# ON THE TREE GRAPH OF A CONNECTED GRAPH

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1 Discussiones Mathematicae Graph Theory 28 (2008 ) ON THE TREE GRAPH OF A CONNECTED GRAPH Ana Paulina Figueroa Instituto de Matemáticas Universidad Nacional Autónoma de México Ciudad Universitaria, México D.F , México and Eduardo Rivera-Campo Departmento de Matemáticas Universidad Autónoma Metropolitana-Iztapalapa Av. San Rafael Atlixco 186, México D.F , México Abstract Let G be a graph and C be a set of cycles of G. The tree graph of G defined by C, is the graph T (G, C) that has one vertex for each spanning tree of G, in which two trees T and T are adjacent if their symmetric difference consists of two edges and the unique cycle contained in T T is an element of C. We give a necessary and sufficient condition for this graph to be connected for the case where every edge of G belongs to at most two cycles in C. Keywords: tree graph, property, property Mathematics Subject Classification: 05C05. 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 T and T are adjacent if T T contains a unique cycle. It is well-known that the T (G) is always connected. Research supported by CONACYT (México).

2 502 A.P. Figueroa and E. Rivera-Campo Several variations of the tree graph have been studied; see for instance the adjacency tree graph studied by Zhang and Chen in [5] and by Heinrich and Liu in [3], and the leaf exchange tree graph of Broersma and Li [1] and Harary et al. [2]. Let G be a connected graph and C be a set of cycles of G. The tree graph of G defined by C is the spanning subgraph T (G, C) of T (G) in which two trees T and T are adjacent if they are adjacent in T (G) and the unique cycle contained in T T is an element of C. A unicycle U of G is a connected spanning subgraph of G that contains exactly one cycle. In [4], Li et al. defined the following property: A cycle σ of G has property with respect to C if for any unicycle U of G containing σ, there are two cycles δ, γ C, contained in U +e for some edge e of G, such that σ = δ γ. The closure cl G (C) of the set C is the set of cycles obtained from C by successively adding new cycles of G that satisfy property until no such cycle remains. A set C is -dense if cl G (C) is the set of all cycles of G. Li et al. proved that if T (G, C) is connected, then C must span the cycle space of G and that if C is -dense, then T (G, C) is connected. In this article we define a weaker property which we call + and show that being + -dense is also a sufficient condition for T (G, C) to be connected. We also prove that if T (G, C) is connected and every edge of G lies in at most two cycles of C, then C is + -dense Dense: A New Sufficient Condition We denote by Γ(G) the cycle space of a graph G. Let σ be a cycle and U be a unicycle of G, we say that U is a σ-unicycle if the unique cycle contained in U is σ. For an integer k, a cycle σ and a σ-unicycle U of G, (σ, U) has property k with respect to C if there exists a set of cycles X = {σ 0,..., σ k } C and a set of edges Y = {e 0,..., e k 1 } E(G)\E(U) such that the following conditions are satisfied: (P 1 ) X is a basis of Γ(U + Y ). (P 2 ) Let ρ X. For every edge x E(ρ)\E(σ), there exists a unique ρ X such that ρ ρ and x E(ρ ). (P 3 ) For every edge x E(σ), there exists a unique ρ X such that x E(ρ).

