(cid:73) Complexity of Improper Twin Edge Coloring of Graphs Paniz Abedina, Saieed Akbaria, Marc Demangeb, Tınaz Ekimc,∗ aDepartment of Mathematical Sciences, Sharif University of Technology, 11155-9415, Tehran, Iran bSchool of Science, RMIT University, Melbourne, Victoria, Australia cDepartment of Industrial Engineering, Bogazici University, Istanbul, Turkey 6 Abstract 1 0 Let G be a graph whose each component has order at least 3. Let s : E(G) → Z for some integer k 2 k ≥ 2 be an improper edge coloring of G (where adjacent edges may be assigned the same color). p If the induced vertex coloring c : V(G) → Z defined by c(v) = (cid:80) s(e) in Z , (where the e k e∈Ev k indicated sum is computed in Z and E denotes the set of all edges incident to v) results in a S k v proper vertex coloring of G, then we refer to such a coloring as an improper twin k-edge coloring. 3 2 The minimum k for which G has an improper twin k-edge coloring is called the improper twin chromatic index of G and is denoted by χ(cid:48) (G). it ] M It is known that χ(cid:48) (G) = χ(G), unless χ(G) = 2 (mod 4) and in this case χ(cid:48) (G) ∈ {χ(G),χ(G)+ it it 1}. Inthispaper, wefirstgiveashortproofofthisresultandweshowthatifGadmitsanimproper D twink-edgecoloringforsomepositiveintegerk, thenGadmitsanimpropertwint-edgecoloringfor . s all t ≥ k; we call this the monotonicity property. In addition, we provide a linear time algorithm to c [ constructanimpropertwinedgecoloringusingatmostk+1colors, wheneverak-vertexcoloringis given. Then we investigate, to the best of our knowledge the first time in literature, the complexity 2 v of deciding whether χ(cid:48) (G) = χ(G) or χ(cid:48) (G) = χ(G)+1, and we show that, just like in case of it it 7 the edge chromatic index, it is NP-hard even in some restricted cases. Lastly, we exhibit several 6 classes of graphs for which the problem is polynomial. 2 2 Keywords: Modular chromatic index, Twin edge - vertex coloring, Odd - even color classes, 0 . NP-hardness 1 0 6 1 1. Introduction : v i 1.1. Definitions and Literature X Throughout this paper all graphs are simple with no loops and multiple edges. Let G be a r a graph. We denote the vertex set and the edge set of G, by V(G) and E(G), respectively (or simply V and E when no ambiguity may occur). For v ∈ V(G), E denotes the set of all edges incident v to v and N (v) denotes the (open) neighborhood of v in G; when no ambiguity may occur we just G denote N(v) = N (v). We denote respectively by K , P and C the complete graph of order n, G n n n the path or order n and the cycle of order n. A k-clique is a set of k vertices inducing a complete graph and an independent set is a set of vertices pairwise not linked by an edge (i.e., inducing a (cid:73)PartofthisresearchwascarriedoutwhileSaieedAkbariwasvisitingIstanbulCenterforMathematicalSciences (IMBM) whose support is greatly acknowledged. ∗Corresponding Author Preprint submitted to Elsevier September 26, 2016 subgraph with only isolated vertices). K will denote the complete bipartite graph with parts of p,q size p and q. If p = 1, then it is star graph and the vertex of the part of size 1 is called center. A (proper) k-vertex coloring of G is an assignment of k colors to the vertices of G such that no two adjacent vertices have the same color (possibly some colors are not used). Given such a k-vertex coloring, a color class is the set of vertices of the same color; it is an independent set. A graph having a k-vertex coloring is said k-vertex colorable. The chromatic number χ(G) of G is the minimum k for which G is k-vertex colorable. A k-edge coloring of G is an assignment of k colors to the edges of G (possibly some colors are not used); it is said to be proper if two edges with a common endpoint do not have the same color, and improper if it is not necessarily proper. Note that the distinction between proper and improper colorings exists as well for vertex colorings but since we will not use it (formally since all our vertex colorings are proper) we specify proper/improper for edge colorings only. The minimum k such that