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31 Aug 2020 Injective Edge-Coloring of Sparse Graphs Injective edge-coloring of sparse graphs Baya Ferdjallah1,4, Samia Kerdjoudj 1,5, and André Raspaud2 1LIFORCE, Faculty of Mathematics, USTHB, BP 32 El-Alia, Bab-Ezzouar 16111, Algiers, Algeria 2LaBRI, Université de Bordeaux, 351 cours de la Libération, 33405 Talence Cedex, France 4Université de Boumerdès, Avenue de l’indépendance, 35000, Boumerdès, Algeria 5Université de Blida 1, Route de Soumâa BP 270, Blida, Algeria September 1, 2020 Abstract An injective edge-coloring c of a graph G is an edge-coloring such that if e1, e2, and e3 are three consecutive edges in G (they are consecutive if they form a path or a cycle of length three), then e1 and e3 receive different colors. The minimum integer k such that, G has an injective edge-coloring with k ′ colors, is called the injective chromatic index of G (χinj(G)). This parameter was introduced by Cardoso et al. [12] motivated by the Packet Radio Network ′ problem. They proved that computing χinj(G) of a graph G is NP-hard. We give new upper bounds for this parameter and we present the rela- tionships of the injective edge-coloring with other colorings of graphs. The obtained general bound gives 8 for the injective chromatic index of a subcubic graph. If the graph is subcubic bipartite we improve this last bound. We prove that a subcubic bipartite graph has an injective chromatic index bounded by arXiv:1907.09838v2 [math.CO] 31 Aug 2020 6. We also prove that if G is a subcubic graph with maximum average degree 7 8 less than 3 (resp. 3 , 3), then G admits an injective edge-coloring with at most 4 (resp. 6, 7) colors. Moreover, we establish a tight upper bound for subcubic outerplanar graphs. 1 Introduction All the graphs we consider are finite and simple. For a graph G, we denote by V (G), E(G), δ(G), and ∆(G) its vertex set, edge set, minimum degree, and maximum degree, respectively. We denote by dG(v) the degree of a vertex v in G. If G is clear from the context, then we may omit the subscript. The girth of a graph G is the length of a shortest cycle contained in the graph. If two edges e and f of a graph G 1 are the two extremities of a path of length 3, we will say that e and f are at distance 2. A proper vertex (respectively, edge) coloring of G is an assignment of colors to the vertices (respectively, edges) of G such that no two adjacent vertices (respectively, edges) receive the same color. Let G = (V, E) be a graph and c be a vertex k-coloring of G. c is a function c : V −→ {1,...,k}. We say that the vertex coloring of G is injective if its restriction to the neighborhood of any vertex is injective. The injective chromatic number χinj(G) of a graph G is the least k such that there is an injective k-coloring. Injective coloring of graphs was introduced by Hahn et. al in [18]. Note that an injective coloring is not necessarily a proper coloring. Three edges e1, e2, and e3 in a graph G are consecutive if they form a path (in this order) or a cycle of length three. An injective edge-coloring of a graph G = (V, E) is a coloring c of the edges of G such that if e1, e2, and e3 are consecutive edges in G, then c(e1) =6 c(e3). In other words, every two edges at distance exactly 2 or belonging to a triangle do not use the same color. The injective chromatic index ′ of G, denoted by χinj(G), is the minimum number of colors needed for an injective edge-coloring of G. Note that an injective edge-coloring is not necessarily a proper edge-coloring. This notion was introduced in 2015 by Cardoso et al. [12] motivated by a Packet Radio Network problem. Two adjacent entities in a network must use different frequencies to send messages to their other neighbors. What is the minimum number of frequencies needed? When we consider proper colorings, it is well known that the edge chromatic number of a graph G is equal to the vertex chromatic number of its line graph L(G), χ′(G)= ′ χ(L(G)). For the injective coloring it is not always true, that χinj(G)= χinj(L(G)). ′ For the star K1,n we have χinj(K1,n)=1 and χinj(L(K1,n)= n. ′ In [12], it is proved that computing χinj(G) of a graph G is NP-hard and the authors gave the injective chromatic index of some classes of graphs (paths, cycles, wheels, Petersen graph and complete bipartite graphs). Proposition 1.1. [12] Let Pn (resp. Cn ) be a path (resp. a cycle) with n vertices. Then ′ 1. χinj(Pn)=2, for n ≥ 4. ′ 2 if n ≡ 0 (mod 4); 2. χinj(Cn)= (3 otherwise. For the case of trees, they gave the following bound: ′ Proposition 1.2. [12] For any tree of order n ≥ 2, 1 ≤ χinj(G) ≤ 3. The injective chromatic index is a parameter interesting by it-self and determine bounds for this parameter and the injective chromatic index of graphs with maximum degree 3 graphs is already a good challenge. It is our motivation. In 2019, Axenovich et al. [4] introduced the notion of induced arboricity of a graph G, denoted by ia(G), it is the smallest number k such that the edges of G can be covered with k induced forests in G. 2 For a graph parameter p and a class of graphs F , let p(F ) = sup{p(G): G ∈ F }. In [4], the authors provide bounds on ia(F ) when F is the class of planar graphs, the class of d-degenerate graphs, or the class of graphs having tree-width at most d. They prove that if F is the class of planar graphs, then 8 ≤ ia(F ) ≤ 10. They establish similar results for the induced star arboricity of classes of graphs. The star arboricity of a graph G, denoted by isa(G), is defined as the smallest number of induced star-forests covering the edges of G. They prove that if F is the class of planar graphs, then 18 ≤ isa(F ) ≤ 30. By applying the Proposition 2.2 of [12] we have the following easy observation: ′ Observation 1. For any graph G, we have χinj(G) = isa(G). In what follows we will use the terminology of injective edge-coloring, as before, we find that it is easier to handle colors. We now present the relationships of the injective edge-coloring with other color- ings of graphs and the deduced bounds for the injective chromatic index. Strong edge-coloring A strong k-edge-coloring of a graph G is an assignment of k colors to the edges of G in such a way that any two edges at distance at most two are assigned distinct colors. The minimum number of colors for which a strong edge-coloring of G exists ′ is the strong chromatic index of G, denoted χs(G). This notion was introduced by Fouquet and Jolivet [15, 16]. In 1985, Erdős and Nešetřil conjectured that the 5 2 1 2 strong chromatic index of a graph is at most 4 ∆ (G) if ∆(G) is even and 4 (5∆ (G)− 2∆(G)+1) if ∆(G) is odd. Andersen [1] proved the conjecture for ∆(G)=3. When ′ 2 ∆ is large enough, Molloy and Reed [23] showed that χs(G) ≤ 1.998∆ (G). This ′ 2 result was improved by Bruhn and Joos [9] who proved χs(G) ≤ 1.93∆ (G) for ∆ large enough. Let us give an immediate remark. For any graph G, we have: ′ ′ χinj(G) ≤ χs(G) Acyclic coloring A proper vertex coloring of a graph G is acyclic if there is no bicolored cycle in G. In other words, the union of any two color classes induces a forest. The acyclic chromatic number, denoted by χa(G), of a graph G is the smallest integer k such that G has an acyclic k-coloring. This notion was introduced by Grünbaum [17] in 1973 and studied by Mitchem [22], Albertson and Berman [2], and Kostochka [20]. In 1979, Borodin [7] confirmed the conjecture of Grünbaum by proving : Theorem 1.3. [7] Every planar graph is acyclically 5-colorable. ′ Since χinj(G) = isa(G), we can say that the authors of [4] (Theorem 8) proved: 3 ′ 3k(k−1) Theorem 1.4. Let G be a graph with χa(G)= k. Then χinj(G) ≤ 2 . ′ By using Theorem 1.3, they deduce that, for any planar graph: χinj(G) ≤ 30. Moreover they construct an example of planar graph with injective chromatic index equal to 18. Borodin, Kostochka, and Woodall [6] proved that, if G is a planar graph with girth g ≥ 5 (respectively, g ≥ 7), then χa(G) ≤ 4 (respectively, χa(G) ≤ 3). Corollary 1.5. Let G be a planar graph with girth g, ′ 1. If g ≥ 5, then χinj(G) ≤ 18. ′ 2. If g ≥ 7, then χinj(G) ≤ 9. Star coloring A star coloring of G is a proper vertex coloring of G such that the union of any two color classes induces a star forest in G, i.e. every component of this union is a star. The star chromatic number of G, denoted by χst(G), is the smallest integer k for which G admits a star coloring with k colors. This notion was first mentioned by Grünbaum [17] in 1973 (see also [14]).
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