# Tools for parsimonious edge-colouring of graphs with maximum degree three. J.L. Fouquet and J.M. Vanherpe. Rapport n o RR

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1 Tools for parsimonious edge-colouring of graphs with maximum degree three J.L. Fouquet and J.M. Vanherpe LIFO, Université d Orléans Rapport n o RR

2 Tools for parsimonious edge-colouring of graphs with maximum degree three J.L. Fouquet and J.M. Vanherpe L.I.F.O., Faculté des Sciences, B.P Université d Orléans, Orléans Cedex 2, FR Abstract In a graph G of maximum degree let γ denote the largest fraction of edges that can be edge-coloured. Albertson and Haas showed that γ 13 when G is cubic [1]. The notion of δ minimum edge colouring was introduced in [3] in order to extend the so called parcimonious 15 edge-colouring to graphs with maximum degree 3. We propose here an english translation of some structural properties already present in [2, 3] (in French) for δ minimum edge colourings of graphs with maximum degree 3. Keywords : Cubic graph; Edge-colouring; 1 Introduction Throughout this note, we shall be concerned with connected graphs with maximum degree 3. We know by Vizing s theorem [7] that these graphs can be edge-coloured with 4 colours. Let φ : E(G) {α, β, γ, δ} be a proper edgecolouring of G. It is often of interest to try to use one colour (say δ) as few as possible. When an edge colouring is optimal, following this constraint, we shall say that φ is δ minimum. In [2] we gave without proof (in French) results on δ minimum edge-colourings of cubic graphs. Some of them have been obtained later and independently by Steffen [5] and [6]. The purpose of Section 2 is to give with their proofs those results as structural properties of δ minimum edge-colourings. An edge colouring of G using colours α, β, γ, δ is said to be δ-improper provided that adjacent edges having the same colours (if any) are coloured with δ. It is clear that a proper edge colouring (and hence a δ minimum edge-colouring) of G is a particular δ-improper edge colouring. For a proper or δ-improper edge colouring φ of G, it will be convenient to denote E φ (x) (x {α, β, γ, δ}) the set of edges coloured with x by φ. For x, y {α, β, γ, δ},x y, φ(x, y) is the partial subgraph of G spanned by these two colours, that is E φ (x) E φ (y) (this subgraph being a union of paths and even cycles where the colours x and y alternate). Since any two δ minimum edge-colourings of G have the same number of edges coloured δ we shall denote by s(g) this number (the colour number as defined in [5]). As usual, for any undirected graph G, we denote by V (G) the set of its vertices and by E(G) the set of its edges and we suppose that V (G) = n and 2

3 E(G) = m. A strong matching C in a graph G is a matching C such that there is no edge of E(G) connecting any two edges of C, or, equivalently, such that C is the edge-set of the subgraph of G induced on the vertex-set V (C). 2 Structural properties of δ minimum edge-colourings The graph G considered in the following series of Lemmas will have maximum degree 3. Lemma 1 [2, 3] Any 2-factor of G contains at least s(g) disjoint odd cycles. Proof Assume that we can find a 2-factor of G with k < s(g) odd cycles. Then let us colour the edges of this 2-factor with α and β, except one edge (coloured δ) on each odd cycle of our 2-factor and let us colour the remaining edges by γ. We get hence a new edge colouring φ with E φ (δ) < s(g), impossible. Lemma 2 [2, 3] Let φ be a δ minimum edge-colouring of G. Any edge in E φ (δ) is incident to α, β and γ. Moreover each such edge has one end of degree 2 and the other of degree 3 or the two ends of degree 3. Proof Any edge of E φ (δ) is certainly adjacent to α, β and γ. Otherwise this edge could be coloured with the missing colour and we should obtain an edge colouring φ with E φ (δ) < E φ (δ). Lemma 3 below was proven in [4], we give its proof for sake of completeness. Lemma 3 [4] Let φ be a δ improper colouring of G then there exists a proper colouring of G φ such that E φ (δ) E φ (δ) Proof Let φ be a δ-improper edge colouring of G. If φ is a proper colouring, we are done. Hence, assume that uv and uw are coloured δ. If d(u) = 2 we can change the colour of uv to α, β or γ since v is incident to at most two colours in this set. If d(u) = 3 assume that the third edge uz incident to u is also coloured δ, then we can change the colour of uv for the same reason as above. If uz is coloured with α, β or γ, then v and w are incident to the two remaining colours of the set {α, β, γ} otherwise one of the edges uv, uw can be recoloured with the missing colour. W.l.o.g., consider that uz is coloured α then v and w are incident to β and γ.since u has degree 1 in φ(α, β) let P be the path of φ(α, β) which ends on u. We can assume that v or w (say v) is not the other end vertex of P. Exchanging α and β along P does not change the colours incident to v. But now uz is coloured α and we can change the colour of uv with β. In each case, we get hence a new δ-improper edge colouring φ 1 with E φ1 (δ) E φ (δ). Repeating this process leads us to construct a proper edge colouring of G with E φ (δ) E φ (δ) as claimed. 3