G admits a proper k-edge coloring is the chromatic index of G denoted by χ(cid:48)(G). Integers will be used to label colors, which will give us the opportunity to compute operations. A walk in G is a sequence W = v e v e v ...,e v , whose terms are alternately vertices and 0 1 1 2 2 k k edges, such that e = v v , for 1 ≤ i ≤ k. The number k is called the length of the walk. A walk i i−1 i is called a closed walk if v = v . The reader is referred to [9] for all definitions in graph theory 0 k which are not defined here. For any integer k ≥ 2, we will denote by Z = Z/kZ the additive group over {0,...,k −1}. k For x,y,z in Z , x+y = z in Z is also denoted z = x+y mod k. For any two integers, we will k k denote x ≡ y (mod k) if 0 = (x−y) mod k. In the literature, we can find a number of results about proper or improper edge colorings inducing a coloring for vertices by mapping, for any vertex v, the multiset of colors of the edges adjacent to v to a vertex color. Such a mapping is called vertex-coloring mapping; see [15] for variations of such colorings. Given a vertex-coloring mapping, an edge-coloring is called vertex- distinguishing if the colors induced by the edge coloring are different for every vertex. The edge- coloring is called neighbor-distinguishing if only adjacent vertices have different induced colors, see e.g. [12] and [1]. Neighbor-distinguishing edge colorings are also called vertex coloring edge partitions in [1]. Chapter 13 of [6] is devoted to distinguishing colorings. Thesenotionsstronglyvarywithrespecttothevertexcoloringmapping, i.e., thewaythevertex colors are induced from the edge colors. For instance, in [1], the color of the vertex v is directly the multiset of the colors in E while in [12], the color of a vertex is obtained as the sum of the colors of v the edges incident to it and the related parameter is called sum chromatic number. This definition gives rise to the “1-2-3 Conjecture” which states that for every connected graph G of order at least 3, there exists a neighbor-distinguishing edge coloring of G using only 3 colors. A variation of the sum chromatic number obtained using Abelian groups is called the group sum chromatic number and studied in [3]; it is the smallest value s such that taking any Abelian group G of order s, there exists a function f : E(G) −→ G such that the sums of edge colors properly color the vertices i.e., with different colors on adjacent vertices. Another popular vertex coloring mapping uses the specific group Z , where k is at least the k number of colors used for the edges. If the edges are colored using colors between 0 and k−1, seen as elements of Z , then one assigns to a vertex v the sum in Z of the colors of the edges in E . k k v More formally, let G be a graph the components of which have order at least 3. Let s : E(G) → Z k 2 for some integer k ≥ 2 be a k-edge coloring of G. We then define c : V(G) → Z as follows: s k (cid:88) ∀v ∈ V,c (v) = s(e) in Z . s k e∈Ev If c is a (proper) k-vertex coloring, then it will be called the vertex coloring induced by s and s is s said to induce c . s A proper edge coloring s inducing a vertex coloring c is called a proper twin k-edge coloring s of G. This concept was defined in [2] where it has been shown that any graph with components of size at least 3 has at least one such coloring. Then, the minimum k for which G has a proper twin k-edge coloring is called the proper twin chromatic index of G and is denoted by χ(cid:48)(G). t Similarly, an improper (i.e. not necessarily proper) edge coloring s inducing a vertex coloring c is called an improper twin k-edge coloring of G. The difference between both notions is only s whether one deals with proper or improper edge colorings, but the related vertex coloring is always required to be proper. Since a proper edge coloring is improper as well, the existence of improper twin k-edge colorings for some k comes from the existence of proper ones. The minimum k for which G has an improper twin k-edge coloring is called the improper twin chromatic index of G and is denoted by