3 On the Tree Graph of a Connected Graph 503 We say a cycle σ of G has property + with respect to C if for every σ-unicycle U of G, there exists a positive integer k such that (σ, U) has property k with respect to C. The closure cl G (C) + of the family C is the set of cycles obtained from C by successively adding new cycles of G that satisfy property + until no such cycle remains. It is not difficult to prove that this closure is well defined. We say C is + -dense if cl + G (C) is the set of all cycles of G. Remark 1. If C is -dense, then C is + -dense. Note that σ has property with respect to C if and only if (σ, U) has property 1 with respect to C for every σ-unicycle U of G. Lemma 1. Let C be a family of cycles of a connected graph G, U be a σ-unicycle of G for some cycle σ C and k be a positive integer. If (σ, U) has property k with respect to C, then every pair of spanning trees of U is connected by a path in T (G, C). P roof. By induction on k. The induction basis follows from the proof of Lemma 3.1 in [4]. Nevertheless, we will include a proof here. Let U be a σ-unicycle of G such that (σ, U) has the property 1 and T and T be two spanning trees of U. There exists an edge e E(U) and two cycles δ, γ C of U + e such that σ = δ γ. Let a and b be edges of U such that T = (T a) + b. If a E(δ)\E(γ) and b E(γ)\E(δ), let Q = (T a) + e. It follows that Q = (T b)+e. Since the unique cycle of G contained in Q T is δ and the unique cycle of G contained in Q T is γ, we have that Q is adjacent to T and to T in T (G, C). Thus, we have found the path {T, Q, T } from T to T in T (G, C). If a E(γ)\E(δ) and b E(δ)\E(γ) we can interchange γ with δ in the previous argument. If a, b E(δ)\E(γ) let c E(γ)\E(δ) and consider the spanning trees Q = (T c) + e and Q = (T c) + e of G. Notice that Q is adjacent to T in T (G, C) because the unique cycle contained in Q T is γ and that Q is adjacent to T in T (G, C) because the unique cycle contained in T Q is also γ. Even more, Q is adjacent to Q in T (G, C) because Q = (Q a) + b and the unique cycle contained in Q Q is δ. Thus, {T, Q, Q, T } is a path connecting T and T in T (G, C). Finally, if a, b E(γ)\E(δ), we interchange γ with δ in the previous argument.

4 504 A.P. Figueroa and E. Rivera-Campo Suppose now that the result is true for every positive integer less than k. Let U be a σ-unicycle of G such that (σ, U) has the property k with respect to C. Then there exists a set of cycles X = {σ 0,..., σ k } C and a set of edges Y = {e 0,..., e k 1 } of G that satisfy (P 1 ), (P 2 ) and (P 3 ). Let T and T be spanning trees of U and a, b E(U) be such that T = (T a) + b. Notice that σ 0 σ k = σ because every edge of σ is in exactly one cycle in X and every edge of a cycle in X that is not an edge of σ is an edge of exactly two cycles in X. Thus, there must exist at least one cycle ρ X such that E(ρ) E(σ). It follows that at least one edge e E(ρ) Y is such that U + e contains two cycles δ and γ such that δ γ = σ. Without loss of generality we assume e = e k 1. Case 1. a E(δ)\E(γ) and b E(γ)\E(δ). Subcase 1.1. δ X. We can rename the cycles in X in such a way that δ = σ k. Let U = (U a) + e k 1, clearly U is a γ-unicycle of G. We shall prove that (γ, U ) has property k 1. Let X = {σ 0,..., σ k 1 } and Y = {e 0,..., e k 2 }. Let x E(ρ), for ρ X, then x E(U) Y = (E(U ) Y ) {a}. Notice that a E(σ k ) E(σ), so the unique cycle in X that contains a is σ k because σ, U, X and Y satisfy (P 3 ). It follows that x a, so x E(U ) Y. Then, every cycle in X is a cycle of U + Y. The dimension of Γ(U + Y ) is k, and X is a linear independent set consisting of k cycles of U + Y, then X is a basis of Γ(U +Y ). Therefore γ, U, X and Y satisfy property (P 1 ). Let ρ X and x E(ρ)\E(γ). If x E(σ), then x E(σ)\E(γ). Since σ = γ σ k and σ, U, X and Y satisfy (P 3 ), the unique cycle in X that contains x is σ k which is impossible because ρ σ k and ρ X. It follows that x E(σ) and x E(σ k ) because σ = σ k γ. Since σ, U, X and Y satisfy (P 2 ), there exists a unique ρ X such that x E(ρ ). Clearly ρ σ k, so ρ X. Therefore γ, U, X and Y satisfy (P 2 ). Let x σ. If x E(γ), there exists a unique cycle ρ X such that x E(ρ) because σ, U, X and Y satisfy (P 3 ). Since σ = δ γ, ρ σ k. It follows that ρ X. If x E(γ)\E(σ), x E(σ k ). We know that σ, U, X and Y satisfy (P 2 ) implying there exists a unique cycle ρ X\{σ k } = X containing x. Therefore γ, U, X and Y satisfy (P 3 ). Since (P 1 ), (P 2 ) and (P 3 ) are satisfied, (γ, U ) has property k 1 and the inductive hypothesis applies. Let Q = (T a)+e k 1, since Q and T are spanning trees of U, by induction Q and T are joined by a path in T (G, C).

