Decomposition of (2k + 1)-regular graphs containing special spanning 2k-regular Cayley graphs into paths of length 2k + 1 F´abio Botler Luiz Hoffmann Programa de Engenharia de Sistemas e Computac¸˜ao Universidade Federal do Rio de Janeiro December 10, 2020 Abstract A Pℓ-decomposition of a graph G is a set of paths with ℓ edges in G that cover the edge set of G. Favaron, Genest, and Kouider (2010) conjectured that every (2k + 1)-regular graph that contains a perfect matching admits a P2k+1- decomposition. They also verified this conjecture for 5-regular graphs without cycles of length 4. In 2015, Botler, Mota, and Wakabayashi verified this conjecture for 5-regular graphs without triangles. In this paper, we verify it for (2k + 1)- regular graphs that contain the kth power of a spanning cycle; and for 5-regular graphs that contain special spanning 4-regular Cayley graphs. keywords: Decomposition, Path, Regular graph, Cayley graph. 1 Introduction All graphs in this paper are simple, i.e., have no loops nor multiple edges. A decom- position of a graph G is a set D of edge-disjoint subgraphs of G that cover its edge set. If every element of D is isomorphic to a fixed graph H, then we say that D is an H-decomposition. In this paper, we focus on the case H is the simple path with 2k + 1 arXiv:2012.05145v1 [math.CO] 9 Dec 2020 edges, which we denote by P2k+1. Note that this notation is not standard. In 1957, Kotzig [9] (see also [4]) proved that a 3-regular graph G admits a P3-decomposition if and only if G contains a perfect matching. In 2010, Favaron, Genest, and Kouider [6] extended this result by proving that every 5-regular graph that contains a perfect matching and no cycles of length 4 admits a P5-decomposition; and proposed the following conjecture to extend Kotzig’s result. Conjecture 1 (Favaron–Genest–Kouider, 2010). If G is a (2k + 1)-regular graph that con- tains a perfect matching, then G admits a P2k+1-decomposition. *E-mails: [email protected] (F. Botler), [email protected] (L. Hoffmann). 1 In 2015, Botler, Mota, and Wakabayashi [3] verified Conjecture 1 for triangle-free 5-regular, and, more recently, Botler, Mota, Oshiro, and Wakabayashi [2] generalized this result for (2k + 1)-regular graphs with girth at least 2k. It is clear that a 5-regular graph contains a perfect matching if and only if it con- tains a spanning 4-regular graph. In fact, by using a theorem of Petersen [10], one can prove that a (2k + 1)-regular graph contains a perfect matching if and only if it contains a spanning 2k′-regular graph for every k′ ≤ k. Theorem 1 (Petersen, 1891). If G is a 2k-regular graph, then G admits a decomposition into spanning 2-regular graphs. In this paper, we explore Conjecture 1 for (2k + 1)-regular graphs that contain special spanning 2k-regular graphs as follows. Throughout the text, Γ denotes a finite group of order n; + denotes the group operation of Γ; and 0 denotes the identity of Γ. As usual, for each x ∈ Γ, we denote by −x the inverse of x, i.e., the element y ∈ Γ for which x + y = 0, and the operation − denotes the default binary operation (x, y) 7→ x + (−y). Let S ⊆ Γ be a set not containing 0, and such that −x ∈ S for every x ∈ S (i.e., S is closed under taking inverses). The Cayley graph X(Γ, S) is the graph H with V(H) = Γ, and E(H) = xy : y − x ∈ S (see [7]). In this paper, we allow S to be a set not generating Γ, and hence X(Γ, S) is not necessarily connected. We say that H is simply commutative if (i) x + y = y + x for every x, y ∈ S, and (ii) −x 6= x for every x ∈ S. Condition (ii) implies that H has no multiple edges, and, since 0∈ / S, H is simple. It is not hard to check that, in such a graph, the neighborhood of a vertex v ∈ Γ is N(v) = {v + x : x ∈ S}. Although the definition of Cayley graphs can be extended to multigraphs and directed graphs, Conjecture 1 considers only simple graphs. In fact, we explore some structure of the colored directed Cayley graph (see [5]) in which the edge set consists of the pairs (x, x + s) with color s ∈ S. We present two results regarding Conjecture 1. We verify it for (2k + 1)-regular graphs that contain the kth power of a spanning cycle (see Section 2); and for 5- regular graphs that contain spanning simply commutative 4-regular Cayley graphs (see Section 3). Since the graphs in these classes may contain cycles of lengths 3 and 4, these results extend the family of graphs for which Conjecture 1 is known to hold. We believe that, due to the underlying group structure, the techniques developed here can be extended for dealing with (2k + 1)-regular graphs that contain more gen- eral spanning Cayley graphs, and also (2k + 1)-regular graphs that contain special spanning Schreier graphs, which could give us significant insight with respect to the general case of Conjecture 1 (see Section 4). Notation. A graph T is a trail if there is a sequence x0,..., xℓ of its vertices for which E(T) = {xi xi+1 : 0 ≤ i ≤ ℓ − 1} and xixi+1 6= xjxj+1, for every i 6= j. Moreover, if xi 6= xj for every i 6= j, we say that T is a path. A subgraph F of a graph G is a factor of G or a spanning subgraph of G, if V(F) = V(G). If, additionally, F is r-regular, then we say that F is an r-factor. In particular, a perfect matching is the edge set of a 1-factor. Moreover, we say that F is an H-factor if F is a factor that consists of vertex-disjoints copies of H. The reader may refer to [1] for standard definitions of graph theory. 2 2 Regular graphs that contain powers of cycles Given a perfect matching M in a graph G, we say that a Pℓ-decomposition D of a graph G is M-centered if for every P = a0a1 ··· aℓ−1aℓ ∈ D, we have aiai+1 ∈ M for i = (ℓ − 1)/2, i.e., if the middle edges of the paths in D are precisely the edges of M. The next results are examples of M-centered decomposition that are used in the proof of Theorems 7 and 12. Proposition 2. If G is a 5-regular graph that contains a spanning copy K of K4,4, and M = E(G) \ E(K), then G admits an M-centered P5-decomposition. Proof. Let G, K, and M be as in the statement. Let (R, L) be the bipartition of K, where R = {r1, r2, r3, r4} and L = {l1, l2, l3, l4}. Since K is a complete bi- partite graph, if xy ∈ M, then either x, y ∈ R or x, y ∈ L. Thus, we may assume, without loss of generality, that M = {r1r2, r3r4, l1l2, l3l4}, and hence, D = {l1r1l3l4r2l2, l3r3l1l2r4l4, r1l2r3r4l1r2, r3l4r1r2l3r4} is an M-centered decomposi- tion of G as desired (see Figure 1). r1 r2 r3 r4 l1 l2 l3 l4 Figure 1: P5-decomposition of a 5-regular graph that contains a spanning copy of a K4,4. Given positive integers k and n, the kth power of the cycle on n vertices, denoted k by Cn, is the graph on the vertex set {0,..., n − 1} and such that, for every vertex v, we have x ∈ N(v) if and only if x = v + r (mod n), where r ∈ {−k,..., −1}∪{1,..., k}. Proposition 3. Let n and k be positive integers for which k < n/2. If G isa simple (2k + 1)- k regular graph on n vertices that contains a copy C of Cn, and M = E(G) \ E(C), then G admits an M-centered P2k+1-decomposition. Proof. Let G, C, and M be as in the statement, and let V(C)= {0,..., n − 1} as above. The operations on the vertices of C are taken modulo n. Since C isa2k-regular graph, M is a perfect matching of G. Given i ∈ V(C), let Qi be the path v0v1 ··· vk in which v0 = i; and, for j = 1,..., k, we have vj = vj−1 + j if j is odd; and vj = vj−1 − j if j is even (see Figure 2). Note that for every j = 1,..., k, the path Qi contains an edge xy such that |x − y| = j. Also, we have V(Qi)= i + r : r ∈ {−⌊k/2⌋, −⌊k/2⌋ + 1,..., ⌈k/2⌉} . It is not hard to check that the set Q = Qi : i ∈ V(C) is a Pk- decomposition of C. Given an edge e = ij ∈ M, let Pe = Qi ∪{ij} ∪ Qj. Since Qi and Qj have, respec- tively, i and j as end vertices, and E(Qi) ∩ E(Qj)= ∅, the graph Pe is a trail of length 2k + 1. Thus, since Q is a Pk-decomposition of C, and M is a perfect matching of G, 3 6 5 7 4 8 3 9 2 0 1 Figure 2: The path Puv, with u = 2 and v = 8, in the proof of Proposition 3 for a 3 7-regular graph that contains a spanning copy of a C10.