# Vertex 3-colorability of claw-free graphs

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1 Vertex 3-colorability of claw-free graphs Marcin Kamiński and Vadim Lozin Abstract The 3-colorability problem is NP-complete in the class of claw-free graphs and it remains hard in many of its subclasses obtained by forbidding additional subgraphs. (Line graphs and claw-free graphs of vertex degree at most four provide two examples.) In this paper we study the computational complexity of the 3-colorability problem in subclasses of claw-free graphs defined by finitely many forbidden subgraphs. We prove a necessary condition for the polynomial-time solvability of the problem in such classes and propose a linear-time algorithm for an infinitely increasing hierarchy of classes of graphs. The algorithm is based on a generalization of the notion of locally connected graphs. 1 Introduction A 3-coloring of a graph G is a mapping that assigns a color from set {0, 1, 2} to each vertex of G in such a way that any two adjacent vertices receive different colors. The 3- colorability problem is that of determining if there exists a 3-coloring of a given graph, and if so, finding it. Alternatively, one can view a 3-coloring of a graph as a partitioning its vertices into three independent sets, called color classes. If such a partition is unique, the graph is said to have a unique coloring. From an algorithmic point of view, 3-colorability is a difficult problem, i.e., it is NP-complete. Moreover, the problem remains difficult even under substantial restrictions, for instance, for graphs of vertex degree at most four [4]. On the other hand, for graphs in some special classes, such as locally connected graphs [5], the problem can be solved efficiently, i.e., in polynomial time. Recently, a number of papers investigated computational complexity of the problem on graph classes defined by forbidden induced subgraphs (see e.g. [7, 8, 9, 10, 11]). In the present paper, we study this problem restricted to the class of claw-free graphs, i.e., graphs containing no claw as an induced subgraph. As a subproblem, this includes edge 3-colorability of general graphs, i.e., the problem of determining whether the edges of a given graph can be assigned colors from set {0, 1, 2} so that any two edges sharing a vertex receive different colors. Indeed, by associating with a graph G its line graph L(G) (i.e. the graph with V (L(G)) = E(G) and two vertices being adjacent in L(G) if and only if the respective edges of G have a vertex in common), one can RUTCOR - Rutgers University Center for Operations Research, 640 Bartholomew Road, Piscataway, NJ 08854, USA. RUTCOR - Rutgers University Center for Operations Research, 640 Bartholomew Road, Piscataway, NJ 08854, USA. 1

