# Group connectivity of graphs with diameter at most 2

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1 European Journal of Combinatorics 27 (2006) Group connectivity of graphs with diameter at most 2 Hong-Jian Lai a, Xiangjuan Yao b a Department of Mathematics, West Virginia University, Morgantown, WV , United States b College of Sciences, China University of Mining And Technology, Jiangsu, Xuzhou , PR China Received 29 January 2004; accepted 16 November 2004 Available online 25 March 2005 Abstract Let G be an undirected graph, A be an (additive) abelian group and A = A {0}. AgraphG is A-connected if G has an orientation D(G) such that for every function b : V (G) A satisfying v V(G) b(v) = 0, there is a function f : E(G) A such that at each vertex v V (G), the amount of f values on the edges directed out from v minus the amount of f values on the edges directed into v equals b(v). In this paper, we investigate, for a 2-edge-connected graph G with diameter at most 2, the group connectivity number Λ g (G) = min{n : G is A-connected for every abelian group A with A n}, and show that any such graph G satisfies Λ g (G) 6. Furthermore, we show that if G is such a 2-edge-connected diameter 2 graph, then Λ g (G) = 6 if and only if G is the 5-cycle; and when G is not the 5-cycle, then Λ g (G) = 5 if and only if G is the Petersen graph or G belongs to two infinite families of well characterized graphs Elsevier Ltd. All rights reserved. 1. Introduction Graphs in this paper are finite and may have loops and multiple edges. Undefined terms and notation are from [1]. Throughout the paper, Z n denotes the cyclic group of order n, for some integer n 2. address: (H.-J. Lai) /\$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi: /j.ejc

2 H.-J. Lai, X. Yao / European Journal of Combinatorics 27 (2006) Let D = D(G) be an orientation of an undirected graph G. If an edge e E(G) is directed from a vertex u to a vertex v, then let tail(e) = u and head(e) = v. Foravertex v V (G),let E + D (v) ={e E(D) : v = tail(e)}, and E D (v) ={e E(D) : v = head(e)}. Let A denote an (additive) abelian group with identity 0, and let F(G, A) denote the set of all functions from E(G) to A, andf (G, A) the set of all functions from E(G) to A {0}. Given a function f F(G, A), let f : V (G) A be given by f (v) = f (e) f (e), e E + D (v) e E D (v) where refers to the addition in A. A function b : V (G) A is called an A-valued zero sum function on G if v V (G) b(v) = 0. The set of all A-valued zero sum functions on G is denoted by Z(G, A). Givenb Z(G, A), ifg has an orientation D and a function f F (G, A) such that f = b, then f is an (A, b)-nowhere-zero flow (abbreviated as an (A, b)-nzf) under the orientation. A graph G is A-connected if for any b, G has an (A, b)-nzf. For an abelian group A,let A be the family of graphs that are A-connected. It is observed in [5] that the property G A is independent of the orientation of G, and that every graph in A is 2-edge-connected. For convenience, we also use 0 to denote the zero value constant function in F(G, A). Then an A-nowhere-zero flow (abbreviated as an A-NZF) in G is an (A, 0)-NZF. For a positive integer k, whena = Z, the additive group of all the integers, a function f F (G, Z) is a nowhere-zero k-flow (abbreviated as a k-nzf) of G if f (e) < k, e E(G). Tutte ([14], also [4]) showed that a graph G has an A-NZF if and only if G has an A -NZF. The concept of A-connectivity was introduced by Jaeger et al. in [5], where A-NZF s were successfully generalized to A-connectivity. A similar concept to the group connectivitywas independentlyintroducedin [7], with a different motivation from [5]. For more on the literature of the nowhere-zero-flow problems, see [14,4] or[15]. For a 2-edge-connected graph G,definetheflow number of G as Λ(G) = min{k : G has a k-nzf} and the group connectivity number of G as Λ g (G) = min{k : if A is an abelian group with A k, then G A }. Tutte conjectured (see [4]) that if a 2-edge-connected graph G does not have a subgraph contractible to P 10, the Petersen graph, then Λ(G) 4. This conjecture has been proved for cubic graphs [12], and the general case remains open. The main purpose of this note is to show that for 2-edge-connected graphs with diameter at most 2, the group connectivity number is at most 6 with the 5-cycle C 5 being the only such graph with Λ g (C 5 ) = 6. Moreover, we will also show that if G is a 2-edge-connected graph with diameter at most 2 such that G C 5,thenΛ g (G) 5. All such graphs whose group connectivity number is exactly 5 will be characterized.

