# Clique coloring B 1 -EPG graphs

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1 Clique coloring B 1 -EPG graphs Flavia Bonomo a,c, María Pía Mazzoleni b,c, and Maya Stein d a Departamento de Computación, FCEN-UBA, Buenos Aires, Argentina. b Departamento de Matemática, FCE-UNLP, La Plata, Argentina. c CONICET d Departamento de Ingeniería Matemática, Universidad de Chile, Santiago, Chile. Abstract We consider the problem of clique coloring, that is, coloring the vertices of a given graph such that no (maximal) clique of size at least two is monocolored. It is known that interval graphs are 2-clique colorable. In this paper we prove that B 1 -EPG graphs (edge intersection graphs of paths on a grid, where each path has at most one bend) are 4-clique colorable. Moreover, given a B 1 -EPG representation of a graph, we provide a polynomial time algorithm that constructs a 4-clique coloring of it. Keywords: clique coloring, edge intersection graphs, paths on grids, polynomial time algorithm. 1 Introduction An EPG representation P, G of a graph G, is a collection of paths P of the twodimensional grid G, where the paths represent the vertices of G in such a way that two vertices are adjacent in G if and only if the corresponding paths share at least one edge of the grid. A graph which has an EPG representation is called an EPG graph (EPG stands for Edge-intersection of Paths on a Grid). In this paper, we consider the subclass B 1 -EPG. A B 1 -EPG representation of G is an EPG representation in which each path in the representation has at most one bend (turn on a grid point). Recognizing B 1 -EPG graphs is an NP-complete problem [11]. Also, both the coloring and the maximum independent set problem are NP-complete for B 1 -EPG graphs [9]. EPG graphs have a practical use, for example, in the context of circuit layout setting, which may be modelled as paths (wires) on a grid. In the knock-knee layout model, two wires may either cross or bend (turn) at a common point grid, but are not allowed to share addresses: This work was partially supported by UBACyT Grant BA, CONICET PIP and CO, and ANPCyT PICT (Argentina), Fondecyt grant (Chile), and MathAmSud Project 13MATH-07 (Argentina Brazil Chile France). 1

2 a grid edge; that is, overlap of wires is not allowed. In this context, some of the classical optimization graph problems are relevant, for example, maximum independent set and coloring. More precisely, the layout of a circuit may have multiple layers, each of which contains no overlapping paths. Referring to a corresponding EPG graph, then each layer is an independent set and a valid partitioning into layers corresponds to a proper coloring. In this paper, we consider the problem of clique coloring, that is, coloring the vertices of a given graph such that no (maximal) clique of size at least two is monocolored. Clique coloring can be seen also as coloring the clique hypergraph of a graph. The question of coloring clique hypergraphs was raised by Duffus et al. in [7]. We prove that B 1 -EPG graphs are 4-clique colorable. Moreover, given a B 1 -EPG representation of a graph, we provide a polynomial time algorithm that constructs a 4- clique coloring of it. 2 Preliminaries All graphs considered here are connected, finite and simple, we follow the notation of [2]. The vertex set V (G) of a graph G is the set of all the vertices of G, and the edge set E(G) of G is the set of all the edges of G. A complete graph is a graph that has all possible edges. A clique of a graph G is a maximal complete subgraph of G. A k-coloring of a graph G is a function f : V (G) {1, 2,..., k} such that f(v) f(w) for all distinct v, w V (G). The chromatic number χ(g) of a graph G is the smallest positive integer k such that G has a k-coloring. A k-clique coloring of a graph G is a function f : V (G) {1, 2,..., k} such that no clique of G with size at least two is monocolored. A graph G is k-clique colorable if G has a k-clique coloring. The clique chromatic number of G, denoted by χ c (G), is the smallest k such that G has a k-clique coloring. Clique coloring has some similarities with usual coloring,. For example, every k-coloring is also a k-clique coloring, and χ(g) and χ c (G) coincide if G is triangle-free. But there are also essential differences, for example, a clique coloring of a graph need not be a clique coloring for its subgraphs. Indeed, subgraphs may have a greater clique chromatic number than the original graph. Another difference is that even a 2-clique colorable graph can contain an arbitrarily large clique. It is known that the 2-clique coloring problem is NPcomplete, even under different constraints [1, 12]. Many families of graphs are 3-clique colorable, for example, comparability graphs, cocomparability graphs, circular arc graphs and the k-powers of cycles [3, 4, 7, 8]. In [1], Bacsó et al. proved that almost all perfect graphs are 3-clique colorable and conjectured that all perfect graphs are 3-clique colorable. This conjecture was recently disproved by Charbit et al. [6], who show that there exist perfect graphs with arbitrarily large clique chromatic number. Previously known families of graphs having unbounded clique chromatic number are, for example, triangle-free graphs, UE graphs, and line graphs [1, 5, 13]. It has been proved that chordal graphs, and in particular interval graphs, are 2-clique colorable [14]. Moreover, the following result holds for strongly perfect graphs, a superclass of chordal graphs. 2