2 transform the question of edge 3-colorability of G into the question of vertex 3-colorability of L(G). In conjunction with NP-completeness of edge 3-colorability [3], this implies NP-completeness of (vertex) 3-colorability of line graphs. It is known that every line graph is claw-free. Moreover, the line graphs constitute a proper subclass of claw-free graphs, which can be characterized by 8 additional forbidden induced subgraphs (see e.g. [2] for the complete list of minimal non-line graphs). In this paper we study computational complexity of the problem in other subclasses of claw-free graphs defined by finitely many forbidden induced subgraphs. First we prove a necessary condition for polynomial-time solvability of the problem in such classes, and then for an infinitely increasing hierarchy of classes that meet the condition, we propose a linear-time solution. To develop such a solution for the basis of this hierarchy, we generalize the notion of locally connected graphs that has been recently studied in the context of the 3-colorability problem. All graphs in this paper are finite, loopless and without multiple edges. The vertex set of a graph G is denoted V (G) and the edge set E(G). A subgraph of G is called induced by a set of vertices A V if it can be obtained from G by deleting the vertices of V A. We denote such a subgraph by G[A]. The neighborhood of a vertex v, denoted by N(v), is the set of all vertices adjacent to v. The number of neighbors of a vertex v is called its degree and is denoted deg(v). (G) is the maximum degree and δ(g) is the minimum degree of a vertex in a graph G. If (G) = δ(g), the graph G is called regular of degree (G). In particular, regular graphs of degree 3 are called cubic. The closed neighborhood of v is the set N[v] := N(v) {v}. As usual, P n, C n and K n stand, respectively, for a chordless path, a chordless cycle and a complete graph on n vertices. K n,m is the complete bipartite graph with parts of size n and m. A wheel W n is obtained from a cycle C n by adding a dominating vertex, i.e., a vertex adjacent to every vertex of the cycle. For some particular graphs we use special names: K 1,3 is a claw, K 4 e (i.e. the graph obtained from K 4 by removing one edge) is a diamond, while a gem is the graph obtained from a P 4 by adding a dominating vertex. Notice that in the context of 3-colorability it is enough to consider connected graphs only, since if a graph is disconnected, then the problem can be solved for each of its connected components separately. Therefore, without loss of generality, we assume all graphs in this paper are connected. We can also assume that δ(g) 3. Indeed, if v is a vertex of G of degree less than three, then G has a 3-coloring if and only if the graph obtained from G by deleting v has one. Moreover, whenever we deal with claw-free graphs, we can restrict ourselves to graphs of vertex degree at most four. Indeed, it is not difficult to verify that every graph with five vertices contains either a triangle or its complement or a C 5. Therefore, every graph with a vertex of degree five or more contains either a claw or K 4 or W 5. Since K 4 and W 5 are not 3-colorable, we conclude that every claw-free graph, which is 3-colorable, has maximum vertex degree at most 4. 2 NP-completeness In this section we establish several results on the NP-completeness of the 3-colorability problem in subclasses of claw-free graphs. We start by recalling the following known fact. 2

3 Lemma 1. The 3-colorability problem on (claw, diamond)-free graphs of maximum vertex degree at most four is NP-complete. The result follows by a reduction from edge 3-colorability of C 3 -free cubic graphs, which is an NP-complete problem [3]. It is not difficult to verify that if G is a C 3 -free cubic graph, then L(G) is a (claw, diamond)-free regular graph of degree four. To establish more results, let us introduce more definitions and notations. First, we introduce the following three operations: replacement of an edge by a diamond (Figure 1); a b a b Figure 1: Replacement of an edge by a diamond implantation of a diamond at a vertex (Figure 2); Figure 2: Diamond implantation implantation of a triangle into a triangle (Figure 3) Figure 3: Triangle implantation Observe that a graph obtained from a graph G by diamond or triangle implantation is 3-colorable if and only if G is. Denote by T i,j,k the graph represented in Figure 4; 3

4 T 1 i,j,k the graph obtained from T i,j,k by replacing each edge, which is not in the triangle, by a diamond; T 2 i,j,k the graph obtained from T 1 i,j,k by implanting into its central triangle a new triangle. i i k 1 j 1 k j Figure 4: The graph T i,j,k Finally, denote by T the class of graphs every connected component of which is an induced subgraph of a graph of the form T i,j,k ; T 1 the class of graphs every connected component of which is an induced subgraph of a graph of the from T 1 i,j,k ; T 2 the class of graphs every connected component of which is an induced subgraph of a graph of the from T 2 i,j,k. Notice none of the classes T 1 and T 2 contains the other. Indeed, T 2 \ T 1 contains a gem, while T 1 \ T 2 contains the graph T0,0,0 (see Figure 5 for the definition of T i,j,k ). On the other hand, both T 1 and T 2 include the class T. Theorem 2. Let X be a subclass of claw-free graphs defined by a finite set M of forbidden induced subgraphs. If M T 1 = or M T 2 =, then the 3-colorability problem is NP-complete for graphs in the class X. Proof. We prove the theorem by a reduction from the class of (claw, gem, W 4 )-free graphs of vertex degree at most 4, where the problem is NP-complete, since this class is an extension of (claw, diamond)-free graphs of maximum degree 4. Let G be a (claw, gem, W 4 )-free graph of vertex degree at most 4. Without loss of generality we can also assume that G is K 4 -free (since otherwise G is not 3-colorable) and every vertex of G has degree at least 3. We will show that if M T 1 = or M T 2 =, then G can be transformed in polynomial time into a graph in X, which is 3-colorable if and only if G is. Assume first that M T 1 =. Let us call a triangle in G private if it is not contained in any diamond. Also, we shall call a vertex x splittable if the neighborhood of x can be partitioned into two disjoint cliques X 1, X 2 with no edges between them. In particular, in a (claw, gem, W 4, K 4 )-free 4

