# Unicyclic Graphs with Given Number of Cut Vertices and the Maximal Merrifield - Simmons Index

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1 Filomat 28:3 (2014), DOI /FIL H Published by Faculty of Sciences Mathematics, University of Niš, Serbia Available at: Unicyclic Graphs with Given Number of Cut Vertices the Maximal Merrifield - Simmons Index Hongbo Hua a,b, Xinli Xu b, Hongzhuan Wang b a Department of Mathematics, Nanjing University, Nanjing , P. R. China b Faculty of Mathematics Physics, Huaiyin Institute of Technology, Huai an, Jiangsu , P. R. China Abstract. The Merrifield-Simmons index of a graph G, denoted by i(g), is defined to be the total number of independent sets in G, including the empty set. A connected graph is called a unicyclic graph, if it possesses equal number of vertices edges. In this paper, we characterize the maximal unicyclic graph w.r.t. i(g) within all unicyclic graphs with given order number of cut vertices. As a consequence, we determine the connected graph with at least one cycle, given number of cut vertices the maximal Merrifield-Simmons index. 1. Introduction In this paper only simple graphs without loops multiple edges are considered. For terminology notation not defined here, the reader is referred to Bondy Murty [2]. Given a graph G with vertex set V(G) edge set E(G). If S V(G) the subgraph induced by S has no edges, then S is said to be an independent set of G. Let i(g) denote the total number of independent sets, including the empty set, in G. Since, for the n-vertex path P n, i(p n ) is exactly equal to the Fibonacci number F n+1, some researchers also call i(g) the Fibonacci number of a graph G (see [1, 17]). Nowdays, i(g) is commonly termed as Merrifield- Simmons index, which originated from [15]. This index is one of the most popular topological indices in chemistry, which has been extensively studied, as can be seen in the monograph [13]. During the past several decades, a number of research results on the Fibonacci number or Merrifield-Simmons index of graphs have been obtained, among which characterization of graphs with extremal i(g) within a given class of graphs with special structure has been one of the most popular tendency. For instance, see [11] [17] for trees, [16] for trees with given number of pendent vertices, [10] for trees with a given diameter, [7] for trees with bounded degree, [14] for unicyclic graphs, [9] for the unicyclic graphs with a given diameter, [12] for the cacti, [8] for the quasi-tree graphs, [4] for connected graphs with given number of cut edges, [5] for connected graphs with given number of cut vertices, [6] for 3-connected 3-edge-connected graphs, [1] for maximal outerplanar graphs, so on Mathematics Subject Classification. Primary 05C35; Secondary 05C90 Keywords. independent set; Merrifield-Simmons index; unicyclic graph; cut vertices; upper bound; extremal graph Received: 16 October 2012; Accepted: 13 September 2013 Communicated by Dragan Stevanović Research supported in part by NSF of the Higher Education Institutions of Jiangsu Province (No. 12KJB110001), NNSF of China (No.s , ), SRF of HIT (No. HGA1010) Qing Lan Project of Jiangsu Province, PR China. address: (Hongbo Hua)

2 H. Hua et al. / Filomat 28:3 (2014), In the current paper, we characterize the maximal unicyclic graph w.r.t. i(g) within all unicyclic graphs with given order number of cut vertices. As a consequence, we determine the connected graph with at least one cycle, given number of cut vertices the maximal Merrifield-Simmons index. Before proceeding, we introduce some notation terminology. For a vertex v V(G), we use N G (v) to denote the set of neighbors of v, let N G [v] = N G (v) {v. For the sake of brevity, we write [v] instead of N G [v]. The degree of v in G, denoted by d(v), is the number of its neighbors. A vertex v is said to be a branched vertex, if d(v) 3. A vertex v is said to be a pendent vertex, if d(v) = 1. A cut vertex of a graph is any vertex that when removed increases the number of connected components of this graph. If S is a subset of V(G), we use G S to denote the subgraph of G obtained by deleting