3 Lemma 1 (Bacsó et al. [1]) Every strongly perfect graph admits a 2-clique coloring in which one of the color classes is an independent set. For chordal graphs, such a coloring can be easily obtained in linear time, by a slight modification of the 2-clique coloring algorithm for chordal graphs proposed in [14]. Namely, let v 1,..., v n be a perfect elimination ordering of the vertices of a chordal graph G, i.e., for each i, N[v i ] is a clique of G[{v i,..., v n }]; color the vertices from v n to v 1 with colors a and b in such a way that v n gets color a and v i gets color b if and only if all of its neighbors that are already colored got color a. A perfect elimination ordering of the vertices of a chordal graph can be computed in linear time [15]. 3 B 1 -EPG graphs are 4-clique colorable In this section, we prove that B 1 -EPG graphs are 4-clique colorable. We need the following definitions and theorem. Let P, G be a B 1 -EPG representation of a graph G. A clique C of G is an edge clique of P, G if all the paths of P that correspond to the vertices of C share a common edge of the grid G. A clique C of G is a claw clique of P, G if there is a point x of the grid and three edges of the grid sharing x (they may be shaped,,, or ), such that each path of P that corresponds to a vertex of C contains two of these three edges, and every pair of these three edges is contained in at least one path P of P (so, it is not an edge clique). We say that the claw clique is centered at x, or that x is the center of the claw clique. An example can be seen in Figure 1. Figure 1: A B 1 -EPG representation of the 3-sun. The central triangle {2, 3, 5} is a claw clique; the other three triangles are edge cliques (figure from [10]). Theorem 2 (Golumbic et al. [10]) Let P, G be a B 1 -EPG representation of a graph G. Every clique in G corresponds to either an edge clique or a claw clique in P, G. Now we can prove the main result of this paper. Theorem 3 Let G be a B 1 -EPG graph. Then, G is 4-clique colorable. Moreover, given a B 1 -EPG representation of G, a 4-clique coloring of G can be obtained in linear time on the number of vertices and edges. 3

4 Proof. Let P, G be a B 1 -EPG representation of the graph G. Each path of P is composed of either a single segment, formed by one or more edges on the same row or column of the grid G, or of two segments sharing a point of the grid, one horizontal (i.e., in a row) and one vertical (i.e., in a column). We will first assign colors independently to the horizontal and vertical segments of each path, and then we will show how to combine those colors into a single color for each path, as required. First, we use Lemma 1 to color the segments on each row and each column of G as if they were vertices of an interval graph, with two colors a and b, such that the segments colored b form an independent set, i.e., a pairwise non intersecting set, on each row (respectively column). We thus obtain four types of paths according to the colors given to their corresponding segments: (a, a), (a, b), (b, a), (b, b), where the first component corresponds to the horizontal part of the path and the second component corresponds to the vertical part of the path; if one of these parts does not exist, we assign an a to the missing component. Observe that the colour class (b, b) is stable. (1) Let us now investigate which cliques could be monocolored. Edge cliques of P, G are also cliques of the interval graph corresponding to the row of the grid, respectively column of the grid, to which the edge (where all the paths of the clique intersect) belongs. Thus, the colors of the paths in such a clique have to be different in the horizontal component (respectively in the vertical component), that is, the clique is not monocolored. Let us now turn to the claw cliques. Suppose that there is a claw clique which is monocolored. Then this clique contains at least two paths whose horizontal segments overlap, and have the same colour, and the same is true for vertical segments. Since the horizontal segments colored b form an independent set, and the same is true for the vertical segments, the only possible coloring of the paths in our monocolored claw clique is (a, a). Now, for each point x of the grid which is the center of one or more claw cliques monocolored (a, a), we will perform a recoloring of one or two paths having a bend at x. In this way, each path will be recolored at most once, as it has at most one bend. Paths without bends will not be recolored. The order in which we process the points x of the grid does not matter: The recolorings are independent of recolorings at other grid points. In the recoloring we will assign color b to some segments that were originally colored a, obeying the following rules, for any fixed point x of the grid: (I) the recolored paths either get color (a, b) or (b, a), (II) every segment of a path with a bend at x that is recolored b is contained in a segment of a path with a bend at x that is colored a; (III) if we recolor with b a segment S of a path with bend at x, then every other path with bend at x that meets S is colored (a, a); (IV) after recoloring, there is no claw clique colored (a, a) centered at x. 4