5 graph every vertex of a private triangle is splittable. Also, it is not difficult to verify that every chordless cycle of length at least 4 contains a splittable vertex. Given a splittable vertex x with cliques X 1, X 2 in its neighborhood, apply the diamond implantation k times, i.e., replace x with two new vertices x 1 and x 2, connect x i to every vertex in X i for i = 1, 2, and connect x 1 to x 2 be a chain of k diamonds. Obviously the graph obtained in this was is 3-colorable if and only if G is. We apply this operation to every splittable vertex of G and denote the resulting graph by G(k). Observe that G(k) is (C 4,..., C k )-free and the distance between any two private triangles is at least k. Let us show that if k is larger than the size of any graph in M, then G(k) belongs to X. Assume by contradiction that G(k) does not belong to X, then it must contain an induced subgraph A M. We know that A cannot contain chordless cycles C 4,..., C k. Moreover, it cannot contain cycles of length greater than k, since A has at most k vertices. For the same reason, each connected component of A contains at most one private triangle. But then A T 1, contradicting our assumption that M T 1 =. Now assume that M T 2 =. In this case, we first transform G into G(k) and then implant a new triangle into each private triangle of G. By analogy with the above case, we conclude that the graph obtained in this way belongs to X, thus completing the necessary reduction. The above theorem not only proves NP-completeness of the problem in certain classes, but also suggests what classes can have the potential for accepting a polynomial-time solution. In particular, for subclasses of claw-free graphs defined by a single additional induced subgraph we obtain the following corollary of Theorem 2 and Lemma 1. Corollary 3. If X is the class of (claw, H)-free graphs, then the 3-colorability problem can be solved in polynomial time in X only if H T, where T is the class of graphs every connected component of which is an induced subgraph of a graph of the form Ti,j,k with at least two non-zero indices (see Figure 5). i i k 1 k j 1 j Figure 5: The graph T i,j,k In the next section, we analyze some of (claw, H)-free graphs with H T and derive polynomial-time solutions for them. 5

6 3 Polynomial-time results We start by reporting several results that are known from the literature. It has been shown in [9] that the 3-colorability problem can be solved in polynomial time for (claw, T 0,0,2 )-free graphs. More generally, we can show that Theorem 4. The 3-colorability problem can be solved in polynomial time in the class of (claw, T i,j,k )-free graphs for any i, j, k. Proof. First, observe that 3-colorability is a problem solvable in polynomial time on graphs of bounded clique-width [1]. From the results in [6] it follows that (claw, T i,j,k )- free graphs of bounded vertex degree have bounded clique-width for any i, j, k. In the introduction, it was observed that claw-free graphs containing a vertex of degree 5 or more are not 3-colorable, which provides the desired conclusion. Another solvable case described in [9] deals with (claw, hourglass)-free graphs, where an hourglass is the graph consisting of a vertex of degree 4 and a couple of disjoint edges in its neighborhood. Unfortunately, the authors of [9] do not claim any time complexity for their solution. In Section 3.1 we show that this case can be solved in linear time by generalizing the notion of locally connected graphs studied recently in connection with the 3-colorability problem. Then in Section 3.2 we extend this result to an infinitely increasing hierarchy of subclasses of claw-free graphs. 3.1 Almost locally connected graphs A graph G is locally connected if for every vertex v V, the graph G[N(v)] induced by the neighborhood of v is connected. The class of locally connected graphs has been studied in [5] in the context of the 3-colorability problem. A notion, which is closely related to 3-coloring, is 3-clique ordering. In a connected graph G, an ordering (v 1,..., v n ) of vertices is called a 3-clique ordering if v 2 is adjacent to v 1, and for each i = 3,..., n, the vertex v i forms a triangle with two vertices preceding v i in the ordering. It is not difficult to see that for a graph with a 3-clique-ordering, the 3-colorability problem is solvable in time linear in the number of edges. (We will refer to such running time of an algorithm as linear time.) In [5], it has been proved that if G is connected and locally connected, then G admits a 3-clique-ordering and it can be found in linear time. Therefore, the 3-colorability problem in the class of locally connected graphs can be solved in linear time. Now we introduce a slightly broader class and show that the 3-colorability of graphs of vertex degree at most 4 in the new class can be decided in linear time. This will imply, in particular, a linear-time solution for (claw, hourglass)-free graphs. We say that a graph G is almost locally connected if the neighborhood of each vertex either induces a connected graph or is isomorphic to K 1 + K 2 (disjoint union of an edge and a vertex). In other words, the neighborhoods of all vertices are connected, or, if the degree of a vertex is 3, we allow the neighborhood to be disconnected, provided it consists of two connected components, one of which is a single vertex. If v is a vertex of degree 3 6

