhood of v, N k (v) = {u u V and d(u, v) = k} denotes the k-th neighborhood of v, Definition.1.3 (Degree of Vertex) Let G = (V, E) be a graph and v V. Set deg G (v) = N G (v), the degree of v in G. The maximum degree is denoted (G), minimum degree is denoted δ(g). A vertex with degree 0 is called an isolated vertex. We call vertices of degree one in G end-vertices of G. A center vertex (or universal vertex) of G is one that is adjacent to all other vertices. Definition.1.4 (Clique and Stable Set) Let G = (V, E) be a graph. A set of vertices Q V is called a clique in G if every two distinct vertices in Q are adjacent; A stable set or an independent set is a set of pairwise non-adjacent vertices. A maximal clique (stable set) is a clique (stable set) that is not properly contained in another clique(stable set). A clique (stable set) is maximum if its cardinality is the maximum possible size of a clique (stable set) in G. The clique number of graph, written ω(g), is the maximum clique size in G. The independence number of graph, written α(g), is the maximum independent set size in G. The chromatic number of graph, written χ(g), is the minimum number of colors needed to label the vertices so that adjacent veritices receive different colors. The clique cover number of graph, written θ(g), is the minimum number of cliques in G needed to cover V G. The next definitions give some special graph classes that we need in later chapters. Definition.1.5 (Intersection Graph) For a given set M of objects (for which intersection makes sense), the intersection graph G M of these objects has M as vertex set, and two objects are adjacent in G M if the intersection of the corresponding objects is non-empty.

19 .1. DEFINITIONS AND NOTATION 9 Definition.1.6 (Some Special Graph Classes) A complete graph is one in which every two distinct vertices are adjacent; a complete graph on k vertices is also denoted by K k. A star is a graph with at least two vertices that has a center vertex and the other vertices are pairwise non-adjacent. Note that a star contains at least one edge and at least one center vertex; the center vertex is unique whenever the star has more than two vertices. A graph is a block graph if it is connected and its blocks (-connected components) are cliques. A graph is a parity graph if for any two induced paths joining the same pair of vertices, the path lengths have the same parity (i.e., they are both odd or both even). A graph G is perfect if χ(h) = ω(h) for every induced subgraph H of G. A graph G is Berge graph if it does not contain an odd hole or an odd antihole. A graph is perfect if and only if it is a Berge graph [3]. A graph G is chordal if it contains no induced cycle of length at least four. A graph G is weakly chordal if G and G contain no induced cycle of length at least five. A chordal graph is strongly chordal if it does not contain any l-sun as an induced subgraph; here a l-sun, l 3, consists of a stable set {u 1, u,...,u l } and a clique {v 1, v,...,v l } such that for i {1,...,l}, u i is adjacent to exactly v i and v i+1 (index arithmetic modulo l). A graph is a split graph if its vertex set can be partitioned into a clique and stable set. Clearly split graphs are chordal. A graph is an interval graph if it has an intersection model consisting of intervals on a straight line. A proper interval graph is an interval graph that has an intersection model in which no interval properly contains another. A graph is bipartite if there is a partition of its vertex set into two disjoint stable sets called the bipartition of G. It is well known that a graph is bipartite if and only if its chromatic number is at most. G is a forest if G contains no cycle. G is a tree if G connected and contains no cycle.

20 10 CHAPTER. BACKGROUND A vertex v is simplicial if its neighborhood is a clique (equivalently, if it belongs to exactly one maximal clique). Let σ = (v 1, v,...,v n ) be an ordering of the vertices in a graph G. We say that σ is a simplicial elimination ordering (or perfect elimination ordering ) if for all i {1,...,n}, the vertex v i is simplicial in G i = G[v i,...,v n ]. It is proven that Theorem.1.7 ([6]) A graph is chordal if and only if it has a simplicial elimination ordering. Furthermore, a simplicial elimination ordering of a chordal graph can be computed in linear time [6].. Graph powers and related works Since the power of a graph H is the union of the powers of the connected components of H, throughout this thesis, we assume that all graphs considered are connected. Graph powers and roots are fundamental graph-theoretic concepts and have been extensively studied in the literature, both in theoretic and algorithmic senses; see e.g., [17, 9, 4, 45, 46, 49] for recent results and the numerous references listed there. Characterizing and recognizing graph powers are two fundamental problems in the study of graph powers. For characterization problem, one gives necessary and sufficient properties of graphs that are powers of some graph. In this way, in 1960, Ross and Harary [63] characterized squares of trees and showed that tree square roots, when they exist, are unique up to isomorphism. In 1968, Rao [59] gave a necessary and sufficient conditions for a graph to be the cube of a tree. He further showed that if G is noncomplete and is the cube of a tree, then G has an unique cube root. Furthermore, he also obtained a criterion for a graph to be the fourth power of a tree [60]. In 1967, Mukhopadhyay [56] provided a characterization of graphs which have a square root. He found that a connected undirected graph G with vertices v 1,...,v n has a square root if and only if G contains a collection of n cliques G 1,...,G n such that for all 1 i, j n: 1. 1 i n G i = G,. v i G i, 3. v i G j if and only if v j G i. In 1997, Flotow [31] gave sufficient conditions for graphs whose powers are chordal graphs and graphs whose powers are interval graphs.