5 On the Tree Graph of a Connected Graph 505 On the other hand, T is adjacent to Q in T (G, C) because δ is the unique cycle of T Q. So, T and T are also joined by a path in T (G, C). Subcase 1.2. γ X. This case can be proved in the same way interchanging γ with δ and a with b. Subcase 1.3. δ, γ X. Since X is a basis of Γ(U + Y ), by reordering X we can also assume that there exists an integer 1 r k 2 such that σ 0 σ r = δ. Since δ γ = σ = σ 0 σ k, it follows that γ = σ r+1 σ k. Let A = {σ 0,..., σ r } and B = {σ r+1,..., σ k }. If an edge e i Y \{e k 1 } is such that e i E(ρ) for some cycle ρ A and e i E(ρ ) for some ρ B, then e i E(δ) E(γ). Since E(δ) E(γ) E(U + e k 1 ), e i = e k 1, which is impossible. Thus, A and B induce a partition of the edges in Y \{e k 1 } and this partition is not trivial because every ρ X contains at least one edge in Y \{e k 1 }. Then, we can reorder the edges of Y \{e k 1 } in such a way that there exists an integer 0 t k 2, such that A = {e 0,..., e t 1 } is the set of edges in Y \{e k 1 } contained in elements of A and B = {e t,..., e k 2 } is the set of edges in Y \{e k 1 } contained in elements of B. Let U 1 = (U b) + e k 1 and U 2 = (U a) + e k 1. Notice that U 1 is a δ-unicycle and U 2 is a γ-unicycle. We will show that (δ, U 1 ) and (γ, U 2 ) have properties r and k r+1, respectively. It is not difficult to see that no cycle of A contains b and that no cycle of B contains a. That means that every cycle in A is contained in U 1 + A and every cycle in B is contained in U 2 + B. It follows that A is a linear independent set of Γ(U 1 +A ) and B is a linear independent set of Γ(U 2 +B ), so A A +1 and B B +1. That is, r+1 t+1 and (k+1) (r+1) ((k 1) t) + 1. Thus, t r t; this implies r = t. Since the dimension of Γ(U 1 + A ) is t + 1 and A is a linear independent set of Γ(U 1 + A ) with t + 1 cycles, A is a basis of Γ(U 1 + A ). Therefore δ, U 1, A and A satisfy (P 1 ). Analogously γ, U 2, B and B satisfy (P 1 ). Let ρ A and x E(ρ)\E(δ). If x E(σ) ((E(ρ)\E(δ)), there exists a unique cycle in X that contains x, because σ, U, X and Y satisfy (P 3 ). Then, ρ is the unique cycle of X that contains x and therefore also the unique cycle of A that contains x. Thus, x is an edge of ρ A ρ = δ which is impossible. It follows that x E(σ). Since σ, U, X and Y satisfy (P 2 ), there exists a unique ρ X such that ρ ρ and x E(ρ ). It is clear that if ρ B, then x E(γ) E(δ) which is impossible. Thus, ρ A.