7 and w is an isolated vertex in the neighborhood of v, then we call the edge vw a pendant edge. A maximal (with respect to set inclusion) subset of vertices that induces a 3-clique orderable graph will be called 3-clique orderable component. Since a pendant edge belongs to no triangle in an almost locally connected graph, the endpoints of the pendant edge form a 3-clique orderable component, in which case we call it trivial. For the non-trivial 3-clique orderable components we prove the following helpful claim. Claim 5. Let G be an almost locally connected graph with (G) 4 and with at least 3 vertices. (1) Every non-trivial 3-clique orderable component of G contains at least 3 vertices. (2) Any two non-trivial 3-clique orderable components are disjoint. (3) Any edge of G connecting two vertices in different non-trivial 3-clique orderable components is pendant. Proof. To see (1), observe that in an almost locally connected graph with at least 3 vertices every non-pendant edge belongs to a triangle. To prove (2), suppose G contains two non-trivial 3-clique orderable components M 1 and M 2 sharing a vertex v. Without loss of generality let v have a neighbor w in M 1 M 2. Since M 1 is not trivial, the edge vw cannot be pendant, and hence there is a vertex u M 1 adjacent both to v and w. Notice that u cannot belong to M 2, since otherwise M 2 {w} induces a 3-clique orderable graph contradicting maximality of M 2. On the other hand, M 2 must have a triangle containing vertex v, say v, x, y. If neither u nor w has a neighbor in {x, y}, then G is not locally connected. If there is an edge between {u, w} and {x, y}, then M 2 is not a maximal set inducing a 3-clique orderable graph. This contradiction proves (2), which in its turn implies (3) in an obvious way. An obvious corollary from the above claim is that an almost locally connected graph admits a unique partition into 3-clique orderable components and such a partition can be found in linear time. Now we present a linear-time algorithm that, given a connected, almost locally connected graph G, decides 3-colorability of G and finds a 3-coloring, if one exists. In the algorithm, the operation of contraction of a set of vertices A means substitution of A by a new vertex adjacent to every neighbor of the set A. ALGORITHM A Step 1. Find the unique partition V 1,..., V k of G into 3-clique orderable components. Step 2. If one of the graphs G[V i ] is not 3-colorable, return the answer G is not 3- colorable. Otherwise, 3-color each 3-clique orderable component. Step 3. Create an auxiliary graph H, contracting color classes in each 3-clique orderable component V i and then removing vertices of degree 2. Step 4. If H is isomorphic to K 4, return the answer G is not 3-colorable. Otherwise, 3-color H. Step 5. Expand the coloring of H to a coloring of G and return it. To show that the algorithm is correct and runs in linear time, we need to prove two properties of the auxiliary graph built in Step 3 of the algorithm. 7