χ(cid:48) (G). The same parameter was already studied under the name of modular it chromatic index, denoted by χ(cid:48) (G), in [10] and [11]1. m Note that χ(cid:48) (G) and χ(cid:48)(G) are not defined for graphs having a component which is K . Fol- it t 2 lowing the terminology of [1] we call nice a graph without a component of size 2. However, the definitions are valid for graphs with isolated vertices by considering that an empty sum is null and then defining c (v) = 0 for all isolated vertices v and all twin k-edge colorings, k ≥ 2. All the s graphs considered from now on will be supposed to be nice. By definition, for every nice graph G we have: χ(cid:48)(G) ≥ χ(cid:48) (G) ≥ χ(G). t it In this paper, we deal with improper twin k-edge colorings of G. Note that although they are very close, the group sum chromatic number and the improper twin chromatic index are not the same. In fact, for a graph G, χ(cid:48) (G) does not exceed the group sum chromatic number of G. Here it is an example where the group sum chromatic number is greater than the improper twin chromatic index. Consider the disjoint union of K and K and make one vertex of K adjacent to one vertex 4 2 4 of K and call the resultant graph by G. Note that G has order 6 with degree sequence: 4,3,3,3,2,1. 2 The group sum chromatic number of G is 5 since this graph is ugly (see Definition 1.1 of [3]) and ugly graphs have group sum chromatic number of χ(G)+1 (see Theorem 1.2 of [3]). However, the improper twin chromatic index of G is 4 by Theorem 1.5 reminded below. Let us conclude the definitions with a concrete example of improper twin edge coloring. Let P be the Petersen graph. Since P contains an odd cycle, χ(P) ≥ 3. The assignment of numbers to E(P) given in Figure 1 shows that χ(cid:48) (P) = 3. it 1.2. Preliminaries Letusnowfirstunderlinethatsomenaturalpropertiesoftheclassicaledgeandvertexcolorings do not hold for the improper twin edge coloring problem. 1Weusetheterm“impropertwinchromaticindex”followingtheterminologyin[2]sinceitallowsustodistinguish between the proper and the improper cases. 3 Figure 1: Improper twin 3-edge coloring of the Petersen graph. Remark 1.1. The notions of improper twin edge coloring and induced vertex coloring strongly depend on the labels assigned to the colors. In particular they are not stable under a reordering of the colors. Consider indeed the star graph K ; the 2-vertex coloring assigning 0 to the center and 1 to 1,2 the two other vertices is induced by the edge coloring assigning 1 to both edges while the 2-vertex coloring assigning 1 to the center and 0 to the two other vertices is not induced by any improper twin edge coloring. Actually this 2-edge coloring is the unique improper 2-edge coloring. Similarly, considering a P , the edge coloring assigning color 1 to two adjacent edges and 0 to the third edge 4 is an improper 2-edge coloring while the edge coloring assigning 0 to two adjacent edges and 1 to the last edge is not. Remark 1.2. Unlike the usual vertex/edge coloring problems, there are some graphs G with a subgraph H such that χ(cid:48) (G) < χ(cid:48) (H). it it Indeed, using Theorem 1.5 (see below), we consider m = 2 mod 4 and n = 0 mod 4 with n > m > 2; then χ(cid:48) (P ) = 3 and χ(cid:48) (C ) = 2. Taking G = C and H = P raises infinitely many it m it n n m examples where taking a subgraph increases the improper twin chromatic index. Remark 1.3. Unlike the usual edge coloring problem, there are some graphs G with an improper twin k-edge coloring s such that the restriction of s to E(G)−e for some edge e is not necessarily an improper twin k-edge coloring of G−e. Indeed, a triangle admits an improper twin 3-edge coloring with colors 0, 1 and 2 on its three edges, however, removing the edge of color 2 leaves a graph with an edge coloring which does not induce a proper vertex coloring. Note however that removing an edge colored with 0 obviously leaves an improper twin k-edge coloring in the remaining graph. Remark 1.4. Unlike the usual vertex/edge coloring problems, an improper twin k-edge coloring is not necessarily an improper twin t-edge coloring for t ≥ k. This is due to the mod t or mod k operations respectively used in the definition of twin t- and twin k-edge coloring. Consider indeed a star with a center vertex a and four pending vertices b,c,d,e. Theedgecoloringassigningcolor1toallthefouredgesisanimpropertwin2-edgecoloring but not an improper twin 3-edge coloring. 