21 .. GRAPH POWERS AND RELATED WORKS 11 On the other hand, the following recognition problem has attracted much attention in recent years. k-th power of graph Instance: A graph G. Question: Is there a graph H such that G = H k? This is motivated by the fact that all above-mentioned characterizations are not polynomial in the sense that they do not lead to a polynomial time recognition algorithms for such graphs. The complexity of k-th power of graph was unresolved until 1994 when Motwani and Sudan [55] proved that it is NP-complete to determine if a given graph has a square root. This result implies that good characterizations of graph powers may not exist in general. Moreover, by the negative results of Motwani and Sudan, a natural way (in theory and in applications) in considering graph powers is to restrict the root graphs H. Formally, given a graph class C and an integer k, the restricted recognition problem is as follows. k-th power of C-graph Instance: A graph G. Question: Is there a graph H in C such that G = H k? The case when C is the class of all trees is well-studied ([63, 34, 49, 4, 17, 45, 15]). In 1995, Lin and Skiena [49] gave an algorithm that recognizes squares of trees in linear time. In 1998, Kearney and Corneil [4] provided an algorithm to recognize k-th powers of trees in cubic time for all fixed k. Recently, in 006, Chang et al. [17] improved Kearney and Corneil s result by reducing the running time to linear. New and simpler linear-time algorithms for recognizing squares of trees are given in [15, 45]. Following this line of research, several important graph classes are of great interest. In 1995, Lin and Skiena [49] gave a linear-time algorithm to find square roots of planar graphs based on the characterization of Harary, Karp and Tutte [36]. In 004, Lau and Corneil [46] also studied recognizing powers of proper interval, split, and chordal graphs. They showed that recognizing squares of chordal graphs and split graphs are NP-complete, whereas recognizing squares of proper interval graphs is polynomial time. Recently, in 006, Lau [45] showed that recognizing squares of bipartite graphs is polynomially solvable, while cubes of bipartite graphs is NP-complete. He conjectured that recognizing k-th powers of graphs for all fixed k and recognizing k-th powers of bipartite graphs for all fixed k 3 are hard. k-th power of bipartite graph Instance: A graph G. Question: Is there a bipartite graph H such that G = H k?

22 1 CHAPTER. BACKGROUND Conjecture..1 ([45]) For all fixed k, k-th power of graph is NPcomplete. Conjecture.. ([45]) For all fixed k 3, k-th power of bipartite graph is NP-complete. Notice that for all fixed k 3, recognizing k-th powers of split graphs is trivial since k-th powers of split graphs, k 3, are exactly the complete graphs. However, the computational complexity of recognizing k-th powers of chordal graphs is unknown so far. Furthermore, we recall that the girth of a graph is the smallest length of a cycle in the graph. Let g be a natural number. Graphs with girth at least g form graph class C(g) that properly contains all trees. In this work, we deal with powers of graphs with girth conditions. In this direction, we have found several results (see Chapters 5, 6, 7).

23 Part II NP-completeness 13

24 Chapter 3 Preliminaries In this chapter, we first will recall the NP-completeness results on graph power recognition problems in the literature. Then we present some tools that will be used in our reductions in Chapters 4 and Introduction Recently Lau and Corneil [46, 45] have shown that the following problems are NPcomplete. square of chordal graph Instance: A graph G. Question: Is there a chordal graph H such that G = H? square of split graph Instance: A graph G. Question: Is there a split graph H such that G = H? square root of chordal graph Instance: A chordal graph G. Question: Is there a graph H such that G = H? cube of bipartite graph Instance: A graph G. Question: Is there a bipartite graph H such that G = H 3? Furthermore, Lau [45] believed that k-th power of graph and k-th power of bipartite graph are also NP-complete (cf. Conjecture..1 and Conjecture..). Chapter 4 deals with Conjecture..1 and Conjecture... First, we show that these conjectures are indeed true. Besides, we generalize the problem square root of chordal graph to k-th of chordal graph and k-th root of chordal 14

