CORE Metadata, citation and similar papers at core.ac.uk Provided by Elsevier - Publisher Connector Discrete Mathematics 307 (2007) 567–578 www.elsevier.com/locate/disc A classification of cubic bicirculants Tomaž Pisanski IMFM, University of Ljubljana, and University of Primorska, Slovenia Received 2 April 2004; received in revised form 12 March 2005; accepted 26 September 2005 Available online 14 September 2006 Abstract The well-known Petersen graph G(5, 2) admits a semi-regular automorphism acting on the vertex set with two orbits of equal size. This makes it a bicirculant. It is shown that trivalent bicirculants fall into four classes. Some basic properties of trivalent bicirculants are explored and the connection to combinatorial and geometric configurations are studied. Some analogues of the polycirculant conjecture are mentioned. © 2006 Elsevier B.V. All rights reserved. Keywords: Bicirculant; Covering graph; Configuration 1. Introduction The object of this study are trivalent (cubic) graphs admitting an action of a cyclic group having two equally sized vertex orbits. Such graphs are called bicirculants. For symmetry properties of more general bicirculants compare [8]. The automorphism that generates the corresponding cyclic group is said to be semi-regular. Let G be a bicirculant and let be the corresponding semi-regular automorphism. By V (G) = V1 ∪ V2 we denote the decomposition of the vertex set into the two orbits where V1 ={u0,u1,...,un−1}, V2 ={v0,v1,...,vn−1} and (ui) = ui+1, (vi) = vi+1,i= 0, 1,...,n− 1. Note that all computations are mod n. There are two types of edges with respect to . An edge e that has both of its endpoints in the same orbit is said to be a rim edge (R), otherwise, if the two endpoints belong to distinct orbits, the edge is said to be a spoke (S). Now we distinguish four classes of bicirculants (with respect to a given semi-regular element ) and classify the vertices according to the type of incident edges. If all the edges are rim edges, we denote the corresponding vertex class by 3R, if two of them are rim edges the class is denoted by 2R + S, if only one edge is a rim edge, the class is denoted by R + 2S, while the remaining class, composed of only spoke edges is denoted by 3S. Recall that the Möbius ladder Mn on 2n vertices is a graph on the vertex set {v0,v1,...,v2n−1} with two types of edges vk ∼ vk+1,vk ∼ vk+n,k= 0, 1,...,n− 1. E-mail address: [email protected]. 0012-365X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.disc.2005.09.053 568 T. Pisanski / Discrete Mathematics 307 (2007) 567–578 Also, a generalized Petersen graph G(n, r), introduced by Mark Watkins in [25], is a graph on the vertex set {u0,u1,...,un−1,v0,v1,...,vn−1} with the adjacencies: uk ∼ uk+1,vk ∼ vk+r ,uk ∼ vk,k= 0, 1,...,n− 1. In particular, G(5, 2) is the Petersen graph, G(10, 2) is the dodecahedron graph, and G(n, 1) is also known as the n-prism n. A circulant graph is a possibly disconnected Cayley graph of a cyclic group. It is well-known that each circulant C(n, S) can be described by two parameters: an integer n and a symbol S ⊂ Zn, such that 0 ∈/ S,S =−S. It follows, that for a cubic circulant we have S ={i, −i, n/2} and therefore n has to be even. Each cubic circulant graph either consists of isomorphic copies of r-prisms r or Möbius ladders. Bicirculants have been studied in the past [19]. They form a subclass of an important class of graphs, called polycir- culants; see for instance [17,19,18] and also [2,22]. We are interested only in simple graphs (graphs with no loops or parallel edges). However in describing them we use general graphs and even pre-graphs, allowing parallel edges, loops and even half-edges. Proposition 1. In any cubic bicirculant graph the type of vertex is constant; i.e. all the vertices have the same type. Proof. Since the type of a vertex is preserved by the automorphism the vertices in the same orbit have the same type. As both vertex orbits have the same size, the number of spokes per vertex is constant. The structure of a cubic bicirculant is highly dependent on the vertex type. We shall explain the structure of each of the four classes. 