## Computational Invariant of Chemical Structures and their Applications

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Tingzeng Wu, Yong Yu, "On the Hosoya Indices of Bicyclic Graphs with Small Diameter", *Journal of Chemistry*, vol. 2021, Article ID 5555700, 14 pages, 2021. https://doi.org/10.1155/2021/5555700

# On the Hosoya Indices of Bicyclic Graphs with Small Diameter

**Academic Editor:**Muhammad Faisal Nadeem

#### Abstract

Let be a graph. The Hosoya index of , denoted by , is defined as the total number of its matchings. The computation of is NP-Complete. Wagner and Gutman pointed out that it is difficult to obtain results of the maximum Hosoya index among tree-like graphs with given diameter. In this paper, we focus on the problem, and a sharp bound of Hosoya indices of all bicyclic graphs with diameter of 3 is determined.

#### 1. Introduction

Hosoya index is an important topological index introduced by Hosoya [1]. It was found that Hosoya index is related to a variety of physicochemical properties of alkanes (= saturated hydrocarbons). In particular, the boiling points of alkanes are well correlated with Hosoya index. Another series of researches revealed the applicability of Hosoya index in the theory of conjugated -electron systems [2, 3]. Jerrum [4] showed that the computing complexity of Hosoya index is NP-Complete. The Hosoya index got much attention by many researchers in the past decades. They have been interested in identifying the extremal value of Hosoya index for various classes of graphs, such as trees [5–7], unicyclic graphs [8–12], bicyclic graphs [13], and -graphs [14, 15]. Wagner and Gutman [16] gave an exhaustive survey for Hosoya index, and they pointed out some open problems (also see [17]) as follows:•It seems difficult to obtain results of the maximum Hosoya index among trees with a given number of leaves or given diameter. However, partial results are available, so the problem might not be totally intractable, and results in this direction would definitely be interesting.•If the aforementioned questions can be answered for trees, then it is also natural to consider the analogous questions for tree-like graphs.

According to the open problems, Liu et al. [17] discussed the problem in which unicyclic graph with diameter of 3 or 4 has the maximum Hosoya index. In this paper, we focus on similar problems to the above. That is, which bicyclic graph with diameter of 3 has the maximum Hosoya index? We give an answer of the problem as follows.

Theorem 1. * and ; each of the following holds:*

The rest of this paper is organized as follows. In Section 2, we shall present some definitions and lemmas. In Section 3, we will prove Theorem 1. Furthermore, some upper bounds for Hosoya index of some special classes of bicyclic graphs with diameter of 3 are also determined.

#### 2. Preliminaries

In this paper, we only consider finite and simple graphs. Let be a graph with vertices and edges. The neighborhood of vertex in a graph , denoted by , is the set of vertices adjacent to . The degree of , denoted by , is the number of neighbors of in . The distance of two vertices is the length of a shortest path from to , denoted by . We will use to represent the graph after deleting the vertex . The diameter of is max .

Let be the set of all bicyclic graphs with vertices and diameter of 3. It is easy to verify that the structure of graph must be isomorphic to , where . The resulting graph can be seen in Figure 1.

Let be the number of -matchings of . It is convenient to denote and for . The Hosoya index of , denoted by , is defined as the sum of all the numbers of its matchings; namely,

Lemma 1 (see [16]). *Let be a graph and let be a vertex of graph . Then*(i)*(ii)**, where is a component of *

Let be a graph obtained by joining the centers of two stars and , denoted by . By Lemma 1, we obtain the following result.

Lemma 2. (i)*(ii)*

#### 3. The Proof of Theorem 1

In order to prove Theorem 1, we first give some lemmas.