the vertices in S the edges incident with them. Suppose that P = v 1 v 2 v s (s 2) is a path lying within a graph G. If d(v 1 ) 3, d(v s ) = 1 d(v j ) = 2(1 < j < s), then we call P a pendant path of G. Denote, as usual, by S n C n the star cycle on n vertices, respectively. Let S l n denote the graph obtained from the cycle C l by attaching n l pendent vertices to any one vertex of it. Let P n, t be the tree obtained by attaching t 2 pendent edges to the second vertex (natural ordering) of the path P n t+2. Let U n, k denote the set of connected unicyclic graphs with n vertices k cut vertices, U l n, k denote a subset of U n, k, in which every graph has girth l. If k = 0, then U n, k contains a single element C n. So we will assume that k 1. Obviously, we have k n 3, since n l + k k + 3. For any graph G in U n, k, we let P(G) be the set of pendent vertices in G C(G) be the set of cut vertices in G. For any graph G in U l, we let OC(G) be the number of cut vertices in G lying outside of the unique n, k cycle C l. 2. Preliminary results The following lemmas are needed in the proof of main results. Lemma 2.1 ([3]). Let G be a graph with m components G 1, G 2,..., G m. Then i(g) = m i(g i ). i=1 Lemma 2.2 ([3]). Let G be a graph, u be a vertex vw be an edge of G. Then (i) i(g) = i(g u) + i(g [u]); (ii) i(g) = i(g vw) i(g {[v] [w]). Lemma 2.2 (ii) implies the following lemma. Lemma 2.3. Let G 1 G 2 be two graphs. If G 1 can be obtained from G 2 by deleting some edges, then i(g 2 ) < i(g 1 ). Recall that F n = F n 1 + F n 2 with initial conditions F 0 = F 1 = 1. Thus, ( ) n+2 ( ) n i(p n ) = F n+1 = Prodinger Tichy [13] gave an upper bound for the i(g) of trees, later Lin Lin [6] characterized the unique tree attaining this upper bound. Their results are summarized as follows: Lemma 2.4. Let T be a tree on n vertices. Then i(t) 2 n 1 + 1, with the equality if only if T S n. Yu Lv proved the following result concerning the i(g) of trees with k pendent vertices. Lemma 2.5 ([16]). Let T be a tree with n vertices k pendent vertices. Then (i) i(t) 2 k 1 F n k+1 + F n k, with equality if only if T P n, k ; (ii) i(p n, k ) > i(p n, k 1 ).

3 By means of Lemma 2.5, we obtain the following result. H. Hua et al. / Filomat 28:3 (2014), Lemma 2.6. Let T be a tree, not isomorphic to S n, with n vertices. Then i(t) 3 2 n 3 + 2, with equality if only if T P n, n 2. Proof. Let T be a tree, not isomorphic to S n, with n vertices. Then T has k n 2 pendent vertices. By Lemma 2.5, we have i(t) i(p n, k ) i(p n, n 2 ). This completes the proof. Lemma 2.7 ([12]). Let X, Y Z be three pairwise disjoint connected graphs with X, Y, Z 2. Suppose that u, v are two vertices of Z, v is a vertex of X, u is a vertex of Y. Let G be the graph obtained from X, Y Z by identifying v with v u with u, respectively. Also, we let G 1 be the graph obtained from X, Y Z by identifying vertices u, v, u G 2 be the graph obtained from X, Y Z by identifying vertices v, v, u ; see Fig. 1 for instance. Then i(g 1 ) > i(g) or i(g 2 ) > i(g). X v Z u Y v Z Y u X v X Y Z u G G 1 G 2 Fig. 1. The graphs G, G 1 G 2. We call the graph transformation from G to G 1 (or G 2 ) Operation I. From Lemma 2.7, we know that Operation I increases the i(g) of graphs under consideration. 3. Unicyclic graph with given number of cut vertices the maximal Merrifield-Simmons index Lemma 2.5 implies the following result. Proposition 3.1. Let T be a tree with n vertices k cut vertices. Then i(t) 2 n k 1 F k+1 + F k, with equality if only if T P n, n k. Proposition 3.2. Let T be a tree with n vertices at least k cut vertices. Then i(t) 2 n k 1 F k+1 +F k, with equality if only if T P n, n k. Proof. For each 1 k n 3, i(p n, n k ) i(p n, n k 1 ) = (2 n k 1 F k+1 + F k ) (2 n k 2 F k+2 + F k+1 ) So, for any 1 k < k n 2, = 2 n k 2 (2F k+1 F k+2 ) (F k+1 F k ) = 2 n k 2 F k 1 F k 1 > 0. i(p n, n k ) > i(p n, n k 1 ) >... > i(p n, n k ). (1) Suppose that T is a tree of n vertices k ( k) cut vertices. Then by Proposition 3.1 the above Ineq. (1), we have i(p n, n k ) i(p n, n k ) i(t), with equality if only if k = k T P n, n k, i.e., T P n, n k. This completes the proof.