4 1.3. Our contribution In this paper we investigate the improper twin edge chromatic index of graphs and we consider, to the best of our knowledge, the first time, the complexity of a neighbor-distinguishing edge coloring problem. The following summarizes the main contribution of [10] and [11]: Theorem 1.5. ([10, 11]) Let G be a connected graph of order at least 3. Then χ(cid:48) (G) = χ(G)+1 it if and only if χ(G) ≡ 2 (mod 4) and every proper χ(G)-vertex coloring of G results in color classes of odd size; and χ(cid:48) (G) = χ(G) in all other cases. it It should be noted that this result is obtained as a combination of several results from [10] and [11]. In Section 2, we provide a shorter and more compact proof of Theorem 1.5. Actually we obtain a more precise result (see Theorem 2.8) describing a constructive way to build in linear time an improper twin edge coloring using k or k+1 colors, whenever a k-vertex coloring is provided. In [11] it is shown that for non-bipartite connected graphs G and odd t ≥ χ(G), any t-vertex coloring can be induced by an improper t-edge coloring. We give an alternative proof of this result (Proposition 2.4) and give a necessary and sufficient condition for a t-vertex coloring with even t being induced by an improper twin t-edge coloring in a non-bipartite connected graph (Proposition 2.6). Moreover, these two results, together with Theorem 2.2 allow us to establish the following monotonicity property of the improper twin chromatic index for graphs: if a graph G admits an improper twin k-edge coloring for some positive integer k, then it admits an improper twin t-edge coloring for all t ≥ k (Theorem 2.7). Note that following Remark 1.4, unlike the classical coloring problems, this property is not straightforward for the improper twin edge coloring problem. Thecharacterizationofgraphshavingχ(cid:48) (G) = χ(G)+1emphasizesthecloselyrelatedproblem it of deciding whether all color classes are of odd size in all optimal colorings. In Section 3, we use this new problem to show that, interestingly enough, just like in the case of usual edge chromatic index which can only take two possible values, it is NP-complete to decide whether χ(cid:48) (G) = χ(G) it even when the graph G is restricted to have χ(G) equal to its maximum clique size and in the bounded degree case. Finally, in Section 4, we give first examples of graph classes for which this question can be decided in polynomial time. These classes include bipartite graphs, split graphs, co-chordal graphs and co-comparability graphs with bounded chromatic number. 2. Bounds and Monotonicity for the Improper Twin Edge Chromatic Index The contribution of this section is two-fold. First, we provide a short and compact proof of the bounds χ(G) ≤ χ(cid:48) (G) ≤ χ(G)+1 together with a description of the graphs having χ(cid:48) (G) = it it χ(G)+1. Second, we prove the monotonicity of the improper twin chromatic index. We first settle both the monotonicity and the bounds for bipartite graphs. More precisely, we show that for every nice bipartite graph G and every positive integer t ≥ 3, G admits an improper twin t-edge coloring. Given a connected graph G = (V,E), a t-vertex coloring c and an improper twin edge coloring s using colors in Z , we will say that s almost induces c if there is a vertex v ∈ V such that t ∀u ∈ V \{v},c(u) = c (u). In this case v is called the defective vertex. Note that if s induces c then s it almost induces it and any vertex can be chosen as defective. Moreover if s is almost inducing c, either it induces it or there is a unique possible defective vertex. The following lemma plays a crucial role in our approach. 