3 438 H.-J. Lai, X. Yao / European Journal of Combinatorics 27 (2006) Preliminaries In this section, we present some of the known results which will be utilized in our proofs. For a subset X E(G), the contraction G/ X is the graph obtained from G by identifying the two ends of each edge e in X and deleting resulting loops. Note that even G is a simple graph, the contraction G/ X may have multiple edges. For convenience, we define G/ =G, and write G/e for G/{e},wheree E(G).IfH is a subgraph of G,then we write G/H for G/E(H ). Proposition 2.1 (Proposition 3.2 of [9]). Let A be an abelian group with A 3. Then A satisfies each of the following: (C1) K 1 A, (C2) if G A and e E(G),thenG/e A, (C3) if H is a subgraph of G and if both H A and G/H A, theng A. Lemma 2.2 ([5] and [9]). Let C n denote a cycle of n vertices. Then C n A if and only if A n + 1. (Equivalently, Λ g (C n ) = n + 1.) Lemma 2.3 ([5]). Let G be a connected graph with n vertices and m edges. Then Λ g (G) = 2 if and only if n = 1 (and so G has m loops with a single vertex). Let O(G) ={odd degree vertices of G}. AgraphG is collapsible if for any subset R V (G) with R 0(mod2),G has a spanning connected subgraph Γ R such that O(Γ R ) = R. Asin[1], κ (G) denotes the edge-connectivity of a graph G. Theorem 2.4 ([2]). Suppose that G is one edge short of having two edge-disjoint spanning trees. Then G is collapsible if and only if κ (G) 2. Lemma 2.5 ([8]). Let G be a collapsible graph and let A be an abelian group with A =4.ThenG A. Lemma 2.6 ([10]). Let A be an abelian group with A 3, and let G be a connected graph. If for each e E(G), G has a subgraph H e A, theng A. Theorem 2.7 ([3]). Let m > n > 0 be integers. If A is an abelian group with A =4, then K 2,n A. Furthermore, 5 if n = 2 Λ g (K m,n ) = 4 if n = 3 3 if n Graphs with diameter at most 2 Let G be a connected graph. For any pair of vertices u,v V (G),letd(u,v)denote the length of a shortest (u,v)-path in G. Then the diameter of G equals max{d(u,v) : u,v V (G)}.

4 H.-J. Lai, X. Yao / European Journal of Combinatorics 27 (2006) Fig. 1. K 2,m and S m,n. In 1989, it was shown [6] thatifg is a 2-edge-connected graph with diameter at most 2, then Λ(G) 5, where equality holds if and only if G = P 10. In this section, we shall consider what would happen if we replaced Λ by Λ g in this result. Lemma 2.2 indicates that Λ g (C 5 ) = 6andTheorem 2.7 suggests that P 10 is not the only diameter 2 graph with group connectivity number equaling 5. Let m, n be two positive integers and let H 1 = K2,m and H 2 = K2,n.Letu 1, u 2 denote the two nonadjacent vertices of degree m in H 1 ; v 1,v 2 denote the two nonadjacent vertices of degree n in H 2.ThenS m,n is the graph obtained from the disjoint union of H 1 and H 2 by identifying u 1 with v 1 and by adding a new edge e joining u 2 to v 2 (see Fig. 1). Note that S 1,1 = C5, the 5-cycle. The main results of the paper are the following. Theorem 3.1. If G is a 2-edge-connected loopless graph with diameter at most 2, then Λ g (G) 6,whereΛ g (G) = 6 if and only if G = C 5, the 5-cycle. Theorem 3.2. If G is a 2-edge-connected loopless graph with diameter at most 2 such that G C 5,thenΛ g (G) 5. Moreover, when G is one such graph, the following are equivalent: (i) Λ g (G) = 5. (ii) G = P 10, the Petersen graph, or for some integers m, n with m > n > 0, G = S m,n, or G has a collapsible subgraph H with V (H ) 2 such that G/H = K 2,m for some integer m > 1. A few lemmas and former results are needed for the proofs of Theorems 3.1 and 3.2. Recall that a graph G is collapsible if for any subset R V (G) with R 0(mod 2),G has a spanning connected subgraph Γ R such that O(Γ R ) = R. For a graph G,letG denote the graph obtained from G by successively contracting all maximal collapsible subgraphs of G. ThenG is called the reduction of G. Catlin [2] showed that the reduction of any graph exists and is unique. Lemma 3.3 (Theorem 3 of [6]). Let G be a 2-edge-connected graph with diameter at most 2, and let G denote the reduction of G. Then exactly one of the following must hold:

5 440 H.-J. Lai, X. Yao / European Journal of Combinatorics 27 (2006) (i) G is collapsible, (ii) G = P 10, (iii) G = C 5, (iv) G = K 2,m for some integer m 2, (v) G = S m,n for some integer m, n with m > n > 0. Theorem 3.4 ([5]). If G has two edge-disjoint spanning trees, then for any abelian group A with A 4, G A. Lemma 3.5. Let G be a loopless graph. If one of the following holds: (i) G = P 10,or (ii) G has a collapsible subgraph H with V (H ) 2 such that G/H = K 2,m for some m 2,or (iii) G = S m,n for some m, n with m > n > 0, then Λ g (G) = 5. Proof. We first show that if (ii) holds, then Λ g (G) = 5. By Theorem 2.7, Λ g (K 2,m ) = 5. Suppose that G has a subgraph H with V (H ) 2 such that G/H = K 2,m.Then Λ g (G) Λ g (G/H ) = Λ g (K 2,m ) = 5. If V (H ) =1, then Λ g (G) = Λ g (K 2,m ) = 5. Therefore, we assume that V (H ) =2. Since H is collapsible and since V (H ) =2, H must have two edge-disjoint spanning trees. Let A be an abelian group A with A 5. By Theorem 3.4, H A. Since G/H = K 2,m A, by(c3)ofproposition 2.1, G A, andsoλ g (G) = 5. Next, we do the same if (iii) holds. Suppose that some S m,n A. S m,n can be contracted to a K 2,m+n, and so by (C2) of Proposition 2.1, K 2,m+n A as well. Thus Λ g (S m,n ) Λ g (K 2,m+n ) = 5. Since m > n > 0, S m,n must contain a subgraph H = K 2,m with m 2. By Theorem 2.7, Λ g (H ) = 5. Every edge in S m,n /H lies in a cycle of length at most 4, and so by Lemmas 2.2 and 2.6, Λ g (S m,n /H ) 5. Since Λ g (H ) = 5 and since Λ g (S m,n /H ) 5, it follows by (C3) of Proposition 2.1 that Λ g (S m,n ) 5, and so we conclude that Λ g (S m,n ) = 5. It remains to show that Λ g (P 10 ) = 5. It is well known that Λ(P 10 ) = 5,andsowehave Λ g (P 10 ) Λ(P 10 ) = 5. To see that Λ g (P 10 ) = 5, it suffices to show that for any abelian group A with A 5, P 10 A. Since every edge of P 10 lies in a 5-cycle, by Lemmas 2.2 and 2.6,if A 6, then P 10 A. It remains to show that P 10 Z 5. First, we shall show that if e, e are incident with the same vertex v V (P 10 ),then H = (P 10 e)/{e } Z 5. This is because that H contains a 4-cycle L such that in H/L, every edge lies in a cycle of length at most 4. Therefore, by Lemmas 2.2 and 2.6, H/L Z 5.ByLemma 2.2, L Z 5, and so by (C3) of Proposition 2.1, H Z 5. Now we can prove that P 10 Z 5.Letb Z(P 10, Z 5 ).Ifb 0, then since Λ(P 10 ) = 5, P 10 has a (Z 5, 0)-NZF. Therefore, we assume that for some vertex v V (P 10 ), b(v) 0 Z 5.Lete be an edge joining v to another vertex u in P 10, and let e, e, e be the three edges incident with v in P 10. Then define H = (P 10 e)/{e }. By the definition of contraction, we may assume that V (H ) = V (P 10 ) {v}.