6 506 A.P. Figueroa and E. Rivera-Campo Therefore δ, U 1, A and A satisfy (P 2 ). Analogously γ, U 2, B and B satisfy (P 2 ). Let x E(δ). If x E(σ), then there exists a unique cycle ρ X such that x E(ρ) because σ, U, X and Y satisfy (P 3 ). Since x E(δ), ρ A. If x E(σ), then x E(γ) E(δ). Since δ = ρ A ρ, x E(ρ) for some ρ A and since γ = ρ B ρ, x E(ρ ) for some ρ B. There are at most two cycles in X containing x because (σ, U) has property k with respect to C. Thus, if x E(δ), there exists a unique cycle ρ A such that x E(ρ). Therefore δ, U 1, A and A satisfy (P 3 ). Analogously γ, U 2, B and B satisfy (P 3 ). We can now apply the inductive hypothesis to (δ, U 1 ) and to (γ, U 2 ). Let R = (T a) + e k 1 = (T b) + e k 1. Since R is a spanning tree of both U 1 and U 2, there exists a path in T (G, C) from T to R and a path from R to T. Therefore T and T can also be joined by a path in T (G, C). Case 2. a E(γ)\E(δ) and b E(δ)\E(γ). This case can be proved as Case 1 by interchanging a with b. Case 3. a, b E(δ)\E(γ). Consider an edge c E(γ)\E(δ) and let Q = (T a) + c = (T b) + c. Notice that Q is a spanning tree of U. Applying Case 1 to T and Q and to Q and T, we have that T and Q are joined by a path in T (G, C) and also there exists a path between Q and T in T (G, C). It follows that there exists a path from T to T in T (G, C). Case 4. a, b E(γ)\E(δ). The proof of this case is analogous to that of Case 3 by interchanging δ with γ. Theorem 1. Let C be a set of cycles of a connected graph G. The graph T (G, C) is connected if and only if T (G, cl G + (C)) is connected. P roof. If T (G, C) is connected then T (G, cl + G (C)) is connected because C cl + G (C). Let σ 1, σ 2,..., σ n be the sequence of cycles added to C while obtaining cl + G (C) and let C 0 = C and C i = C {σ 1, σ 2,..., σ i } for 1 i n. Let 1 i n. Since σ i has property + with respect to C i 1, for every σ i -unicycle U of G there exists a positive integer k such that (σ i, U) has property k with respect to C i 1.

7 On the Tree Graph of a Connected Graph 507 Suppose that T (G, C i ) is connected and let T and T be two spanning trees of G adjacent in T (G, C i ). If T and T are not adjacent in T (G, C i 1 ), then T T is a σ i -unicycle. By Lemma 1, there exists a T T -path in T (G, C i 1 ). Therefore if T (G, C i ) is connected, then T (G, C i 1 ) is connected. Clearly this implies that if T (G, cl + G (C)) is connected, then T (G, C) is connected. Corollary 1. If C is + -dense, then T (G, C) is connected. P roof. If C is + -dense, then T (G, cl + G (C)) = T (G) which is always connected. By Theorem 1, T (G, C) is connected. 2. Main Result Let G be a connected graph and C be a family of cycles of G such that T (G, C) is connected. For any spanning trees T and T of G, let d C (T, T ) denote the distance between T and T in T (G, C). For any edge x of G, we denote by A x the set of spanning trees of G containing x. For a spanning tree T of G such that x E(T ), the distance d C (T, A x ) from T to A x is the minimum distance d C (T, R) with R A x. Lemma 2. Let C be a family of cycles of a connected graph G such that every edge of G is in at most two cycles of C. Let U be a σ-unicycle of G for some cycle σ C. If T (G, C) is connected, then (σ, U) has property k for some positive integer k. P roof. Let T be a spanning tree of U, and let x be the unique edge of U not in T. Since T (G, C) is connected, there exists a spanning tree T x such that the path between T and T x has length d = d C (T, A x ). Since σ C, d 2. We shall prove by induction on d that (σ, U) has property k for some integer k. Suppose d = 2 and let S A x be such that {T, S, T x } is a path in T (G, C). Since T is adjacent to S in T (G, C), there exist a E(T )\E(S), e E(S)\E(T ) and δ C such that S = (T a)+e and S T is a δ-unicycle of G. On the other hand, since S is adjacent to T x in T (G, C), there exist b E(S)\E(T x ) and γ C such that T x = (S b) + x and T x S is a γ-unicycle of G. Notice that x E(γ)\E(δ) since S A x. Thus δ γ and U +e contains three different cycles: δ, γ and σ. Therefore σ = δ γ. We conclude that (σ, U) has property 1.