4 H. Hua et al. / Filomat 28:3 (2014), Lemma 3.3. For any 3 l n 1, i(s l n) i(s 3 n), with equality if only if l = 3. Proof. Let u be a vertex of degree 2, adjacent to the vertex of degree n l + 2 in S l n. Write S l n u = T n 1. Then we have i(s l n) = i(t n 1 ) + i(p l 3 (n l)k 1 ) i(s 3 n) = i(s n 1 ) + i((n 3)K 1 ). By Lemma 2.4, i(t n 1 ) i(s n 1 ) with equality if only if T n 1 S n 1, i.e., l = 3. Note that P l 3 (n l)k 1 contains (n 3)K 1 as a proper spanning subgraph if l 4. By Lemma 2.3, i(p l 3 (n l)k 1 ) i((n 3)K 1 ) with equality if only if l = 3. Consequently, i(s l n) i(s 3 n), with equality if only if l = 3. This completes the proof. Let C 3 (n, k) be a unicyclic graph constructed as follows. For k = 1, we let Q 3 (n, 1) = S 3 n. For k 2, we let C 3 (n, k) be the graph obtained by attaching a path of length k 1 to a vertex of degree 2 in C 3 (n k + 1, 1). See Fig. 2 for instance. n k {{ k 1 Fig. 2. The graph C 3 (n, k). According to the above definition for C 3 (n, k), we have i(c 3 (n, k)) = { 3 2 n 3 + 1, k = 1; 2 n k 2 F k+2 + F k, k 2. Before proceeding, we prove the following two lemmas. Lemma 3.4. Let G be a unicyclic graph in U l n, k. If OC(G) = 0, then i(g) i(c 3(n, k)), with equality if only if G C 3 (n, k). Proof. If k = 1, then G S l n. Thus, by Lemma 3.3, we have i(g) i(s 3 n) = i(c 3 (n, 1)) with equality if only if l = 3, that is, G C 3 (n, 1). So, we may assume that k 2. If k = 2, then G is the graph G s, t (l), obtained by attaching s t pendent edges to any two vertices of the cycle C l, where s + t + l = n. If min{s, t 2, then by Lemma 2.6, we have i(g) = i(g s, t (l)) < i(g 1, s+t 1 (l)). Thus, i(g) = i(g s, t (l)) i(g 1, s+t 1 (l)). In view of Lemmas (i), i(c 3 (n, 2)) = i(s 3 n 1 ) + i(s n 2)

5 H. Hua et al. / Filomat 28:3 (2014), i(g 1, s+t 1 (l)) = i(s l n 1 ) + i(t n 2), where T n 2 is a tree of order n 2. Evidently, i(t n 2 ) i(s n 2 ), with equality if only if T n 2 S n 2, i(s l n 1 ) i(s3 ), with equality if n 1 only if l = 3. So i(g) i(g 1, s+t 1 (l)) i(c 3 (n, 2)) with equality if only if T n 2 S n 2, l = 3 G G 1, s+t 1 (l), that is, G C 3 (n, 2), as claimed. So, we