5 Lemma 2.1. Given connected graph G = (V(G),E(G)), a vertex t-coloring c of G (t ≥ 1), a vertex v ∈ V and a spanning tree T, there is an improper twin edge coloring s that almost induces c with v as defective vertex. Moreover ∀e ∈ E(G)\E(T),s(e) = 0. Proof. We direct the edges of T from the root to the leaves using v as the root. We set ∀e ∈ E(G)\E(T),s(e) = 0. Then, starting from the deepest level of T and going up to the root, color the edges e of T with colors s(e) in {0,...,t−1} to ensure that for every vertex u (cid:54)= v, the color of u corresponds to the sum in Z of the colors of edges of T incident to u. Since s(e) = 0 for the t edges e ∈ E(G)\E(T), it means that ∀u (cid:54)= v,c(u) = c (u). s IntheconfigurationdescribedinLemma2.1, wewillsaythatsisassociated with T and v. Note that this notion is not unique: several different improper twin t-edge colorings can be associated with the same tree T and v. Theorem 2.2. If G = (X ∪Y,E) is a nice bipartite graph with vertex parts X and Y, then the following statements hold: i) [10] χ(cid:48) (G) = 2 if and only if G does not have any connected component with both parts of it the bipartition of odd size. ii) For any integer t ≥ 3 the graph G admits an improper twin t-edge coloring. Proof. Without loss of generality, suppose that G is connected with at least 3 vertices. For a vertex v ∈ X ∪Y consider a spanning tree T of G and direct the edges from the root of the leaves using v as the root. Set c(x) = 1 for all vertices x in the same part of the bipartition as v and c(x) = 0 for all vertices in the other part. Using Lemma 2.1, we consider an improper twin t-edge coloring s of G associated with T and v. i) Consider t = 2. Without loss of generality we assume that |X| is even and then, we chose v ∈ X. We then have: (cid:88) (cid:88) 0 ≡ c(w) ≡ c(u) (mod 2), (1) w∈X u∈Y where the first summation is the sum of an even number of 1’s and the second summation is the sum of 0’s. On the other hand, by construction of c we have:   (cid:88) (cid:88) (cid:88) (cid:88) c(u) ≡ s(e) ≡  c(u)+ s(e) (mod 2). (2) u∈Y e∈E(T) u∈X\{v} e∈Ev (cid:80) Using Relations 1 and 2 we deduce c(v) ≡ s(e) (mod 2) = c (v). So, c = c and s is an e∈Ev s s improper twin 2-edge coloring of G. To prove the other direction, we proceed by contradiction: assume that |X| and |Y| are both odd and we have an improper twin 2-edge coloring of G. Since G is connected, (X,Y) is the unique bipartition of G which should be also induced by the improper twin 2-edge coloring. By double- counting the color of all edges as previously, we obtain a contradiction to the fact that both |X| and |Y| are odd. 6 ii) We assume now that v is of degree at least 2 in G (G is nice) and consider T such that v is of degree at least 2 in T. Let a ,a ,...a be the colors (among 0,1,...,t−1) of the edges of T incident to v (r ≥ 2). If 1 2 r a +a +...+a (cid:54)≡ 0 (mod t), then s is an improper twin t-edge coloring of T that almost induces 1 2 r c. Now, if a +a +...+a ≡ 0 (mod t), then we consider two cases. If t (cid:54)= 4, then change the color 1 2 r of edge e = vx from a to a +2 mod t and the color of edge e = vx from a to a +2 mod t 1 1 1 1 2 2 2 2 for some x ,x ∈ N (v) to obtain an improper twin t-edge coloring s(cid:48). c (x ) = c (x ) = 2 and 1 2 T s(cid:48) 1 s(cid:48) 2 c (v) = 4 mod t (note that 4 mod t ∈/ {0,2}). If t = 4, then change the color of edge e = vx s(cid:48) 1 1 from a to a +3 mod 4 and change the color of edge e = vx from a to a +3 mod 4 to obtain 1 1 2 2 2 2 an improper twin t-edge coloring s(cid:48). We have c (x ) = c (x ) = 3 and c (v) = 2. s(cid:48) 1 s(cid:48) 2 s(cid:48) Note that in the case ii), the induced t-vertex coloring has at most 3 vertices with a color greater than 1. The following is a key lemma for the establishment of the monotonicity property and the bounds for non-bipartite case in a constructive way. Lemma 2.3. Let G = (V,E) be a connected non-bipartite graph. Let c be a k-vertex coloring of G almost induced by an improper twin k-edge coloring s with defective vertex v. For any x ∈ Z such k that ∀u ∈ N (v),c(u) (cid:54)= c (v)+2x mod k, the k-vertex coloring