# A counterexample to a result on the tree graph of a graph

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1 AUSTRALASIAN JOURNAL OF COMBINATORICS Volume 63(3) (2015), Pages A counterexample to a result on the tree graph of a graph Ana Paulina Figueroa Departamento de Matemáticas Instituto Tecnológico Autónomo de México (ITAM) Mexico City Mexico Eduardo Rivera-Campo Departamento de Matemáticas Universidad Autónoma Metropolitana Iztapalapa Mexico City Mexico Abstract Given a set of cycles C of a graph G, the tree graph of G, defined by C, is the graph T (G, C) whose vertices are the spanning trees of G and in which two trees R and S are adjacent if R S contains exactly one cycle and this cycle lies in C. Li et al. [Discrete Math. 271 (2003), ] proved that if the graph T (G, C) is connected, then C cyclically spans the cycle space of G. Later, Yumei Hu [Proc. 6th Int. Conf. Wireless Communications Networking and Mobile Comput. (2010), 1 3] proved that if C is an arboreal family of cycles of G which cyclically spans the cycle space of a 2-connected graph G, then T (G, C) is connected. In this note we present an infinite family of counterexamples to Hu s result. 1 Introduction The tree graph of a connected graph G is the graph T (G) whose vertices are the spanning trees of G, in which two trees R and S are adjacent if R S contains exactly one cycle. Li et al. [2] defined the tree graph of G with respect to a set of Partially supported by CONACyT, México Projects and

2 A.P. FIGUEROA ET AL. / AUSTRALAS. J. COMBIN. 63 (3) (2015), cycles C as the spanning subgraph T (G, C) of T (G) where two trees R and S are adjacent only if the unique cycle contained in R S lies in C. A set of cycles C of G cyclically spans the cycle space of G if for each cycle σ of G there are cycles α 1, α 2,..., α m C such that: σ = α 1 α 2... α m and, for i = 2, 3,..., m, α 1 α 2... α i is a cycle of G. Li et al. [2] proved the following theorem: Theorem 1. If C is a set of cycles of a connected graph G such that the graph T (G, C) is connected, then C cyclically spans the cycle space of G. A set of cycles C of a graph G is arboreal with respect to G if for every spanning tree T of G, there is a cycle σ C which is a fundamental cycle of T. Yumei Hu [1] claimed to have proved the converse theorem: Theorem 2. Let G be a 2-connected graph. If C is an arboreal set of cycles of G that cyclically spans the cycle space of G, then T (G, C) is connected. In this note we present a counterexample to Theorem 2 given by a triangulated plane graph G with 6 vertices and an arboreal family of cycles C of G such that C cyclically spans the cycle space of G, while T (G, C) is disconnected. Our example generalises to a family of triangulated graphs G n with 3(n + 2) vertices for each integer n 0. If α is a face of a plane graph G, we denote, also by α, the corresponding cycle of G as well as the set of edges of α. 2 Preliminary results Let G be a plane graph. For each cycle τ, let k(τ) be the number of faces of G contained in the interior of τ. A diagonal edge of τ is an edge lying in the interior of τ having both vertices in τ. The following lemma will be used in the proof of Theorem 4. Lemma 3. Let G be a triangulated plane graph and σ be a cycle of G. If k(σ) 2, then there are two faces φ and ψ of G, contained in the interior of σ, both with at least one edge in common with σ, and such that σ φ and σ ψ are cycles of G. Proof. If k(σ) = 2, let φ and ψ be the two faces of G contained in the interior of σ. Clearly σ φ = ψ and σ ψ = φ which are cycles of G. Assume k = k(σ) 3 and that the result holds for each cycle τ of G with 2 k(τ) < k. If σ has a diagonal edge uv, then σ together with the edge uv define two cycles σ 1 and σ 2 such that k(σ) = k(σ 1 ) + k(σ 2 ). If σ 1 is a face of G, then σ σ 1 is a cycle of G and, if k(σ 1 ) 2, then by induction there are two faces φ 1 and ψ 1 of G, contained in the interior of σ 1, both with at least one edge in common with σ 1, and such that σ 1 φ 1 and σ 1 ψ 1 are cycles of G. Without loss of generality, we assume uv is not an edge of φ 1 and therefore φ = φ 1 has at least one edge in common

3 A.P. FIGUEROA ET AL. / AUSTRALAS. J. COMBIN. 63 (3) (2015), with σ and is such that σ φ is a cycle of G. Analogously σ 2 contains a face ψ with at least one edge in common with σ and such that σ ψ is a cycle of G. For the remaining of the proof we may assume σ has no diagonal edges. Since k = k(σ) 3, there is a vertex u of σ which is incident with one or more edges lying in the interior of σ. Let v 0, v 1,..., v m be the vertices in σ or in the interior of σ which are adjacent to u. Without loss of generality we assume v 0 and v m are vertices of σ and that v 0, v 1,..., v m 1, v m is a path joining v 0 and v m, see Figure 1. Figure 1: Cycle σ with no diagonal edges. As σ has no diagonal edges, vertices v 1, v 2,..., v m 1 are not vertices of σ and therefore faces φ = uv 0 v 1 and ψ = uv m v m 1 are such that σ φ and σ ψ are cycles of G, each with one edge in common with σ. Theorem 4. Let G be a triangulated plane graph and α and β be two internal faces of G with one edge in common. If C is the set of internal faces of G with cycle α replaced by the cycle α β, then C cyclically spans the cycle space of G. Proof. Let σ be a cycle of G. If k(σ) = 1, then σ C or σ = α in which case σ = (α β) β. In both cases σ is cyclically spanned by C. We proceed by induction assuming k = k(σ) 2 and that if τ is a cycle of G with k(τ) < k, then τ is cyclically spanned by C. By Lemma 3, there are two faces φ and ψ of G, contained in the interior of σ, such that both σ φ and σ ψ are cycles of G; without loss of generality we assume φ α. Clearly k(σ φ) < k; by induction, there are cycles τ 1, τ 2,..., τ m C such that: σ φ = τ 1 τ 2... τ m and, for i = 2, 3,..., m, τ 1 τ 2... τ i is a cycle of G. As σ = (σ φ) φ = τ 1 τ 2... τ m φ, cycle σ is also cyclically spanned by C. 3 Main result Let G be the skeleton graph of a octahedron (see Figure 2) and C be the set of cycles that correspond to the internal faces of G with cycle α replaced by cycle α β.