may assume that k 3. Since OC(G) = 0, for k 3, we must have G C 3 (n, k). By Lemmas (i), i(c 3 (n, k)) = i(p n 1, n k 1 ) + 2 n k 2 i(p k 1 ) i(g) = i(g v 0 ) + i(g [v 0 ]). Notice that G v 0 is a tree of order n 1 at least k cut vertices; then, by Proposition 3.2, we have i(g v 0 ) i(p n 1, n k 1 ), with equality if only if G v 0 P n 1, n k 1. By our assumption that OC(G) = 0, we know that all k cut vertices of G lie on C l. If k = 2, then G [v 0 ] is an empty graph of n 3 isolated vertices. If k 3, G [v 0 ] is a forest composed of x (0 x n k 3) isolated vertices a nontrivial component of n x 3 vertices at least k 2 cut vertices. Also, the largest component of G [v 0 ] contains the path P k as a subgraph. Thus, G [v 0 ] contains (n k 2)K 1 P k 1 as a spanning subgraph. Then by Lemma 2.3, i(g [v 0 ]) i((n k 2)K 1 P k 1 ) = 2 n k 2 i(p k 1 ), with equality if only if k = 2. So, i(g) i(c 3 (n, k)), with equality if only if G C 3 (n, k) (k = 2). Lemma 3.5. Let G be a graph in U l. If OC(G) 1 there exists a pendent path of length 2 in G, then n, k i(g) i(c 3 (n, k)), with the equality if only if G C 3 (n, k). Proof. Obviously, V(G) \ V(C l ). For any given vertex x V(G) \ V(C l ), we let d G (x, C l ) = min{d G (x, y) y V(C l ). By the assumption that OC(G) 1, we have k 2. We shall complete the proof by induction on OC(G). We first check the validity of the lemma for OC(G) = 1. Let v be the unique cut vertex, not belonging to C l, in G. Since OC(G) = 1, we have d G (v, C l ) = 1. Also, d(v) = 2, for otherwise, G has no pendent path of length 2, a contradiction. Let u be the pendent vertex adjacent to v. Clearly, OC(G u) = OC(G [u]) = 0, C(G u) = k 1 C(G [u]) = k 2 or k 1. If k = 2, then C(G u) = 1 C(G [u]) = 0 or 1. Thus, G u S l n 1 G [u] C n 2 or S l n 2. By Lemma 2.7, i(s l n 1 ) i(s3 n 1 ). Also, i(c n 2) < i(p n 2 ) < i(s n 2 ) i(s l n 2 ) i(s3 n 2 ) < i(s n 2) by Lemmas 2.3, Thus, i(g) = i(g u) + i(g [u]) < i(s 3 n 1 ) + i(s n 2) = i(c 3 (n, 2)), as claimed. Assume now that k 3., Since d G (u, C l ) = 2, C(G u) = k 1 C(G [u]) = k 2 or k 1, we have G u U l n 1, k 1 G [u] Ul n 2, k 2 or Ul n 2, k 1.