6 H.-J. Lai, X. Yao / European Journal of Combinatorics 27 (2006) Define b : V (H ) Z 5 by { b b(u) + b(v) if z = u (z) = b(z) if z u. Then b Z(H, Z 5 ).SinceH Z 5, under a certain orientation of H, there exists f F (H, Z 5 ) such that f = b. By the definition of a contraction, we can view E(H ) = E(P 10 ) {e, e }, and so this orientation of H can be viewed as an orientation of the edges of P 10 {e, e } in P 10. Without loss of generality, we may assume that in this orientation of P 10, e is directed into v. We extend this orientation to an orientation of P 10 by orienting both e and e from v. Extend f to a function f F (P 10, Z 5 ) as follows: z E(P 10 ), f (z) if z E(P 10 ) {e, e } f (z) = f (e ) if z = e b(v) if z = e. Then f F (P 10, Z 5 ) such that f = b. Therefore, P 10 Z 5, as desired. Lemma 3.6. Let A be an abelian group with A =4, and let G be a 2-edge-connected graph of diameter at most 2. Then G A if and only if either G = C 5 or (ii) of Theorem 3.2 holds. Proof. Suppose first that G A. Thenby(C2)ofProposition 2.1 and by Lemma 3.5, G cannot be contractible to a K 2,m or to P 10. In particular, G S m,n. Thus we cannot have G = C 5 and neither can (ii) of Theorem 3.2 hold. We apply Lemma 3.3 to show the converse. Assume that G A. ByLemma 2.5, (i) of Lemma 3.3 does not hold. As (ii) of Lemma 3.3 implies (ii) of Theorem 3.2, wemay assume that (i) and (ii) of Lemma 3.3 do not hold either. Hence one of (iii), (iv) or (v) of Lemma 3.3 must hold. If G, the reduction of G, is isomorphic to an S m,n for some positive integers m and n (including S 1,1 = C 5 ), and if G G,thenG has some maximal nontrivial subgraphs H 1, H 2,...,H k such that G/(H 1 H 2 H k ) = S m,n. For each i,sinceh i is nontrivial, H i must have at least two vertices and so the diameter of G would exceed 2, contrary to the assumption that the diameter of G is at most 2. Therefore, if (iii) or (v) of Lemma 3.3 holds, then G = S m,n for some positive integers m and n, and so (ii) of Theorem 3.2 must hold. Finally, we assume that G = K 2,m for some m 2. Then either G = K 2,m and we are done, or G has maximal nontrivial collapsible subgraphs H 1, H 2,...,H k such that G/(H 1 H 2 H k ) = K 2,m. Since the diameter of G is at most 2, we must have k = 1 with V (H 1 ) 2, and so (ii) of Theorem 3.2 holds. This completes the proof of Lemma 3.6. Theorem 3.7 ([11]). Let δ(g) denote the minimum degree of a graph G. If δ(g) 4, then G has a nontrivial subgraph which has two edge-disjoint spanning trees. Theorem 3.8 ([13]). Let d > 0 be an integer. If G has diameter d and if every cycle of G has length at least 2d + 1, then G must be a regular graph.