4 Lemma 4 [2, 3] Let φ be a δ minimum edge-colouring of G. For any edge e = uv E φ (δ) there are two colours x and y in {α, β, γ}such that the connected component of φ(x, y) containing the two ends of e is an even path joining these two ends. Proof Without loss of generality assume that u is incident to α and β and v is incident to γ (see Lemma 2). In any case (v has degree 3 or degree 2) u and v are contained in paths of φ(α, γ) or φ(β, γ). Assume that they are contained in paths of φ(α, γ). If these paths are disjoint then we can exchange the two colours on the path containing u, e will be incident hence to only two colours β and γ in this new edge-colouring and e could be recoloured with α, a contradiction since we consider a δ minimum edge-colouring. Lemma 5 [2, 3] If G is a cubic graph then A φ B φ C φ s(g) (mod 2). Proof φ(α, β) contains 2 A φ + B φ + C φ vertices of degree 1 and must be even. Hence we get B φ C φ (mod 2). In the same way we get A φ B φ (mod 2) leading to A φ B φ C φ s(g) (mod 2) Remark 6 An edge of E φ (δ) is in A φ when its ends can be connected by a path of φ(α, β), B φ by a path of φ(β, γ) and C φ by a path of φ(α, γ). It is clear that A φ, B φ and C φ are not necessarily pairwise disjoint since an edge of E φ (δ) with one end of degree 2 is contained in 2 such sets. Assume indeed that e = uv E φ (δ) with d(u) = 3 and d(v) = 2 then, if u is incident to α and β and v is incident to γ we have an alternating path whose ends are u and v in φ(α, γ) as well as in φ(β, γ). Hence e is in A φ B φ. When e A φ we can associate to e the odd cycle C Aφ (e) obtained by considering the path of φ(α, β) together with e. We define in the same way C Bφ (e) and C Cφ (e) when e is in B φ or C φ. In the following lemma we consider an edge in A φ, an analogous result holds true whenever we consider edges in B φ or C φ as well. Lemma 7 [2, 3] Let φ be a δ minimum edge-colouring of G and let e be an edge in A φ then for any edge e C Aφ (e) there is a δ minimum edge-colouring φ such that E φ (δ) = E φ (δ) {e} {e }, e A φ and C Aφ (e) = C Aφ (e ). Moreover, each edge outside C Aφ (e) but incident with this cycle is coloured γ, φ and φ only differ on the edges of C Aφ (e). Proof By exchanging colours δ and α and δ and β successively along the cycle C Aφ (e), we are sure to obtain an edge colouring preserving the number of edges coloured δ. Since we have supposed that φ is δ-minimum, at each step, the resulting edge colouring is proper and δ-minimum (Lemma 3). Hence, there is no edge coloured δ incident with C Aφ (e), which means that every such edge is coloured with γ. We can perform these exchanges until e is coloured δ. In the δ minimum edge-colouring φ hence obtained, the two ends of e are joined by a path of φ(α, β). Which means that e is in A φ and C Aφ (e) = C Aφ (e ). For each edge e E φ (δ) (where φ is a δ minimum edge-colouring of G) we can associate one or two odd cycles following the fact that e is in one or two 4

5 sets among A φ, B φ or C φ. Let C be the set of odd cycles associated to edges in E φ (δ). Lemma 8 [2, 3] For each cycle C C, there are no two consecutive vertices with degree two. Proof Otherwise, we exchange colours along C in order to put the colour δ on the corresponding edge and, by Lemma 2, this is impossible in a δ minimum edge-colouring. Lemma 9 [2, 3] Let e 1, e 2 E φ (δ) and let C 1, C 2 C be such that C 1 C 2, e 1 E(C 1 ) and e 2 E(C 2 ) then C 1 and C 2 are (vertex) disjoint. Proof If e 1 and e 2 are contained in the same set A φ, B φ or C φ, we are done since their respective ends are joined by an alternating path of φ(x, y) for some two colours x and y in{α, β, γ}. Without loss of generality assume that e 1 A φ and e 2 B φ. Assume moreover that there exists an edge e such that e C 1 C 2. We have hence an edge f C 1 with exactly one end on C 2. We can exchange colours on C 1 in order to put the colour δ on f. Which is impossible by Lemma 7. Lemma 10 [2, 3] Let e 1 = uv 1 be an edge of E φ (δ) such that v 1 has degree 2 in G. Then v 1 is the only vertex in N(u) of degree 2 and for any edge e 2 = u 2 v 2 E φ (δ), {e 1, e 2 } induces a 2K 2. Proof We have seen in Lemma 2 that uv 1 has one end of degree 3 while the other has degree 2 or 3. Hence, we have d(u) = 3 and d(v 1 ) = 2. Let v 2 and v 3 the other neighbours of u. From Remark 6, we know that v 2 and v 3 are not pendant vertices. Assume that d(v 2 ) = 2 and uv 2 is coloured α, uv 3 is coloured β and, finally v 1 is incident to an edge coloured γ. The alternating path of φ(β, γ) using the edge uv 3 ends with the vertex v 1 (see Lemma 4), then, exchanging the colours along the component of φ(β, γ) containing v 2 allows us to colour uv 2 with γ and uv 1 with α. The new edge colouring φ so obtained is such that E φ (δ) E φ (δ) 1, impossible. Lemma 11 [2, 3] Let e 1 and e 2 be two edges of E φ (δ). If e 1 and e 2 are contained in two distinct sets of A φ, B φ or C φ then {e 1, e 2 } induces a 2K 2 otherwise e 1, e 2 are joined by at most one edge. Proof Assume in a first stage that e 1 A φ and e 2 B φ. Since C e1 (φ) and C e2 (φ) are disjoint by Lemma 9, we know that e 1 and e 2 have no common vertex. The edges having exactly one end in C e1 (φ) are coloured γ while those having exactly one end in C e2 (φ) are coloured α. Hence there is no edge between e 1 and e 2 as claimed. Assume in a second stage that e 1 = u 1 v 1, e 2 = u 2 v 2 A φ.since C e1 (φ) and C e2 (φ) are disjoint by Lemma 9, we can consider that e 1 and e 2 have no common vertex. The edges having exactly one end in C e1 (φ) (or C e2 (φ)) are coloured γ. Assume that u 1 u 2 and v 1 v 2 are edges of G. We may suppose 5