6 H. Hua et al. / Filomat 28:3 (2014), Note that OC(G u) = OC(G [u]) = 0; then by Lemma 3.4, i(g u) i(c 3 (n 1, k 1)), with the equality if only if G u C 3 (n 1, k 1). Also, by Lemma 3.4, i(g [u]) i(c 3 (n 2, k 2)), with the equality if only if G [u] C 3 (n 2, k 2), or i(g [u]) i(c 3 (n 2, k 1)), with the equality if only if G [u] C 3 (n 2, k 1). We shall prove that for k 3, i(c 3 (n 2, k 1)) < i(c 3 (n 2, k 2)). (5) The above Ineq. (5) is equivalent to 2 n k 3 F k+1 + 2F k 2 < 2 n k 2 F k + 2F k 3. 2 n k 3 F k+1 2 n k 2 F k < 2F k 3 2F k 2. 2 n k 3 F k 2 > 2F k 4. The last inequality holds due to the fact that n k 3 0. By Ineqs. (2)-(5), for k 4, we obtain i(g) i(c 3 (n 1, k 1)) + i(c 3 (n 2, k 2)) = i(c 3 (n, k)) with the equality if only if G u C 3 (n 1, k 1) G [u] C 3 (n 2, k 2), i.e., G C 3 (n, k). If k = 3, then G C 3 (n, 3), as OC(G) = s 1. Since C 3 (n 2, 1) = S 3 n 2 contains P n 2, n 4 as a proper spanning subgraph, i(c 3 (n 2, 1)) < i(p n 2, n 4 ) by Lemma 2.3. By Ineqs. (2)-(5), for k = 3, we have i(g) i(c 3 (n 1, 2)) + i(c 3 (n 2, 1)) < i(c 3 (n 1, 2)) + i(p n 2, n 4 ) = i(c 3 (n, 3)), as claimed. Suppose now that OC(G) = s 2 the statement of lemma is true for smaller values of OC(G). Then k s Let P be a pendent path of length 2 in G with pendent vertex u N(u) = v. By the definition of pendent path, we have d(v) = 2. Then we have OC(G u) = s 1 OC(G [u]) = OC(G u v) = s 2 or s 1, C(G u) = k 1 C(G [u]) = k 2 or k 1. Note that G u U l n 1, k 1 G [u] Ul n 2, k 1 or Ul ; thus by the induction hypothesis, we have n 2, k 2 i(g u) i(c 3 (n 1, k 1)) with the equality if only if G u C 3 (n 1, k 1). Also, by Lemma 3.4 (when OC(G [u]) = s 2 = 0) or by the induction assumption (when OC(G [u]) = s 2 1), we have: if C(G [u]) = k 2, then i(g [u]) i(c 3 (n 2, k 2)) with the equality if only if G [u] C 3 (n 2, k 2), if C(G [u]) = k 1, then i(g [u]) i(c 3 (n 2, k 1)) with the equality if only if G [u] C 3 (n 2, k 1). Now, by the same way as used in the case of OC(G) = 1, we can obtain the desired result. This completes the proof. (2) (3) (4)

7 H. Hua et al. / Filomat 28:3 (2014), A graph is called a sun graph if it can be obtained by attaching a pendent edge to each vertex of a cycle. A damaged sun graph is a graph obtained from sun graph by deleting part of its pendent edges. Denoted by DSG(N, l) the set of damaged sun graphs having N vertices a cycle of length l. Obviously, we have l + 1 N 2l 1. DSG(n s t, l) d(w) = 3... w {{. t( 1) s( 2) Fig. 3. The graph occurred in the proof of Theorem 1. (a) n k 2 (b)... {{.... {{. k k 1 n k 2 (c)... {{. k 2 n k 2 Fig. 4. The graphs occurred in the proof of Theorem 1. n k 2 {.... {{ k 1 Fig. 5. The graph H 3 (n, k) occurred in the proof of Theorem 1. Now, we are in a position to state prove our maim theorem. Theorem 3.6. Let G be a graph in U n, k. Then i(g) i(c 3 (n, k)), with the equality if only if G C 3 (n, k). Proof. Let G max be a graph chosen from U l n, k for some l (3 l n k) such that i(g max) i(g) for any G U n, k \ {G max. Next, we shall prove that G max C 3 (n, k). By contradiction. Suppose that G max C 3 (n, k). If OC(G max ) = 0, then i(g max ) < i(c 3 (n, k)) by Lemma 3.4, a contradiction to our choice of G max. So we may suppose that OC(G max ) 1. We first prove the following two claims.