8 508 A.P. Figueroa and E. Rivera-Campo We proceed by induction assuming d 3 and that (γ, V ) has property s for some positive integer s for each cycle γ C and each γ-unicycle V of G whenever there exists a spanning tree R of V such that R + z = V for some edge z E(V )\E(R) and d C (R, A z ) < d. Let {T = T 0, T 1,..., T n = T d } be a T T x -path of length d in T (G, C). Since T and T 1 are adjacent in T (G, C), there exists a E(T )\E(T 1 ), b E(T 1 )\E(T ) and a cycle δ C such that T 1 = (T a) + b and T T 1 is a δ-unicycle of G. Let γ be the unique cycle in V = T 1 + x. Since x E(T 1 ), d C (T 1, A x ) = d 1. By the inductive hypothesis, (γ, V ) has property s for some integer s. Therefore there exists a set of cycles X 1 = {γ 0,..., γ s } C and a set of edges Y 1 = {e 0,..., e s 1 } E(G)\E(V ) such that γ, V, X 1 and Y 1 satisfy (P 1 ), (P 2 ) and (P 3 ). Case 1. γ = σ. If b / E(ρ) for any ρ X 1 then E(ρ) (E(V )\{b}) E(Y 1 ) = E(U) E(Y 1 ). So (σ, U) has the property s because σ, U, X 1 and Y 1 satisfy (P 1 ), (P 2 ) and (P 3 ). Then, we can assume b E(ρ) for some ρ X 1. Clearly b E(σ) because b E(U) and U contains σ. Since γ, V, X 1 and Y 1 satisfy (P 2 ) there exists ρ X 1 such that b E(ρ ). Since b is also an edge of δ and b is in at most two cycles of C, without loss of generality we can assume ρ = δ. Since a E(δ)\E(V ), a = e t for some 0 t s 1. Then, it is not difficult to prove that σ, U, X and Y satisfy (P 1 ), (P 2 ) and (P 3 ), where X = X 1 and Y = (Y 1 \{a}) {b}. This also implies that (σ, U) has property s. Case 2. ρ σ. In this case U + b contains exactly three cycles σ, δ, γ. Since b E(σ), b E(δ) E(γ) and δ γ = σ. Subcase 2.1. a E(ρ) for any ρ X. Let X = X 1 {δ} and Y = Y 1 {a}. Since δ is the unique cycle in X containing a, then X is a linear independent set of Γ(G). Note that every cycle in X is a cycle of U + Y and Γ(U + Y ) has dimension s + 1, then X is a basis of Γ(U + Y ). Let x E(ρ)\E(σ) for some ρ X. If x E(γ), then there exists ρ X 1 such that x E(ρ ) because γ, V, X 1 and Y 1 satisfy (P 2 ). If x E(γ) then x E(δ) because σ = δ γ. It follows that if x E(ρ), then x lies in at least two cycles of X. Since every edge is in at most two cycles