25 3.. PRELIMINARIES 15 graph. In particular, we prove that the following problems are NP-complete. k-th power of chordal graph Instance: A graph G. Question: Is there a chordal graph H such that G = H k? k-th root of chordal graph Instance: A chordal graph G. Question: Is there a (perfect) graph H such that G = H k? Finally, in Chapter 5 we will use similar techniques to Chapter 4 to prove the NP-completeness results of the following problems. square of graph with girth 4 Instance: A graph G. Question: Is there a graph H with girth 4 such that G = H? k-th power of graph with girth k + Instance: A graph G. Question: Is there a graph H with girth k + such that G = Hk? 3. Preliminaries In proving NP-completeness results we will consider the well-known NP-complete problem set splitting ([3, Problem SP4]), also known as hypergraph - colorability. set splitting Instance: Collection D of subsets of a finite set S. Question: Is there a partition of S into disjoint subsets S 1 and S such that each subset in D intersects both S 1 and S? We will consider the following small instance of set splitting to illustrate our reductions in Chapter 4 and Chapter 5. Example. S = {u 1, u, u 3, u 4, u 5, u 6, u 7 } and D = {d 1, d, d 3 } with d 1 = {u, u 3, u 4 }, d = {u 1, u 5 } and d 3 = {u 3, u 4, u 6, u 7 }. In this example, S 1 = {u 1, u, u 3 } and S = {u 4, u 5, u 6, u 7 } is a possible solution. We also apply the tail structure of a vertex v, first described in [55], and generalized later in [45]. The tail structure of a vertex v enables us to pin down exactly the neighborhood of v in any k-th root H of G. Lemma 3..1 ([55]) Let a, b, c be vertices of a graph G such that the only neighbors of a are b and c, the only neighbors of b are a, c, and d,

26 16 CHAPTER 3. PRELIMINARIES c and d are adjacent. Then the neighbors, in V G {a, b, c}, of d in any square root of G are the same as the neighbors, in V G {a, b, d}, of c in G; see Figure 3.1. a b c d a b c d Figure 3.1: Tail in H (left) and in G = H (right) Lemma 3.. ([45]) Let G = (V G, E G ) be a connected graph with {v 1,...,v k+1 } V G where N G (v 1 ) = {v,...,v k+1 } and N G (v i ) N G [v i+1 ] for all 1 i k. Then in any k-th root H of G, N H (v 1 ) = {v }, N H (v i ) = {v i 1, v i+1 } for all i k, N H (v k+1 ) v k = N G (v ) {v 1,...,v k+1 }. The vertices v 1,..., v k are tail vertices of v k+1 ; see Figure 3. for an illustration. H v 1 v v i v k v k+1 G = H k v 1 v v i Clique v k v k+1 Figure 3.: Tail in k-th root H of G and in G We remark that k-th power of C-graph is obviously in NP whenever recognizing C is polynomially because guessing a k-th root H, verifying if H is in C and checking if G = H k can be done in polynomial time. This is the case for all graph classes considered in this thesis.

27 3.. PRELIMINARIES 17 In our reductions we say that vertex u reaches v in k steps if u has a path in H of length at most k to v. Moreover, we say also that u reaches v in exactly k steps, meaning u has a path in H of length k to v.

28 Chapter 4 Powers of Bipartite and Chordal Graphs In this chapter, we first prove that both Conjectures..1 and.. are indeed true. Next, in Section 4. we show that recognizing k-th powers of chordal graphs is NP-complete. Finally, in Section 4.3 we provide a reduction to prove the NP-completeness of finding k-th roots of chordal graphs. 4.1 Powers of bipartite graphs This section shows that the following are NP-complete. k-th power of bipartite graph Instance: A graph G. Question: Is there a bipartite graph H such that G = H k? k-th power of graph Instance: A graph G. Question: Is there a graph H such that G = H k? We prove that for fixed k 3, k-th power of bipartite graph is NP-complete by reducing set splitting to it. Our reduction generalizes the one in [45] for cube of bipartite graph. Let S = {u 1,...,u n }, D = {d 1,...,d m } where d j S, 1 j m, be an instance of set splitting, and let k 3 be a fixed integer. We construct an instance G = G(D, S) for k-th power of bipartite graph as follows. The vertex set of G consists of: U i, for all 1 i n. Each element vertex U i corresponds to the element u i 18