4 A.P. FIGUEROA ET AL. / AUSTRALAS. J. COMBIN. 63 (3) (2015), Figure 2: Graph G with internal faces α, β, γ, δ, σ, τ and ρ and outer face ω. By Theorem 4, C cyclically spans the cycle space of G. Suppose C is not arboreal and let P be a spanning tree of G with none of its fundamental cycles in C. For this to happen, each of the cycles β, γ, δ, σ, τ and ρ of G, must have at least two edges which are not edges of P and since P has no cycles, at least one edge of cycle α and at least one edge of cycle ω are not edgs of P. Therefore G has at least 7 edges which are not edges of P. These, together with the 5 edges of P sum up to 12 edges which is exactly the number of edges of G. This implies that each of the cycles ω and α has exactly two edges of P and that each of the cycles β, γ, δ, σ, τ and ρ has exactly one edge of P. If edges xy and xz are edges of P, then vertex x cannot be incident to any other edge of P and therefore cycle α can only have one edge of P, which is not possible. If edges xy and yz are edges of P, then vertex y cannot be incident to any other edge of P. In this case, the edge in cycle σ, opposite to vertex y, must be an edge of P and cannot be incident to any other edge of P, which again is not possible. The case where edges xz and yz are edges of P is analogous. Therefore C is an arboreal set of cycles of G. Let T, S and R be the spanning trees of G given in Figure 3. The graph T (G, C) has a connected component formed by the trees T, S and R since cycle ρ is the only cycle in C which is a fundamental cycle of either T, S or R. This implies that T (G, C) is disconnected. Figure 3: Trees T (left), S (center) and R (right). We proceed to generalise the counterexample to graphs with arbitrary large number of vertices. Let G 0 = G, x 0 = x, y 0 = y, z 0 = z and for t 0 define G t+1 as the graph obtained by placing a copy of G t in the inner face of the skeleton graph of an octahedron as in Figure 4.

5 A.P. FIGUEROA ET AL. / AUSTRALAS. J. COMBIN. 63 (3) (2015), Figure 4: G t+1. Notice that each graph G n contains a copy G of G in the innermost layer. We also denote by α, β, γ, δ, σ, τ and ρ the cycles of G n that correspond to the cycles α, β, γ, δ, σ, τ and ρ of G. Let ω n denote the cycle given by the edges in the outer face of G n. For n 0 let C n be the set of cycles that correspond to the internal faces of G n with cycle α replaced by cycle α β. By Theorem 4, C n cyclically spans the cycle space of G n. We claim that for n 0, set C n is an arboreal set of cycles of G n. Suppose C t is arboreal but C t+1 is not and let P t+1 be a spanning tree of G t+1 such that none of its fundamental cycles lies in C t+1. As in the case of graph G and tree P, above, each cycle in C t+1, other than α β, has exactly one edge in P t+1, while cycles α and ω t+1 have exactly two edges in P t+1. The reader can see that this implies that the edges of P t+1 which are not edges of G t form a path with length 3. Then P t+1 {x t+1, y t+1, z t+1 } is a spanning tree of G t and by induction, one of its fundamental cycles lies in C t C t+1 which is a contradiction. Therefore C t+1 is arboreal. Let T 0 = T and for t 0 define T t+1 as the spanning tree of G t+1 obtained by placing a copy of T t in the inner face of the skeleton graph of an octahedron as in Figure 5. Trees S t+1 and R t+1 are obtained from S t and R t in the same way with S 0 = S and R 0 = R respectively. We claim that, for each integer n 0, cycle ρ is the only cycle in C n which is a fundamental cycle of either T n, S n or R n. Therefore T n, S n and R n form a connected component of G n which implies that T (G n, C n ) is disconnected.

6 A.P. FIGUEROA ET AL. / AUSTRALAS. J. COMBIN. 63 (3) (2015), Figure 5: t = 3k (left), t = 3k + 1 (centre) and t = 3k + 2 (right). References [1] Y. Hu, A necessary and sufficient condition on the tree graph defined by a set of cycles, in Proc. 6th Int. Conf. Wireless Communications Networking and Mobile Computing (WiCOM) (2010), IEEE (2010), 1 3. [2] X. Li, V. Neumann-Lara and E. Rivera-Campo, On the tree graph defined by a set of cycles, Discrete Math. 271 (2003), (Received 9 Jan 2015; revised 17 July 2015)

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