8 H. Hua et al. / Filomat 28:3 (2014), Claim 3.1. G max has exactly one branched vertex lying outside C l. Also, all neighbors but one of this branched vertex are pendent vertices. Proof. If G max has no branched vertices lying outside C l, then G max must contain a pendent path of length 2, as OC(G max ) 1. Then by Lemma 3.5, i(g max ) < i(c 3 (n, k)), a contradiction to our choice of G max. So G max has at least one branched vertex lying outside C l. Suppose that G max has two branched vertices lying outside C l. Then we can employ Operation I on G max obtain a new graph G such that G U l n, k. But, i(g max) < i(g ) by Lemma 2.6, a contradiction to the maximality of G max. Consequently, G max has exactly one branched vertex lying outside C l. Suppose that the unique branched vertex, lying outside C l, has two neighbors of degree 2. Then G max must contain a pendent path of length 2, as G max has exactly one branched vertex lying outside C l. As above, we can obtain a contradiction. This proves the claim. Claim 3.2. Each vertex on the cycle C l of G max is either of degree 2 or of degree 3. Also, all vertices but one, of degree 3, on the cycle C l are adjacent to a pendent vertex. Proof. Suppose to the contrary that G max has a branched vertex, of degree 4, lying along C l. By Claim 3.1, G max has a branched vertex lying outside C l. Then we can employ Operation I on G max obtain a new graph G such that G U l n, k. But then, i(g max) < i(g ) by Lemma 2.6, a contradiction to the maximality of G max. Thus, each vertex on the cycle C l of G max is either of degree 2 or of degree 3. Assume that there are two branched vertices on the cycle C l whose all neighbors are not pendent vertices. By Claim 3.1, G max has exactly one branched vertex lying outside C l. Thus, G max must contain a pendent path of length 2. Then by Lemma 3.5, i(g max ) < i(c 3 (n, k)), a contradiction to the maximality of G max. This proves the claim. By Claims , G max must be isomorphic to the graph as shown in Fig. 3. If l = 3, then G max must be isomorphic to one of the graphs (a), (b) (c), as shown in Fig. 4. G max is the graph (a). By our assumption that OC(G max ) 1, we have k 2. Also, we have n k 3, since n l + k. Thus, i(g max ) = i(p n 1, n k 1 ) + i(p n 3, n k 1 ) = 2 n k 2 F k+1 + F k + 2 n k 2 F k 1 + F k 2 < 2 n k 2 F k+2 + 2F k 1 = i(c 3 (n, k)), a contradiction to the maximality of G max. G max is the graph (b). Since OC(G max ) 1, we have k 3. Also, we have n k 3. Thus, i(g max ) = i(p n 1, n k 1 ) + 2i(P n 4, n k 1 ) = 2 n k 2 F k+1 + F k + 2(2 n k 2 F k 2 + F k 3 ) < 2 n k 2 F k+2 + 2F k 1 = i(c 3 (n, k)), a contradiction to the maximality of G max. G max is the graph (c). Since OC(G max ) 1, we have k 4. Also, we have n k 4 (see Fig. 4). Thus, i(g max ) = 2i(P n 2, n k 1 ) + 2i(P n 5, n k 1 ) = 2(2 n k 2 F k + F k 1 ) + 2(2 n k 2 F k 3 + F k 4 ) < 2 n k 2 F k+2 + 2F k 1 = i(c 3 (n, k)),