29 4.1. POWERS OF BIPARTITE GRAPHS 19 in S. D j, for all 1 j m. Each subset vertex D j corresponds to the subset d j in D. Dj 1,..., Dk j, for all 1 j m. k tail vertices D1 j,...,dk j D j. of the subset vertex P1 1 k,...,p1 and P 1 k,...,p are k pairs of partition vertices. Connection vertex: X. The edge set of G consists of: Edges of tail vertices: (E 1 ) D j, D 1 j,...,dk j form a clique; (E ) For all 1 t k 1: D t j {U i u i d j, 1 i n}; (E 3 ) For all 1 t k : D t j X, Dt j {D j d j d j }, D t j {P h 1 1 h k t 1} and D t j {P h 1 h k t 1}; (E 4 ) For all 1 t k 3: D t j {U i 1 i n}, D t j {D h j 1 h k t, d j d j }; (E 5 ) For all 1 t k 4: D t j {D j 1 j m}; (E 6 ) For all 1 t k 5: D t j {Dh j 1 h k t 4, 1 j m}. Edges of subset vertices: (E 7 ) D j {X, P 1 1,...,P k 1, P 1,...,P k }, D j {U i 1 i n}, D j {D j d j d j }. (E 8 ) If k 4: D j {D j 1 j m}. Edges of element vertices: (E 9 ) U 1,..., U n form a clique, and U i {X, P1 1 k,...,p1, P 1 k,...,p }. Edges of partition vertices: (E 10 ) P1 1,...,P1 k, X, U 1,...,U n form a clique; k,...,p, X, U 1,...,U n form a clique. P 1 (E 11 ) For all 1 t k 3, P t 1 {P h 1 h k t }, P t {P h 1 1 h k t }.

30 0 CHAPTER 4. POWERS OF BIPARTITE AND CHORDAL GRAPHS D 4 1 D 3 1 D 1 D 1 1 D 1 D 4 D 3 D D 1 D D 4 3 D 3 3 D 3 D 1 3 D 3 Clique Clique Clique Clique U 1 U U 3 U 4 U 5 U 6 U 7 X P 1 1 P 1 P 1 P Figure 4.1: The graph G for the example instance of set splitting and k = 4 Clearly, G can be constructed from D, S in polynomial time. For an illustration, in case k = 4, the example instance yields the graph G depicted in Figure 4.1. In this and other figures in this chapter, each ellipse corresponds to a clique and we omit the clique edges to keep the figures simpler. The two dotted lines from a vertex to the cliques mean that the vertex is adjacent to all vertices in those cliques. The 4-th root graph H of G to the solution S 1, S is shown in Figure 4.. Lemma If there exists a partition of S into two disjoint subsets S 1 and S such that each subset in D intersects both S 1 and S, then there exists a bipartite graph H such that G = H k. Proof. Let H have the same vertex set as G. The edges of H are as follows; see also Figure 4.3. Edges of subset vertices and their tail vertices: For all t k, D t j Dt 1 j and D 1 j D j, and D j {U i u i d j, 1 i n}. Edges of partition vertices: P1 1 {U i u i S 1, 1 i n} and P 1 {U i u i S, 1 i n}, and for all t k, P1 t P 1 t 1 and P t P t 1.

31 4.1. POWERS OF BIPARTITE GRAPHS 1 Edges of connection vertex: X {U 1,..., U n }. D 4 1 D 4 D 4 3 D 3 1 D 3 D 3 3 D 1 D D 3 D 1 1 D 1 D 1 3 D 1 D D 3 U 1 U U 3 U 4 U 5 U 6 U 7 X P 1 1 P 1 P 1 P Figure 4.: The bipartite 4-th root graph H of G to the solution S 1, S Now we verify that the edge set of H k is equal to the edge set of G. We do this by following the order of the presentation of the edge set of G; cf. (E 1 ) (E 11 ). For Dj k, it is clear that D j, Dj, 1...,Dj k N H k(dj k) = N G(Dj k). form a clique in H k for all j, hence For 1 t k 1, the vertex D t j reaches D j in t steps. By the construction of H, D j U i whenever u i d j. Thus within t + 1 steps, D t j reaches U i whenever u i d j, therefore D t j U i in H k whenever u i d j. By comparing with (E ), N H k(d t j) = N G (D t j). For 1 t k, the vertex Dj t reaches D j in t steps. Since D j U i whenever u i d j and for all i, U i X in H, so within k steps, Dj t reaches U i whenever u i d j, and Dj t reaches X. Also, in H, D j reaches D j in two steps whenever d j d j, thus within k steps, Dj t reaches D j whenever d j d j. Furthermore, since we have a solution for set splitting, every D j has a common neighbor with P1 1 and a common neighbor with P 1. So Dj t reaches P1 1,...,P1 k t 1 and Dj t reaches k t 1,..., P within k steps. By comparing with (E 3 ), N H k(dj t) = N G(Dj t). P 1 For 1 t k 3, the vertex D t j reaches D j in t steps. Moreover, D j reaches U i within three steps for all i. So D t j reaches U i within k steps for all i. Also, in two steps, D j reaches D j whenever d j d j, within k steps, D t j reaches