8 Claim 6. Let H be the auxiliary graph built in Step 3 of the algorithm. G is 3-colorable if and only if H is. Proof. If G is 3-colorable, then each 3-clique orderable component has a unique coloring. Therefore, the graph obtained by contracting color classes of each 3-clique orderable component (and, possibly, removing vertices of degree 2) is 3-colorable. If H is 3-colorable, then vertices of degree 2 that were deleted in Step 3 can be restored and receive colors that are not assigned to their neighbors. Let a be a vertex of a triangle in H corresponding to a 3-clique orderable component in G. The edges that a is incident to (but not the triangle edges) correspond to the pendant edges of this component. Each is incident to two vertices in different color classes in H and will be in G. From the definition of 3-clique-ordering one can derive the following simple observation. Claim 7. If G has a 3-clique-ordering, then m 2n 3. Claim 8. Let G be a connected and almost locally connected graph of vertex degree at most 4, and H be the auxiliary graph built in Step 3 of the algorithm. Then, (H) 3. Proof. Let T be a 3-clique orderable component of G. Denote by n 2, n 3, n 4 the number of vertices of degree 2, 3 and 4 in G[T ], respectively. Also, let m be the number of edges in G[T ]. Then, 2m = 2n 2 + 3n 3 + 4n 4. In addition, m 2(n 2 + n 3 + n 4 ) 3 (by Claim 7). Therefore, n 2 3. Notice that any pendant edge in G is adjacent to two vertices of degree 3 that are of degree 2 in the graphs induced by their 3-clique orderable components. Therefore, the number of pendant edges of a 3-clique orderable component is at most 3. If a, b, c are vertices of a triangle corresponding to a locally connected component, and three pendant edges are incident to a, then b and c are removed in Step 4 of the algorithm, as both have degree 2, and a has degree 3 in H. If a is incident to two pendant edges, and b to one, then c is removed (as has degree 2) and both b and c have degree at most 3 in H. If each of vertices a, b, c is incident to one pendant edge only, then the degree of each is at most 3. Theorem 9. Algorithm A decides 3-colorability of a connected, almost locally connected graph G with maximum vertex degree at most 4 and finds a 3-coloring of G, if one exists, in linear time. Proof. The correctness of the algorithm follows from Claims 6, 8 and the observation that by the Brooks theorem the only graph of maximum degree 3 that is not 3-colorable is K 4. The first step of the algorithm can be implemented as a breadth first search and performed in linear time. 3-coloring each of the locally connected components in linear time can be done by applying the algorithm presented in [5]. Also, building the auxiliary graph and testing if it is isomorphic to K 4 are linear time tasks. As was proved in [4], Step 4 may be performed in linear time as well, and Step 5 is also clearly linear. Corollary 10. If G is (claw, hourglass)-free, then there exists a linear time algorithm to decide 3-colorability of G, and find a 3-coloring, if one exists. 8