Lemma 3. *Let be a graph with vertices. Then*

*Proof. *Consider that vertex of degree 1 is adjacent to in . By Lemmas 1 and 2, we have . In the following, we use the method of Lagrange multipliers to find the sharp bound of Hosoya index of . First, we make auxiliary function as follows: , where , , and at most one of them is 1. Taking the partial derivatives of , and in , we can obtain the following equations:Solving the equations as above, we obtain that , , and . Because is a unique stable point, must be a unique extreme point. Since , , and are integers, we consider three cases. Case 1: assume that . We know that , , and ; , , and ; or , , and . Thus, . It is easy to verify that when , , and ; , , and ; or , , and . This implies that and the equality holds iff . Case 2: suppose that . We have , , and . So . To simplify the calculation, we know that when , , and . This means that and the equality holds iff . Case 3: assume that . Then , , and ; or , , and . Thus, . It is easy to check that when , , and ; or , , and . This indicates that and the equality holds if and only if .

Lemma 4. *Let be a graph with vertices. Then*

*Proof. *Similar to the proof of Lemma 3, by Lemmas 1 and 2, we have . By the method of Lagrange multipliers, we make an auxiliary function , where , , and . Taking the partial derivatives of , , and in , we get the following equations:Solving the equations as above, we obtain that and . Because is a unique stable point, must be the unique extreme point. Since and are integers, in the following we consider two cases: Case 1: suppose that is even. Then and . Thus, . To simplify calculation, we know that when and . This implies that and the equality holds iff . Case 2: assume that is odd. We obtain and , or and . So, . It is easy to check that when and , or and . This means that and the equality holds iff .

Lemma 5. *Let be a graph with vertices. Then*

*Proof. *Similar to the proof of Lemma 3, by Lemmas 1 and 2, we get that . According to the method of Lagrange multipliers, we make an auxiliary function , where , , and . Taking the partial derivatives of , and in , we obtain the following equations:Solving the equations as above, we obtain that and . Because is a unique stable point, must be the unique extreme point. Since and are integers, we only consider two cases. Case 1: assume that is even. We obtain and , or and . Thus, . It is easy to check that when and , or and . This implies that and the equality holds if and only if . Case 2: consider that is odd. We have and . So, . It is easy to verify that when and . This means that and the equality holds if and only if .

Lemma 6. *Let be a graph with vertices. Then*

*Proof. *Similar to the proof of Lemma 3, by Lemmas 1 and 2, we get . According to the method of Lagrange multipliers, we make an auxiliary function , where , , and . Taking the partial derivatives of , and in , we can obtain the following equations:Solving the equations as above, we obtain that and . Because is a unique stable point, must be the unique extreme point. Since and are integers, we only consider two cases: Case 1: consider that is even. We have and . Thus . To simplify calculation, we get when and . This implies that and the equality holds iff . Case 2: assume that is odd. Then and , or and . So . It is easy to check that when and , or and . This implies that and the equality holds if and only if .

Lemma 7. *Let be a graph with vertices. Then*

*Proof. *Similar to the proof of Lemma 3, by Lemmas 1 and 2, we get . By the method of Lagrange multipliers, we make an auxiliary function , where , , and . Taking the partial derivatives of , and in , we can obtain the following equations:Solving the equations as above, we obtain that and . Because is a unique stable point, must be the unique extreme point. Since and are integers, in the following we only consider two cases. Case 1: consider that is even. We have and . Thus . To simplify calculation, we get when and . This implies that and the equality holds iff . Case 2: assume that is odd. Then and , or and . So . It is easy to check that when and , or and . This implies that and the equality holds if and only if .

Lemma 8. *Let be a graph with vertices. Then*

*Proof. *Similar to the proof of Lemma 3, by Lemmas 1 and 2, direct computing yields . By the method of Lagrange multipliers, we make auxiliary function , where , , and at most one of them is 1. Taking the partial derivatives of , , , and in , we can obtain the following equations:Solving the equations as above, we get . Since is an integer, we know that or . By and , we can obtain that , , and . Similarly, by and , we also can obtain that , , and . Thus, we discuss two cases as follows: Case 1: , , and . Because , , and is a unique root of (14), , , and must be the unique extreme point. Since , , and are integers, we consider three subcases: Subcase 1: assume that . Then , , and , or , , and . Thus . It is easy to verify that when , , and , or , , and . This implies that and the equality holds if and only if . Subcase 2: suppose that . Then , , and . Thus . It is easy to check that when