32 CHAPTER 4. POWERS OF BIPARTITE AND CHORDAL GRAPHS D k 1 D k D k 3 D k 1 1 D k 1 D k 1 3 D 1 1 D 1 D 1 3 D 1 D D 3 U 1 U U 3 U 4 U 5 U 6 U 7 X P 1 1 P 1 P 1 P P 3 1 P 3 P k 3 1 P k 3 P k 1 P k Figure 4.3: A bipartite k-th root H in Lemma to the example solution S 1, S D 1 j,..., Dk t j whenever d j d j. By comparing with (E 4 ), for all 1 t k 3, N H k(dj) t = N G (Dj). t For 1 t k 4, the vertex Dj t reaches D j in t steps. Also, D j reaches D j for all j j in only four steps, thus within k steps, Dj t reaches D j for all j j. By comparing with (E 5 ), for all 1 t k 4, N H k(dj) t = N G (Dj). t For 1 t k 5, the vertex Dj t reaches D j in t steps. Also, D j reaches Dj 1 for all j j in only five steps, thus within k steps, Dj t reaches D1 j,...,dk t 4 j for all j j. By comparing with (E 6 ), for all 1 t k 5, N H k(dj t) = N G(Dj t). Next, the vertex D j reaches U i in one step whenever u i d j. In H, X U i, so within k steps, D j reaches X and D j reaches U i for all i. Moreover, since we have a solution for set splitting, every D j has a common neighbor with P1 1 and a common neighbor with P 1. In H, {P1 1,...,P1 k } and {P 1,...,P k } are paths of length k 3. Thus, within k steps, D j reaches P1 1 k,...,p1, P 1 k,...,p. Also, it is clear that D j reaches D j in only two steps whenever d j d j. Furthermore, if k 4, D j and D j reach X in two steps, therefore D j is adjacent to D j for all j j. By comparing with (E 7 ) and (E 8 ), N H k(d j ) = N G (D j ). Now consider U i. In H, U i X, hence U 1,...,U n form a clique in H k. Moreover, in H, U i P 1 1 whenever u i S 1, and U i P 1 whenever u i S,

33 4.1. POWERS OF BIPARTITE GRAPHS 3 and {P1 1,...,P1 k } and {P 1,...,P k } are paths of length k 3 in H. Thus, within k steps U i reaches P1 1 k,...,p1, P 1 k,...,p. By comparing with (E 9 ), N H k(u i ) = N G (U i ) for all i. Finally, for partition vertices, it is clear that P1 1,...,P1 k, X, U 1,...,U n form a clique in H k and P 1 k,...,p, X, U 1,..., U n form a clique in H k. Moreover, for all 1 t k 3, since P1 1 and P 1 have at least one neighbor U i in H. Also, since U i X in H, P1 t reaches P 1,...,P k t and P t reaches P1 1,...,P1 k t. By comparing with (E 10 ) and (E 11 ), N H k(p1 t) = N G(P1 t) and N H k(p t) = N G(P t) t. We have checked that the edge set of H k is equal to the edge set of G. Now we will show that H is a bipartite graph. We do this by showing a -coloring of H. The vertices get color 1 consist of D j, Dj,.. k.,d j, X, P1 1 k 3,...,P +1 1, P 1,..., P k The other vertices of H get color. It is easy to check that vertices in the same color class are not adjacent. This completes the proof of Lemma For the example instance, the k-th root graph H corresponds to the solution S 1, S is shown in Figure 4.3. Now we show that if G has a k-th root H (not necessarily bipartite), then there is a partition of S into two disjoint subsets S 1 and S such that each subset in D intersects both S 1 and S. We first need: Proposition 4.1. If H is a k-th root of G, then, in H: (i) For all i, j: D j is only adjacent to U i whenever u i d j ; (ii) Dj k is only adjacent to Dj k 1, Dj 1 is only adjacent to D j and Dj, and Dj t is only adjacent to D t 1 j and D t+1 j, t k 1; (iii) If k 4, P k l adjacent to P t 1 l and P t+1 is only adjacent to P k 3 l l, l = 1,., and for all t k 3: P t l is only Proof. By the construction of G, we have for all j: N G (Dj k) = {D j, Dj 1,...,Dk 1 j }, N G (Dj 1) N G[D j ] and N G (Dj t) N G[D t 1 j ] for all t k 1. Thus, (i) and (ii) follow immediately from Lemma 3... For (iii), we only verify for l = 1; the case l = is similar. Since G = H k and by the construction of G, and by (i), (ii), we have for all i, j: d H (D k j, P h 1 ) > k for h k. (4.1) d H (P k 1, X) k 3, d H (P k 1, U i ) k. (4.) d H (P1 k, D j ) k, d H (P1 k, Dj 1 ) k. (4.3)