5 In this way, the segments colored b may no longer be an independent set, but properties (II) and (III) prevent us from creating new monocolored cliques. Indeed, property (II) ensures that we create no monocolored edge cliques, and property (III) guarantees we have no new monocolored claw clique. Property (IV) ensures that after going through all grid points, we have found a 4-clique coloring of G. By Property (I), the colouring has a stable colour class. Let us now explain how we find the recoloring with properties (I), (II), (III) and (IV), for a fixed grid point x. We distinguish three cases. Let us say a shape is missing at x if not all paths with bend at x that have this shape are colored (a, a). Case 1: Two or more of the shapes,,, are missing at x. If there is no (a, a)-colored claw clique centered at x, we do not recolor anything. Clearly, (I) (IV) hold. Otherwise, there is a unique (a, a)-colored claw clique at x, and symmetry allows us to assume this clique is shaped. Both shapes and are missing at x. Of all - or -shaped paths with bend at x, choose the one with the shortest vertical segment, and recolor it (a, b). Then (I) (IV) hold. Case 2: Exactly one of the shapes,,, is missing at x. By symmetry, we can assume is missing at x. Let P be the set of all paths with bend at x that have the shape. If there is a path P P whose horizontal segment is contained in another path with bend at x, then recolor P with (b, a). Otherwise, if there is a path P in P whose vertical segment is contained in another path with bend at x, then recolor P with (a, b). In both cases, the choice of P ensures that (I) (IV) hold. It remains to consider the case that for each of the paths in P, their horizontal (vertical) segment contains all horizontal (vertical) segments of paths with bend at at x (in particular, P = 1). Then, choose any -shaped path P 1 and any -shaped path P 2 with bend at x, recolor P 1 with (a, b) and P 2 with (b, a) and observe that (I) (IV) hold by the choice of P 1 and P 2. Case 3: None of the shapes,,, is missing at x. Consider the shortest of all segments of paths with bend at x, and let Q be the path it belongs to. By symmetry, we may assume Q is shaped, and the shortest segment is the horizontal segment. As in the previous case, let P be the set of all -shaped paths with bend at x. If there is a path P P whose horizontal (vertical) segment is contained in another path with bend at x, then recolor P with (b, a) (or with (a, b), respectively), and recolor Q with (b, a). The choice of P and Q guarantees (I) (IV). Otherwise, for each of the paths in P, their horizontal (vertical) segment contains all horizontal (vertical) segments of paths with bend at at x. Choose any -shaped path P 1 and any -shaped path P 2 with bend at x, and recolor P 1 with (a, b) and P 2 with (b, a). Again, (I) (IV) hold. The presented algorithm gives a 4-clique coloring of G, where one colour class is stable. The algorithm can be implemented to run in linear time in the number of vertices and edges of G. 5