9 H. Hua et al. / Filomat 28:3 (2014), a contradiction to the maximality of G max. Now, we assume that l 4. Let w be the branched vertex of G max such that w lies along C l the unique neighbor, not belonging to C l, of w is of degree 2. Since l 4, there is always a vertex v V(C l ) such that d Gmax (v, w) 2. Let A = {v V(C l ) d Gmax (v, w) 2. Consider the following two cases. Case 3.1. There exists a vertex v in A such that d(v) = 3. By Claim 3.2, v has a pendent vertex as one of its neighbors in G max. Let u v be a pendent edge in C 3 (n, k) such that d(u ) = 1 d(v ) = 3 (note that when n = k + 3 or k = 3, the way of choosing vertices v u in C 3 (n, k) is not unique). By Lemmas (i), we obtain i(c 3 (n, k)) = i(c 3 (n, k) v ) + i(c 3 (n, k) [v ]) = 2i(P n 2, n k 1 ) + 2 n k 2 i(p k 2 ) i(g max ) = i(g max v) + i(g max [v]) = 2i(T 1 ) + i(g max [v]), where T 1 is a subtree of G max v with n 2 vertices. Obviously, T 1 has at least k 1 cut vertices. Let G max [v] = xk 1 T 2 (0 x 2, x + n = n 4), where T 2 is the largest component of G max [v] with n vertices. Then T 2 has at least k 3 cut vertices. By Proposition 3.2, we obtain i(t 1 ) i(p n 2, (n 2) (k 1) ) i(t 2 ) i(p n, n (k 3)). Thus, i(g max [v]) = i(xk 1 T 2 ) = 2 x i(t 2 ) 2 x i(p n, n (k 3)) = i(xk 1 P n, n (k 3)). Note that P n, n (k 3) contains (n k + 1)K 1 P k 1 as a proper spanning subgraph. Thus, xk 1 P n, n (k 3) contains (n k 2)K 1 P k 2 as a proper spanning subgraph. By Lemma 2.3, we have i(g max [v]) i(xk 1 P n, n (k 3)) < i((n k 2)K 1 P k 2 ) = 2 n k 2 i(p k 2 ). So, i(g max ) < i(c 3 (n, k)), a contradiction to our choice of G max. Case 3.2. For each vertex v in A, we have d(v) = 2. Let v be a vertex in A. Then G max v is a tree having n 1 vertices at least k cut vertices. Let G max [v] = xk 1 T 0 (0 x 2, x + n = n 3), where T 0 is the largest component of G max [v] with n vertices. Evidently, T 0 has at least k 2 cut vertices. We shall prove that i(g max ) < i(h 3 (n, k)) in the following, see Fig. 5 for H 3 (n, k). By Lemmas (i), we obtain i(h 3 (n, k)) = i(p n 1, n k 1 ) + 2 n k 2 i(p k 1 )

10 H. Hua et al. / Filomat 28:3 (2014), i(g max ) = i(g max v) + i(g max [v]) = i(g max v) + i(xk 1 T 0 ). By Proposition 3.2, we obtain i(g max v) i(p n 1, (n 1) k ) i(t 0 ) i(p n, n (k 2)). Thus, i(g max [v]) = i(xk 1 T 0 ) = 2 x i(t 0 ) 2 x i(p n, n (k 2)) = i(xk 1 P n, n (k 2)). Note that P n, n (k 2) contains (n k + 1)K 1 P k 1 as a proper spanning subgraph. Thus, xk 1 P n, n (k 2) contains (n k 2)K 1 P k 1 as a proper spanning subgraph. By Lemma 2.3, we have i(g max [v]) i(xk 1 P n, n (k 2)) < i((n k 2)K 1 P k 1 ) = 2 n k 2 i(p k 1 ). So, i(g max ) < i(h 3 (n, k)), a contradiction to our choice of G max. By discussions above, we conclude that G max C 3 (n, k), as claimed. 4. Connected graph with at least one cycle, given number of cut vertices the maximal Merrifield- Simmons index Let U n, k + be the set of unicyclic graphs of order n at least k cut vertices. According to Theorem 3.6, we have the following consequence. Corollary 4.1. Let G be a graph in U n, k +. Then i(g) i(c 3 (n, k)), with the equality if only if G C 3 (n, k). Proof. Let G be a graph in U n, k (1 k n 3). Then by Theorem 3.6, we have i(g) i(c 3 (n, k )), with the equality if only if G C 3 (n, k ). We need only to prove that if k k, then i(c 3 (n, k )) i(c 3 (n, k)). If k = 1, the result is obvious. So we may suppose that k 2. Then k k 2. For 1 k n 4, we have i(c 3 (n, k + 1)) = 2 n (k+1) 2 F (k+1)+2 + 2F (k+1) 1 = 2 n k 3 F k+3 + 2F k < 2 n k 2 F k+2 + 2F k 1 = i(c 3 (n, k)). Thus, for any 1 k < k n 3, we have as claimed. i(c 3 (n, k )) < < i(c 3 (n, k + 1)) < i(c 3 (n, k)), Theorem 4.2. Let G be a connected graph, not isomorphic to a tree, of n vertices k cut vertices. Then i(g) i(c 3 (n, k)), with the equality if only if G C 3 (n, k).