6 without loss of generality that u 1 ans u 2 are incident to α while v 1 and v 2 are incident to β (if necessary, colours α and β can be exchanded on C e1 (φ) and C e2 (φ)). We know that u 1 u 2 and v 1 v 2 are coloured γ. Let us colour e 1 and e 2 with γ and u 1 u 2 with β and v 1 v 2 with α. We get a new edge colouring φ where E φ (δ) E φ (δ) 2, contradiction since φ is a δ minimum edge-colouring. Lemma 12 [2, 3] Let e 1, e 2 and e 3 be three distinct edges of E φ (δ) contained in the same set A φ, B φ or C φ. Then {e 1, e 2, e 3 } induces a subgraph with at most four edges. Proof Without loss of generality assume that e 1 = u 1 v 1, e 2 = u 2 v 2 and e 3 = u 3 v 3 A φ. From Lemma 11 we have just to suppose that (up to the names of vertices) u 1 u 3 E(G) and v 1 v 2 E(G). Possibly, by exchanging the colours α and β along the 3 disjoint paths of φ(α, β) joining the ends of each edge e 1, e 2 and e 3, we can suppose that u 1 and u 3 are incident to β while v 1 and v 2 are incident to α. Let φ be obtained from φ when u 1 u 3 is coloured with α, v 1 v 2 with β and u 1 v 1 with γ. It is easy to check that φ is a proper edge-colouring with E φ (δ) E φ (δ) 1, contradiction since φ is a δ minimum edge-colouring. Lemma 13 [2, 3] Let e 1 = u 1 v 1 be an edge of E φ (δ) such that v 1 has degree 2 in G. Then for any edge e 2 = u 2 v 2 E φ (δ) {e 1, e 2 } induces a 2K 2. Proof From Lemma 11 we have to consider that e 1 and e 2 are contained in the same set A φ, B φ or C φ. Assume without loss of generality that they are contained in A φ. From Lemma 11 again we have just to consider that there is a unique edge joining these two edges and we can suppose that u 1 u 2 E(G). Possibly, by exchanging the colours α and β along the 2 disjoint paths of φ(α, β) joining the ends of each edge e 1, e 2, we can suppose that u 1 and u 2 are incident to β while v 1 and v 2 are incident to α. We know that u 1 u 2 is coloured γ. Let φ be obtained from φ when u 1 u 2 is coloured with α and u 1 v 1 is coloured with γ. It is easy to check that φ is a proper edge-colouring with E φ (δ) E φ (δ) 1, contradiction since φ is a δ minimum edge-colouring. References [1] M.O. Albertson and R. Haas. Parsimonious edge colouring. Discrete Mathematics, 148:1 7, [2] J-L Fouquet. Graphes cubiques d indice chromatique quatre. Annals of Discrete Mathematics, 9:23 28, [3] J-L Fouquet. Contribution à l étude des graphes cubiques et problèmes hamiltoniens dans les graphes orientés. PhD thesis, Université Paris SUD, [4] J-L Fouquet and J-M Vanherpe. On parcimonious edge-colouring of graphs with maximum degree three. Submitted to Graphs and Combinatorics. 6

7 [5] E. Steffen. Classifications and characterizations of snarks. Discrete Mathematics, 188: , [6] E. Steffen. Measurements of edge-uncolorability. Discrete Mathematics, 280: , [7] V.G. Vizing. On an estimate of the chromatic class of p-graphs. Diskret. Analiz, 3:25 30,

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