6 HERE IS THE OLD VERSION OF THE PROOF Now, for each point x of the grid which is the center of one or more claw cliques monocolored (a, a), we will perform a recoloring of one or more paths having a bend at x. In this way, each path will be recolored at most once, as it has at most one bend. The order in which we process the points x of the grid does not matter: The recolorings are independent of recolorings at other grid points. In the recoloring we will assign color b to some segments that were originally colored a, obeying the following rule: ( ) every segment of a path P with a bend at x that is recolored b is contained in a segment of a path P with a bend at x that keeps the color a. In this way, the segments colored b may no longer be an independent set, but we will prove that the aforementioned property prevents us from creating new monocolored cliques. Case 1: There are two claw cliques shaped and, respectively, centered at x and monocolored (a, a), such that all the paths whose vertical segments contain x as an internal point are colored (a, b). Notice that in this case the possible cliques shaped and and centered at x are not monocolored, since all the paths having a bend at x are colored (a, a), while all the vertical paths going through x are coloured (a, b). Our aim is to fix the cliques shaped and without creating a new problem by creating a situation in which a claw clique shaped or is monocolored (a, b). We recolor two paths, according to some subcases. From now on, all the paths we are analyzing are those having a bend at x. Let P 1 be the path or which has the shortest vertical segment (in this and the following cases, if there are more than one of the same length, we just choose one of them). Let P 2 be the path or which has the shortest vertical segment. (a) If P 1 and P 2 are shaped and, respectively, or and, respectively, we recolor the vertical component of P 1 and P 2 with the color B (where B = b, but this indicates that it comes from a recoloration). (b) If P 1 and P 2 are shaped and, respectively, in order to avoid creating a claw clique monocolored (a, b), let P 3 be the path or which has the shortest horizontal segment. We recolor the horizontal segment of P 3 and the vertical segment of P 2 (if P 3 is shaped ) or P 1 (if P 3 is shaped ) with color B. If P 1 and P 2 are shaped and, respectively, the case is symmetric. (Then P 3 is the path or which has the shortest horizontal segment.) An example is shown in Figure 2. Case 2: There are two claw cliques shaped and, respectively, centered at x and monocolored (a, a), such that all the paths whose horizontal segments contain x as an internal point are colored (b, a). 6

7 The case is symmetric to Case 1. Observe that Case 1 and Case 2 cannot arise at the same time. Case 3: Case 1 and Case 2 do not arise, but there is at least one claw clique centered at x and monocolored (a, a) (there may be at most four of them). For a claw clique shaped, we recolor with B the vertical component of the path shaped or belonging to this clique which has the shortest vertical segment; for a claw clique shaped, we recolor with B the vertical component of the path shaped or belonging to this clique which has the shortest vertical segment; for a claw clique shaped, we recolor with B the horizontal component of the path shaped or belonging to this clique which has the shortest horizontal segment; and for a claw clique shaped, we recolor with B the horizontal component of the path shaped or belonging to this clique which has the shortest horizontal segment. For each x, we perform the minimum number of such changes in order to fix all claw cliques centered at x. It is easy to see that the minimum number is at most two, and on different paths. For instance, if the vertical component of a path P is selected for recoloring to fix a claw clique C v shaped or, and the horizontal component of P is selected for recoloring to fix a claw clique C h shaped or, we will recolor just one of the components of P, and that fixes at the same time both cliques. So, we never need to recolor both components of a path. Similarly, if we have selected to recolor paths shaped,, and to fix three cliques, the recoloring of the path shaped can be avoided, since the recoloring of the paths shaped and fix all the four possible claw cliques centered at x. The remaining cases are symmetric, showing that the minimum number of changes needed is at most two. Some examples can be seen in Figure 2. Summarizing, observe that (i) in all the three cases, no path gets color (B, B), and we recolor at most two segments, from which at most one segment on each direction (up, down, left, and right, with respect to x). (ii) by the choice of the paths to be recolored, each segment colored B of a path having a bend at x is contained in a segment colored a of another path having a bend at x. Let us see that after this recoloring process, and identifying (a, B) with (a, b) and (B, a) with (b, a), we obtain a 4-clique coloring of G. All the claw cliques monocolored (a, a) were fixed, and no path got as a new color (b, b). So we only have to check that no clique became monocolored (a, b) or (b, a). For edge cliques, this cannot happen because by ( ), each segment colored B is contained in a segment colored a. For claw cliques, without loss of generality, assume that the claw clique is shaped. If it is monocolored (a, b), as segments originally colored b formed an independent set, one of 7