obtained by replacing c(v) with G s c (v)+2x mod k is induced by an improper twin k-edge coloring. s Proof. Since χ(G) > 2, the graph G contains an odd cycle, say C. Since G is connected, there exists a path between v and C, say P. We construct an odd closed walk W passing by v as follows: traverse P starting at v and go to C and then traverse C and come back to v via P. Note that each edge of P is traversed twice. Given x ∈ Z , add alternately +x,−x mod k to s(e), for each e in W starting from v and k consider the vertex coloring induced by this new edge coloring s(cid:48). All vertices u ∈ W,u (cid:54)= v have the same color since the summation of the colors of edges incident to u is the same (s(u) = s(cid:48)(u)). Vertices outside W are not affected at all and the color c(v) is changed into c (v)+2x mod k = s c (v). This new vertex coloring is a proper vertex coloring since the color c(v)+2x mod k is not s(cid:48) used in N (v) and it is induced by s(cid:48). G From now on, we deal with the non-bipartite case. We first settle the bounds and the mono- tonicity for all improper twin edge colorings with odd number of colors. Lemmas 2.1 and 2.3 give an alternative proof of the following result. Proposition 2.4. [11] If G is a nice graph without bipartite component and t is an odd positive integer such that t ≥ χ(G) ≥ 3, then any t-vertex coloring of G with colors in {0,1,...,t−1} can be induced by an improper twin t-edge coloring of G. Proof. We assume without loss of generality that G is connected since otherwise the same proof applies to every connected component (note that if the hypotheses hold for G they hold for each component). Let f be a t-vertex coloring of G with colors in {0,1,...,t−1} with t ≥ χ(G). We show that there is an improper twin t-edge coloring s of G satisfying c (v ) = f(v ) for all vertices s i i v ∈ V(G),i = 1,...,n. i Consider a spanning tree T of G and direct the edges from the root of the leaves using v as 1 the root. Using Lemma 2.1, we consider an improper t-edge coloring s of G associated with T and v. It satisfies ∀i ≥ 2,c (v ) = f(v ). Let a ,a ,...,a be the colors (among {0,...,t−1}) of the s i i 1 2 r edges in E(T) incident to v . We have c (v ) = a +a +...+a mod t. If c (v ) = f(v ), then 1 s 1 1 2 r s 1 1 7 f is induced by the improper twin t-edge coloring s. Otherwise, since k is odd (gcd(k,2) = 1), the equation c (v )+2x = f(v ) mod k has a solution x ∈ Z . Lemma 2.3 ensures that we can modify s 1 1 k s into s(cid:48) such that f = c (replacing f(v ) by c (v )+2x does not change the vertex coloring f). s(cid:48) 1 s 1 It completes the proof. Remark 2.5. Note that the non-bipartite condition in Proposition 2.4 is required. In particular we cannot improve Theorem 2.2 ii) in the sense that, for bipartite components, only some t-vertex colorings are induced by an improper twin t-edge twin coloring. Indeed, by Theorem 2.2 i) a 2-vertex coloring of the star graph K cannot be induced by an 1,3 impropertwin2-edgecoloring. Moreover, foranyevenp ≥ 4, thep-vertexcoloringofthestargraph K assigning color 0 to the center and different colors from 1 to p−1 for the other vertices 1,p−1 p(p−1) is not induced by an improper twin p-edge coloring since (cid:54)= 0 mod p. For any odd p ≥ 3, 2 the p-vertex coloring of the star graph K assigning color 1 to the center and different colors 1,p−1 in {0,2,...,p−1} for the other vertices is not induced by an improper twin p-edge coloring since (cid:16) (cid:17) p(p−1) −1 (cid:54)= 1 mod p. 2 The following proposition will enable us to obtain the monotonicity of the improper twin edge coloring for any number of colors (Theorem 2.7) and the bounds on χ(cid:48) (G) for graphs with even it chromatic number at least 4. Proposition 2.6. Let G be a nice non-bipartite connected graph and t an even number such that t ≥ χ(G) ≥ 3. Then, a t-vertex coloring f with colors in {0,...,t−1} is induced by an improper (cid:80) twin t-edge coloring if and only if f(v) is even. v∈V(G) Proof. (⇐) Denote n = |V(G)| and V = {v ,...,v } and let f be a t-vertex coloring of G with 1 n colors in {0,...,t−1} such that t and (cid:80)nf(v ) are even. i i As in the proof of Proposition 2.4 we consider a spanning tree T of G and direct the edges from the root of the leaves using v as the root. Using Lemma 2.1, we consider an improper t-edge 1 coloring s of G associated with T and v . It satisfies ∀i ≥ 2,c (v ) = f(v ). 