9 Proof. If (G) 5, then G is not 3-colorable and it can be checked in linear time. Notice that the only disconnected neighborhoods that are allowed in a 3-colorable, claw-free graphs are 2K 2 and K 1 + K 2. Since G is H-free, none of the neighborhoods is isomorphic to 2K 2 ; otherwise, the graph induced by a vertex together with its neighborhood would be isomorphic to H. Hence, all neighborhoods in G are either connected or isomorphic to K 1 + K 2. Therefore, the graph is almost locally connected and by Lemma 9, the 3-colorability problem can be solved for it in linear time. 3.2 More general classes In this section we extend the result of the previous one to an infinitely increasing hierarchy of subclasses of claw-free graphs. The basis of this hierarchy is the class of (claw, hourglass)-free graphs. Now let us denote by H k the graph obtained from a copy of hourglass H and a copy of P k by identifying a vertex of degree 2 of H and a vertex of degree 1 of P k. In particular, H 1 = H. Theorem 11. For any k 1, there is a linear time algorithm to decide 3-colorability of a (K 1,3, H k )-free graph G, and to find a 3-coloring, if one exists. Proof. Without loss of generality we consider only connected K 4 -free graphs of vertex degree at most 4 in the class under consideration. For those graphs that are hourglassfree, the problem is linear-time solvable by Corollary 10. Assume now that a (claw, H k )- free graph G contains an induced hourglass H (which implies in particular that k > 1) and let v denote the center of H. There are only finitely many connected graphs of bounded vertex degree that do not have vertices of distance k +1 from v. Therefore, without loss of generality, we may suppose that G contains a vertex x k+1 of distance k + 1 from v, and let x k+1, x k,..., x 1, v be a shortest path connecting x k+1 to v. In particular, x 1 is a neighbor of v. Since G is H k -free, x 2 has to be adjacent to at least one more vertex, say u, in the neighborhood of v. If u is not adjacent to x 1, then u, x 1, x 2, x 3 induce a claw. Thus, u is adjacent to x 1, while x 2 has no neighbors in N(v) other than x 1 and u. Let us show that u has degree 3 in G. To the contrary, assume z is the fourth neighbor of u different from v, x 1, x 2. To avoid the induced claw G[u, v, x 2, z], z has to be adjacent to x 2. This implies z is not adjacent to x 1 (else a K 4 = G[x 1, u, x 2, z] arises), and z is adjacent to x 3 (else x 2, x 1, z, x 3 induce a claw). But now the path x k,..., x 3, z, u together with v and its neighbors induce an H k ; a contradiction. By symmetry, x 1 also has degree 3 in G. Notice that in any 3-coloring of G, vertices v and x 2 receive the same color. Therefore, by identifying these two vertices (more formally, by replacing them with a new vertex adjacent to every neighbor of v and x 2 ) and deleting u and x 1, we obtain a new graph G which is 3-colorable if and only if G is. Moreover, it is not hard to see that the new graph is again (claw, H k )-free. By applying this transformation repeatedly we reduce the initial graph to a graph G which is either (claw, hourgalss)-free or contains no vertices of distance k + 1 from the center of an hourglass. In both cases the problem is linear-time solvable, which completes the proof. 9

10 References [1] B. Courcelle, J.A. Makowsky and U. Rotics, Linear time solvable optimization problems on graphs of bounded clique-width, Theory Comput. Systems 33 (2000) [2] F. Harary, Graph Theory, (Addison-Wesley, Reading, MA, 1969). [3] Ian Holyer, The NP-completness of edge-coloring, SIAM J. Comput. Vol. 10, No. 4, [4] M. Kochol, V. Lozin, B. Randerath, The 3-colorability problem on graphs with maximum degree 4, SIAM J. Comput. Vol. 32, No. 5, [5] M. Kochol, 3-coloring and 3-clique-ordering of locally connected graphs, Journal of Algorithms 54 (2005) [4] L.Lovász, Three Short Proofs in Graph Theory, Journal of Combinatorial Theory (B) (1975) [6] V. Lozin and D. Rautenbach, On the band-, tree- and clique-width of graphs with bounded vertex degree, SIAM J. Discrete Mathematics, 18 (2004) [7] F. Maffray and M. Preissmann, On the NP-completeness of the k-colorability problem for triangle-free graphs, Discrete Math. 162 (1996), no. 1-3, [8] B. Randerath, 3-colorability and forbidden subgraphs. I. Characterizing pairs, Discrete Math. 276 (2004) [9] B. Randerath, I. Schiermeyer, M. Tewes, Three-colorability and forbidden subgraphs. II: polynomial algorithms, Discrete Math. 251 (2002) [10] B. Randerath, I. Schiermeyer, 3-colorability P for P 6 -free graphs, Discrete Appl. Math. 136 (2004) [11] B. Randerath and I. Schiermeyer, Vertex colouring and forbidden subgraphs a survey, Graphs Combin. 20 (2004)

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