9 On the Tree Graph of a Connected Graph 509 of C, x belongs to exactly two cycles of X. In other words, if x E(ρ) for some ρ X, then there exists a unique cycle ρ X such that x E(ρ ). Let x E(σ). If x E(δ), then x E(γ) because σ = δ γ. Since γ, V, X 1, and Y 1 satisfy (P 3 ), there exists a unique cycle ρ X 1 such that x E(ρ). Thus there exists a unique cycle ρ X such that x E(ρ) because x E(δ). Now suppose that x E(δ), then x E(γ) because σ = σ γ. If x E(ρ) for some ρ X 1, then there exists ρ X 1 such that x E(ρ ) because γ, V, X 1 and Y 1 satisfy (P 2 ). Then x is an edge of δ, ρ and ρ which is impossible because x can not be an edge of three cycles of C. Thus, the unique cycle in X containing x is δ. Therefore σ, U, X and Y satisfy (P 1 ), (P 2 ) and (P 3 ). This implies that (σ, U) has property s+1. Subcase 2.2. a E(ρ) for some ρ X. In this case we can suppose w.l.o.g that a = e s 1. By property s of (γ, V ), the edge a is in exactly two cycles of X 1. Since a E(δ) and a is in at most two cycles of C, then we can assume δ = γ s. Let X = X 1 \{γ s } and Y = Y 1 \{e s 1 }. Since b E(δ)\E(σ) and σ = δ γ, then b E(δ) E(γ). Thus, there exists a unique cycle ρ X 1 containing b because γ, V, X 1 and Y 1 satisfy (P 3 ). Therefore, δ = ρ. It follows that E(ρ) E(V + Y 1 ) = (E(U) E(Y ))\{b} for every ρ X. Then, it is clear that X is a basis of Γ(U + Y ). Let x E(ρ)\E(σ) for some ρ X. Since every edge in E(γ)\E(σ) is an edge of δ, it follows that E(γ) E(ρ) = because γ, V, X 1 and Y 1 satisfy (P 3 ). Thus, x E(γ). It follows that there exists a unique ρ X 1 such that x E(ρ ) because V, X 1 and Y 1 satisfy (P 2 ). If ρ = δ, then x E(δ)\(E(σ) E(γ)) which is impossible because σ = δ γ. Therefore ρ X. Let x E(σ). If x E(σ) E(γ), there exists a unique ρ X 1 such that x E(ρ) because γ, V, X 1 and Y 1 satisfy (P 3 ). Clearly ρ δ because E(σ) E(γ) E(δ) =. If x E(σ)\E(γ) = E(σ) E(δ). Since δ X 1 and γ, V, X 1 and Y 1 satisfy (P 2 ), there exists a unique ρ X 1, ρ δ such that x E(ρ ). Thus, we have proved that there exists a unique ρ X such that x E(ρ). Therefore σ, U, X and Y satisfy (P 1 ), (P 2 ) and (P 3 ), which, in this case, implies that (σ, U) has property s 1.

10 510 A.P. Figueroa and E. Rivera-Campo Theorem 2. Let C be a family of cycles of G such that every edge of G lies in at most two cycles of C. The graph T (G, C) is connected if and only if every cycle σ C has property + with respect to C. P roof. If every cycle σ C has property + with respect to C, then C is + -dense. By Corollary 1, T (G, C) is connected. Conversely, let σ be a cycle of G not in C. By Lemma 2, if T (G, C) is connected, then for each σ-unicycle of G, (σ, U) has property k for some integer k. This implies that σ has property + with respect to C. References [1] H.J. Broersma and X. Li, The connectivity of the of the leaf-exchange spanning tree graph of a graph, Ars. Combin. 43 (1996) [2] F. Harary, R.J. Mokken and M. Plantholt, Interpolation theorem for diameters of spanning trees, IEEE Trans. Circuits and Systems 30 (1983) [3] K. Heinrich and G. Liu, A lower bound on the number of spanning trees with k endvertices, J. Graph Theory 12 (1988) [4] X. Li, V. Neumann-Lara and E. Rivera-Campo, On a tree graph defined by a set of cycles, Discrete Math. 271 (2003) [5] F.J. Zhang and Z. Chen, Connectivity of (adjacency) tree graphs, J. Xinjiang Univ. Natur. Sci. 3 (1986) 1 5. Received 12 February 2008 Revised 18 June 2008 Accepted 18 June 2008

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