1 s i i If c (v ) = f(v ), then the proof is complete. Else we claim that f(v )−c (v ) is even. Indeed, s 1 1 1 s 1 by definition of s and denoting by E(cid:48) ⊂ E the set of edges in E(T) not adjacent to v we have: 1 n (cid:88) (cid:88) f(v ) = c (v )+2 s(e) (3) i s 1 i=2 e∈E(cid:48) (cid:80) Since f(v) is even, this implies c (v )+f(v ) is even and consequently f(v )−c (v ) v∈V(G) s 1 1 1 s 1 is even as well. We set f(v )−c (v ) 1 s 1 x = mod t, 2 so f(v ) = c (v )+2x mod t and use Lemma 2.3 to modify s in s(cid:48) such that f = c . It completes 1 s 1 s(cid:48) the proof of the first implication. (⇒) Conversely, suppose that χ(cid:48) (G) = χ(G). Consider an improper twin edge coloring s it (cid:80) using the colors {0,1,...,χ(G)−1}. This induces a vertex coloring f. We have f(v) ≡ v∈V(G) (cid:80) 2( s(e)) (mod k). Now, since k is even, the proof is complete. e∈E(G) Now, we are ready to show the monotonicity property of the improper twin edge chromatic index for the general case. 8 Theorem 2.7. Let G be a nice graph. If G admits an improper twin k-edge coloring (k ≥ 2), then G admits an improper twin t-edge coloring for all t ≥ k. Proof. We assume without loss of generality that G is connected since otherwise the same proof applies to every connected component. The assertion vacuously holds for t = k for all graphs. If G is bipartite, the assertion holds by Theorem 2.2. Assume now G is not bipartite and t > k ≥ 3. If t is odd, then the assertion holds by Proposition 2.4. Assume t is even. Obviously, χ(G) ≤ k. (cid:80) Consider a proper vertex coloring of G, say f : V(G) → {0,...,k−1}. If the sum f(v) is v∈V(G) even then Proposition 2.6 allows to conclude. Otherwise, consider the maximum color used in f, say (cid:96) and change the color of exactly one vertex with color (cid:96) to (cid:96)+1 and call f(cid:48) the resultant proper coloring. Clearly, (cid:80) f(cid:48)(v) is even and apply Proposition 2.6 to conclude the proof. v∈V(G) Let us finally note that Lemma 2.3 and its consequence Proposition 2.6 give a comprehensive and direct proof of Theorem 1.5 in the case where χ(G) is even of size at least 4, as stated below. The other cases are immediate consequences of Theorem 2.2 and Proposition 2.4 for which we gave alternative simple proves as well. Moreover, as stated in Theorem 2.8 the arguments can be turned into a linear-time algorithm. Alternative Proof of Theorem 1.5. If G is bipartite, the statement holds by Theorem 2.2. For a non-bipartite connected graphs G of order at least 3, if χ(G) is odd then the statement holds by Proposition 2.4. To complete the proof of Theorem 1.5, it remains to show that the statement holds for χ(G) even. Let us show this under two cases. Case 1: χ(G) ≡ 0 (mod 4). Then, for some positive integer r, there are 2r colors with an even label and the same holds for colors of odd labels. Consider a χ(G)-vertex coloring of G, say f. If (cid:80) all color classes have odd size, then f(v) is even and by Proposition 2.6 (G is not bipartite) v∈G we are done. Similarly, if all classes have even size we are done as well. Thus assume that we have χ(G) color classes C ,...,C in which, without loss of generality, |C | and |C | have different 0 χ(G)−1 0 1 parity. Now, color C ,i = 2,...,χ(G)−1, arbitrarily using the colors 2,3,...,χ(G)−1. We have i two possibilities for coloring C and C by two colors 0 and 1, and obviously for one of them, the 0 1 total sum of colors of vertices are even and Proposition 2.6 completes the proof. Case 2: χ(G) ≡ 2 (mod 4). Then there is an odd number of odd colors and the same odd number of even colors in a χ(G)-vertex coloring of G. We will show that if there is a color class of even size in some χ(G)-vertex coloring of G then χ(cid:48) (G) = χ(G), and