35 4.1. POWERS OF BIPARTITE GRAPHS 5 Case : N H k 3(P1 k ) = {P1 1,...,P1 k 3, X}. Then by (4.), d H (P1 k, X) = k 3, hence X B := N k 3 k H (P1 ). Set A := 1 t k 4 Nt k H (P1 ), and note that NH t k (P1 ) for 1 t k 3, hence A k 4, and 1 B. Subcase.1: B = {X, P t 0 1 } for some 1 t 0 k 3. In this case, A contains the k 4 vertices P1 t, for 1 t k 3, t t 0. Similar to Case 1, it can be shown that P1 1 k,..., P1 form a path of length k 3 in H with end-vertices P1 1 k and P1. Subcase.: B = {X}. That is, NH 1 k (P1 )... N k 4 k H (P1 ) = {P1 1,...,P1 k 3 }. Note that, NH t k (P1 ) for 1 t k 3, therefore there is uniquely a set N t 0 k H (P1 ) for some 1 t 0 k 4 such that N t 0 k H (P1 ) = {P ta 1, P t b 1 } for some 1 t a, t b k 3. Let C := 1 t k 4,t t 0 NH t k (P1 ) containing k 5 vertices exactly of {P1 t 1 t k 3, t {t a, t b }}. By NH t k (P1 ) for 1 t k 3, hence N t H (P1 k ) = 1 for 1 t k 3 and t {ta, t b }. Therefore, P1 t for all 1 t k 3 and t {t a, t b }, form a path of length k 6 in H. Since P ta 1, P t b 1 N t 0 k H (P1 ) for some 1 t 0 k 4, hence d H (P1 1, P1 k ) k 4. Therefore, d H (P1 k, Dj ) d H(P1 k, P1 1) + d H(P1 1, D j ) (k 4) + 4, this contradicts to the fact that, in G, P1 k is non-adjacent to Dj. Thus, Subcase. cannot occur. Now we are ready to prove the reverse direction. Lemma If H is a k-th root of G, then there exists a partition of S into two disjoint subsets S 1 and S such that each subset in D intersects both S 1 and S. Proof. From Proposition 4.1. and the fact that, in G, P1 1 P k, P 1 P1 k, it follows that, in H, P1 1 are not adjacent to P 1 k,...,p and P 1 are not adjacent to P1 1 k,...,p1. Also, since, in G, P1 1 Dk 1 j and P 1 Dk 1 j, it follows that, in H, P1 1 and P 1 are not adjacent to the tail vertices and the subset vertices. Thus, S 1 = N H (P1 1) \ {X, P 1 k,...,p1 } and S = N H (P 1) \ {X, P k,...,p } consist of element vertices only. We will show that S 1 and S define a desired partition of the element set S. Claim 1: S 1 S =. Proof of Claim 1: In case k = 3, by the construction of G, P1 1 is non-adjacent to P 1 in G, therefore P 1 1 and P 1 have no common element neighbor in H, thus S 1 S =. Let k 4, by Proposition 4.1. (iii), {Pl 1 k,..., Pl } form an induced path in H of length k 3, l = 1,. Thus, if P1 1 and P 1 have a common neighbor in H then P k reaches P1 1 in k steps, which contradicts to the fact that, in G, P 1 1 is non-adjacent to P k. Claim : For all j, S 1 d j and S d j.

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