8 Figure 2: Examples of the recoloring cases. the or shaped paths in the claw clique was recolored on its vertical component. Each segment colored B of a path having a bend at x is contained in a segment colored a of another path having a bend at x. But then that path belongs also to the claw clique, a contradiction because it is colored either (b, a) or (a, a). Suppose now that the claw clique is monocolored (b, a). If there is a path in the clique whose horizontal segment contains x as an internal point and has been recolored B, that segment is contained in a segment colored a of another path, that intersects every path in the clique, a contradiction with the assumption that the clique is maximal and monocolored. As segments originally colored b formed an independent set, there is just one path in the clique whose horizontal segment contains x as an internal point, that was originally colored (b, a), and all the paths having a bend at x (at least one shaped and one shaped ) were recolored on its horizontal segment. If we have recolored them in the context of Case 3, then there were two claw cliques shaped and, respectively, centered at x and monocolored (a, a), such that all the paths whose horizontal segments contain x as an internal point are colored (b, a). But this situation corresponds with Case 2 and recoloring the horizontal segments of one path shaped and one shaped is not the way we dealt with Case 2. The presented algorithm gives a 4-clique coloring of G. The algorithm can be implemented to run in linear time in the number of vertices and edges of G. 4 Conclusion In this paper we have proved that B 1 -EPG graphs are 4-clique colorable, and that such coloring can be obtained in linear time in the number of vertices and edges of the graph, given a B 1 -EPG representation of it. This algorithm may use 4 colors in graphs that are 8

9 in fact 2-clique colorable or 3-clique colorable. We conjecture that indeed B 1 -EPG graphs are 3-clique colorable. Examples of B 1 -EPG graphs that require 3 colors for a clique coloring are the odd chordless cycles, and we could not find examples of B 1 -EPG graphs having clique chromatic number 4 (The Mycielski graph with chromatic number 4, line graphs of big cliques and the graph in [6] having clique chromatic number 4 are not B 1 -EPG). Further open questions are the computational complexity of 2-clique coloring and 3- clique coloring on B 1 -EPG graphs. References [1] G. Bacsó, S. Gravier, A. Gyárfás, M. Preissmann and A. Sebő, Coloring the maximal cliques of graphs, SIAM Journal on Discrete Mathematics. 17 (2004) [2] J. Bondy and U. Murty, Graph Theory, Springer, New York. (2007). [3] C. N. Campos, S. Dantas and C. P. de Mello, Colouring clique-hypergraphs of circulant graphs, Electronic Notes in Discrete Mathematics. 30 (2008) [4] M. R. Cerioli and A. L. Korenchendler, Clique coloring circular-arc graphs, Electronic Notes in Discrete Mathematics. 35 (2009) [5] M. R. Cerioli and P. Petito, Clique coloring UE and UEH graphs, Electronic Notes in Discrete Mathematics. 30 (2008) [6] P. Charbit, I. Penev, S. Thomassé and N. Trotignon, Perfect graphs of arbitrarily large clique-chromatic number, Journal of Combinatorial Theory, Series B. 116 (2016) [7] D. Duffus, B. Sands, N. Sauer and R. E. Woodrow, Two-colouring all two-element maximal antichains, Journal of Combinatorial Theory Series A. 57 (1991) [8] D. Duffus, H. A. Kierstead and W. T. Trotter, Fibres and ordered set coloring, Journal of Combinatorial Theory Series A. 58 (1991) [9] D. Epstein, M. C. Golumbic, and G. Morgenstern, Approximation Algorithms for B 1 - EPG graphs, In: Algorithms and Data Structures, WADS 2013 Proceedings, Vol of Lecture Notes in Computer Science, Springer (2013), [10] M. C. Golumbic, M. Lipshteyn and M. Stern, Edge intersection graphs of single bend paths on a grid, Networks. 54 (2009) [11] D. Heldt, K. Knauer and T. Ueckerdt, Edge-intersection graphs of grid paths: the bend number, Discrete Applied Mathematics. 167 (2014)

10 [12] J. Kratochvíl and Zs. Tuza, On the complexity of bicoloring clique hypergraphs of graphs, in Proceedings of the 11th Annual ACM-SIAM Symposium on Discrete Algorithms, SIAM, Philadelphia, PA, 2000, pp [13] J. Mycielski, Sur le coloriage des graphes, Colloquium Mathematicum. 3 (1955) [14] H. Poon, Coloring Clique Hypergraphs, Master s thesis, West Virginia University (2000). [15] D. Rose, R. Tarjan and G. Lueker, Algorithmic aspects of vertex elimination on graphs, SIAM Journal on Computing. 5 (1976)

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