otherwise, that is if all color it classes in all χ(G)-vertex colorings of G are of odd size, then χ(cid:48) (G) = χ(G)+1. it Consider a χ(G)-vertex coloring of G, say f. If all color classes have even size, then obviously (cid:80) f(v) is even and by Proposition 2.6 we are done. If f has two color classes, say without loss v∈G of generality C and C whose cardinalities are of different parity, then Proposition 2.6 allows to 1 2 conclude exactly as in part (i). Now, assume that all color classes in all χ(G)-vertex coloring of G are of odd sizes. Then (cid:80) f(v) is odd for all χ(G)-vertex colorings f of G and by Proposition 2.6, χ(cid:48) (G) > χ(G). v∈V(G) it Now, χ(G) + 1 is an odd number and by the method used in the proof of Proposition 2.4, one can obtain an improper twin edge coloring of G with χ(G) + 1 colors, implying that χ(cid:48) (G) = it χ(G)+1. Finally, the following theorem shows that the arguments can be turned into a linear-time algo- rithm which constructs an improper twin k or (k+1)-edge coloring whenever a k-vertex coloring 9 is given. Theorem 2.8. There is a O(|V|+|E|) algorithm that computes, for a nice graph G = (V,E) and a k-vertex coloring of G, k ≤ n: i) if k ≡ 2 (mod 4) and there is at least one connected component with an odd number of vertices of each color, an improper twin (k+1)-edge coloring; ii) else an improper twin k-edge coloring. Proof. Given the graph G = (V,E) the connected components can be determined in O(|V|+|E|); we can assume G is connected and apply, for a non connected graph, the same algorithm on each connected component. There is a O(|V|+|E|) algorithm deciding whether G is bipartite and, in this case, computing its bipartition. Note first that operations in Z can be performed in O(k) time k and k ≤ |V|. In particular, if k is odd, determining x such that 2x+y = z requires for instance the Euclidean algorithm [7]. Lemma 2.1 (including the determination of the spanning tree T) gives a O(|V|+|E|) algorithm to compute, for a t-vertex coloring c of G and v ∈ V an improper t-edge coloring that almost induces c with v as defective vertex. Indeed, it requires ( 1) computing a spanning tree T, (2) assigning the color 0 to edges outside the trees and (3) computing the colors of the tree-edges proceeding from the leaves to the root. All these operations can be respectively performed in O(|V|+|E|),O(|E|) and O(|V|+|E|). Suppose that G is bipartite. If k = 2 and one part of G is of even size or if k ≥ 3, we use the method described in the proof of Theorem 2.2 to compute an improper twin k-edge coloring inducing c. It requires an application of Lemma 2.1 with the right choice of the root v (either of degree 2 if k ≥ 3 or in an even part of the bipartition if k = 2). This choice requires O(|V|) operations. If k = 2 and both parts of the bipartition are odd, then we apply the previous method replacing k by 3. Let us now suppose G = (V,E) is not bipartite (of course k ≥ 3). Lemma 2.3 gives a O(|V|+|E|) strategy to modify a given k-edge coloring accordingly. It requires determining an odd-cycle C, a path P from v to C and performing modifications along the related closed walk W. These operations require respectively O(|V|+|E|), O(|V|+|E|) and O(|E|) oper- ations. Ifkisodd,thenwerevisittheproofofProposition2.4tocomputeanimpropertwink-edgecoloring. It requires one application of Lemma 2.1 and eventually one equation in Z and one application of k Lemma 2.3. If k ≡ 0 (mod 4) or k ≡ 2 (mod 4), k ≥ 4, then we use similar arguments as in the proof of Theorem 1.5, replacing χ(G) with k. If k ≡ 0 (mod 4) or if k ≡ 2 (mod 4) and at least one color class of even size, then determining an improper twin k-edge coloring requires, using the method in Proposition 2.6, an application of Lemma 2.1 and, eventually, one application of Lemma 2.3. Finally, if k ≡ 2 (mod 4) and all color classes are of odd size, then we use the case where k is odd considering the k-vertex coloring as a (k+1)-vertex coloring. Since the chromatic number of planar graphs is bounded by 4 [4, 14], we trivially have the following: 10