2. Pre-graphs, voltage graphs, covering graphs In order to define the four classes of graphs in a simple way we need to apply covering graph technique. First we have to slightly generalize the familiar notion of a graph to pre-graphs. Pre-graphs are allowed to have loops and parallel edges and also half-edges (or pending edges). A pre-graph G is a quadruple G = (V,S,i,r)where V is the set of vertices, S is the set of arcs (or semi-edges, darts, sides, ...), i is a mapping i : S → V , specifying the origin or initial vertex for each half-edge, while r is the reversal involution: r : S → S,r2 = 1. We may also define t : S → V as t(s) := i(r(s)), specifying the terminal vertex for each arc. An arc s with r(s) = s forms an edge e ={s,r(s)}, which is called proper if |e|=2 and is called a half-edge if |e|=1. Define j(e) ={i(s),t(s)}. A pre-graph without half-edges is called a general graph. Note that G is a graph if and only if the involution has no fixed points. A proper edge e with |j(e)|=1 is called a loop and two edges e, e are parallel if j(e) = j(e). A graph without loops and parallel edges is called simple. The valence of a vertex v is defined as val(v) =|{s ∈ S|i(s) = v}|. A pre-graph of valence 3 is called cubic. There are two cubic pre-graphs on a single vertex and there are seven cubic pre-graphs on two vertices. Let us consider four pre-graphs on 2 vertices. In all cases we have V ={u, v}, S ={s1,s2,s3,s4,s5,s6}. The four pre-graphs T,H,I,F can be defined by the tables. Ts1 s2 s3 s4 s5 s6 Hs1 s2 s3 s4 s5 s6 i uuuvvviuuuvvv rs1 s3 s2 s4 s6 s5 rs4 s5 s6 s1 s2 s3 Is1 s2 s3 s4 s5 s6 Fs1 s2 s3 s4 s5 s6 iuuuvvviuuuvvv rs4 s3 s2 s1 s6 s5 rs4 s5 s3 s1 s2 s6 T. Pisanski / Discrete Mathematics 307 (2007) 567–578 569 Now we shall briefly introduce voltage graphs. Voltage graphs are obtained from pre-graphs by assigning group elements to arcs. More precisely, a voltage graph X is a 6-tuple X=(V,S,i,r,,a)where (V,S,i,r)is the underlying pre-graph, is a group and a is a mapping a : S → satisfying the following axiom: For each s ∈ S we have a(r(s)) = a−1(s). Any voltage graph X defines the so-called derived graph or regular covering graph Y as follows: V(Y):= V × , S(Y ) := S × , i(s,) := (i(s), ) for any (s, ) ∈ S(Y), r(s,) := (r(s), a(s) ◦ ) for any (s, ) ∈ S(Y). If s ∈ S is a half-edge, i.e. if r(s) = s its voltage a(s) is of order at most two: a(s) = a−1(s). Hence a = a−1 or equivalently, a2 = 1. In this paper we restrict our attention to the case when the group is cyclic = Zn. In such a case the only voltages that can be assigned to a half-edge are 0 and n/2. A voltage 0 on a half-edge implies that the derived graph remains a pre-graph having half-edges. Since we are interested in simple covering graphs it means that the only admissible voltage for a half-edge remains n/2; furthermore n must be even. The above tables for T,H,I,F can be augmented to describe voltage graphs. Ts1 s2 s3 s4 s5 s6 Hs1 s2 s3 s4 s5 s6 iu uuv vv iuuuvv v rs1 s3 s2 s4 s6 s5 rs4 s5 s6 s1 s2 s3 an/2 i −in/2 j −j aij0 −i −j 0 Is1 s2 s3 s4 s5 s6 Fs1 s2 s3 s4 s5 s6 iuuu vvv iuuu vvv rs4 s3 s2 s1 s6 s5 rs4 s5 s3 s1 s2 s6 a 0 i −i 0 j −j a 0 in/20 in/2 The corresponding covering graphs are denoted by T (n, i, j), H(n, i, j), I (n, i, j), and F (n, i). As mentioned earlier, we are only interested in simple graphs. Since T (n, i, j), T (n, j, i), T (n, n − i, n − j) and T (n, n − j,n − i) are isomorphic we may assume without loss of generality that in T (n, i, j) we have 0 <ij n − 1,i+ j n.Inthe similar way we may restrict parameters for H(n, i, j) to 0 <i<jn−1,i+j n, for I (n, i, j) to 0 <ij n−1,i = n/2,j = n/2 and for F (n, i) to n even and 0 <i<n/2. Note that the family of graphs F (n, i) differs from the other three families since it can be described by only two parameters.