11 H. Hua et al. / Filomat 28:3 (2014), Proof. If G is a graph in U n, k, then by Theorem 3.6, we have completed the proof. Now, we may assume that G is a connected graph of n vertices, k cut vertices at least two cycles. We can always obtain a connected unicyclic spanning subgraph of G by deleting edges along some cycles of G. Let USS(G) denote a connected unicyclic spanning subgraph of G. Note that USS(G) is a connected unicyclic graph of n vertices at least k cut vertices. Thus, by Lemma 2.3 Corollary 4.1, we have as claimed. i(g) < i(uss(g)) i(c 3 (n, k)), References [1] A.F. Alameddine, Bounds on the Fibonacci number of a maximal outerplanar graph, The Fibonacci Quarterly 36 (1998) [2] J.A. Bondy, U.S.R. Murty, Graph Theory with Applications, Macmillan London Elsevier, New York, [3] I. Gutman, O.E. Polansky, Mathematical Concepts in Organic Chemistry, Springer, Berlin, [4] H. Hua, A sharp upper bound for the number of stable sets in graphs with given number of cut edges, Applied Mathematics Letters 22 (2009) [5] H. Hua, S. Zhang, Graphs with given number of cut vertices extremal Merrifield-Simmons index, Discrete Applied Mathematics 159 (2011) [6] H. Hua, S. Zhang, The number of independent sets in 3-connected 3-edge-connected graphs, submitted for publication. [7] C. Heuberger S. Wagner, Maximizing the number of independent subsets over trees with bounded degree, Journal of Graph Theory 44 (2008) [8] S. Li, X. Li, W. Jin, On the extremal Merrifield-Simmons index Hosoya index of quasi-tree graphs, Discrete Applied Mathematics 157 (2009) [9] S. Li, Z. Zhu, The number of independent sets in unicyclic graphs with a given diameter, Discrete Applied Mathematics 157 (2009) [10] X. Li, H. Zhao, I. Gutman, On the Merrifield-Simmons index of trees, MATCH Communications in Mathematical in Computer Chemistry 54 (2005) [11] S. Lin, C. Lin, Trees forests with large small independent indices, Chinese Journal of Mathematics 23 (1995) [12] H. Liu, M. Lu, A unified approach to extremal cacti for different indices, MATCH Communications in Mathematical in Computer Chemistry 58 (2007) [13] R.E. Merrifield, H.E. Simmons, Topological Methods in Chemistry, Wiley, New York, [14] A.S. Pedersen, P.D. Vestergaard, The number of independent sets in unicyclic graphs, Discrete Applied Mathematics 152 (2005) [15] H. Prodinger, R.F. Tichy, Fibonacci numbers of graphs, The Fibonacci Quarterly 20 (1982) [16] A. Yu, X. Lv, The Merrifield-Simmons Hosoya indices of trees with k pendent vertices, Journal of Mathematical Chemistry 41 (2007) [17] H. Zhao, X. Li, On the Fibonacci numbers of trees, The Fibonacci Quarterly 44 (2006)

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