7 442 H.-J. Lai, X. Yao / European Journal of Combinatorics 27 (2006) Lemma 3.9. Let G be a 2-edge-connected graph with diameter at most 2. Suppose that G does not have a cycle of length at most 4. If G does not have a nontrivial subgraph which has two edge-disjoint spanning trees, then G is isomorphic to C 5 or to P 10. Proof. By Theorem 3.7, wehave2 δ(g) 3. By Theorem 3.8, G must be either 2-regular or 3-regular, and so, in these cases, G is isomorphic to either C 5 or to P 10 (see page 82 of [6] for a detailed proof). Proofof Theorem 3.2. Let A be an abelian group of order at least 4. Suppose first that A 5. We argue by induction on V (G) to show that G A. ByLemma 2.2, any 2-edge-connected graph with at most four vertices must be in A. Therefore, we assume that V (G) 5. If G has a nontrivial subgraph H which has two edge-disjoint spanning trees, or which is isomorphic to a cycle of length at most 4, then by Theorem 3.4 or Lemma 2.2, H A. As the diameter will not increase under contraction, G/H is also 2-edge-connected with diameter at most 2. By induction, G/H A andsoby(c3)of Proposition 2.1, G A. Therefore, we assume that G satisfies the hypotheses of Lemma 3.9, andsog {C 5, P 10 }. By the assumption of Theorem 3.2, G C 5, and so we must have G = P 10.By Lemma 3.5, G A. This proves that Λ g (G) 5. Suppose that Λ g (G) = 5. If G is collapsible, then by Lemma 2.5, we would have Λ g (G) 4, contrary to the assumption that Λ g (G) = 5. Therefore, G cannot be collapsible. Since G cannot be C 5, (ii), (iv) or (v) of Lemma 3.3 must hold and so (i) of Theorem 3.2 implies (ii) of Theorem 3.2. Conversely,Lemma 3.5 shows that (ii) of Theorem 3.2 implies (i) of Theorem 3.2. Proofof Theorem 3.1. Since G is 2-edge-connected with diameter at most 2, every edge lies in a cycle of length at most 5, and so by Lemmas 2.2 and 2.6, Λ g (G) 6. If G C 5, then by Theorem 3.2, Λ g (G) 5. On the other hand, Lemma 2.2 shows that Λ g (C 5 ) = 6. This completes the proof. Acknowledgements The authors would like to thank the referees for their helpful suggestions that improved the presentation of this paper. References [1] J.A. Bondy, U.S.R. Murty, Graph Theory with Applications, American Elsevier, New York, [2] P.A. Catlin, A reduction method to find spanning eulerian subgraphs, J. Graph Theory 12 (1988) [3] J.J. Chen, Generalized nowhere zero flows, Masters Thesis, West Virginia University, [4] F. Jaeger, Nowhere zero flow problems, in: L. Beineke, R. Wilson (Eds.), Selected Topics in Graph Theory, vol. 3, Academic Press, London, New York, 1988, pp [5] F. Jaeger, N. Linial, C. Payan, N. Tarsi, Group connectivity of graphs a nonhomogeneous analogue of nowhere zero flow properties, J. Combin. Theory Ser. B 56 (1992) [6] H.-J. Lai, Reduced graphs of diameter Two, J. Graph Theory 14 (1990) [7] H.-J. Lai, Reduction towards collapsibility, in: Y. Alavi et al. (Eds.), Graph Theory, Combinatorics, and Applications, John Wiley and Sons, 1995, pp

8 H.-J. Lai, X. Yao / European Journal of Combinatorics 27 (2006) [8] H.-J. Lai, Extending a partial nowhere-zero 4-flow, J. Graph Theory 30 (1999) [9] H.-J. Lai, Group connectivity of 3-edge-connected chordal graphs, Graphs Combin. 16 (2000) [10] H.-J. Lai, Nowhere-zero 3-flows in locally connected graphs, J. Graph Theory 42 (2003) [11] C.St.J.A. Nash-Williams, Decompositions of finite graphs into forests, J. London Math. Soc. 39 (1961) 12. [12] N. Robertson, D. Sanders, P.D. Seymour, R. Thomas, in press. [13] R.P. Singleton, There are no regular Moore graphs, Amer. Math. Math. 75 (1968) [14] W.T. Tutte, A contribution to the theory of chromatic polynomials, Canad. J. Math. 6 (1954) [15] C.Q. Zhang, Integer Flows and Cycle Covers of Graphs, Marcel Dekker, Inc, 1997.

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