1. Introduction

In this work we consider simple graphs with vertices and edges. Let be the vertex set of . For every vertex , represents the degree of the vertex in . The maximum and minimum degree of the vertices of will be denoted by and , respectively.

The Randić index of a graph was introduced in 1975 [1] and defined as This graph invariant, sometimes referred to as connectivity index, has been successfully related to a variety of physical, chemical, and pharmacological properties of organic molecules, and it has became into one of the most popular molecular-structure descriptors. After the publication of the first paper [1], mathematical properties of were extensively studied, see [2–6] and the references cited therein.

The higher-order Randić indices are also of interest in chemical graph theory. For , the th order Randić index of a graph is defined as where denotes the set of paths of length contained (as subgraphs) in . Of the higher-order Randić indices the most frequently applied is [7–10]. Estimations of the higher-order Randić index of regular graphs and semiregular bipartite graphs are given in [10]. In this paper we are interested in studying the higher-order Randić index, , for corona product graphs. Roughly speaking, we study the cases , for arbitrary graphs and the case when the second factor of the corona product is an empty graph. As an example of a chemical compound whose graph is obtained as a corona product graph we consider the Cycloalkanes with a single ring, whose chemical formula is , and whose molecular graph can be expressed as , where is the cycle graph of order and is the empty graph of order two. We recall that, given two graphs and of order and , respectively, the corona product is defined as the graph obtained from and by taking one copy of and copies of and then joining by an edge each vertex of the copy of with the vertex of .

2. Estimating for Corona Graphs

Theorem 2.1. For , let be a graph of minimum degree , maximum degree , order and size . Then,

Proof. Let , , and let . We have where Thus, the lower bound follows. Analogously we deduce the upper bound.

Corollary 2.2. For , let be a -regular graph of order . Then,

Theorem 2.3. For , let be a graph of minimum degree , maximum degree , order , and size . Then,

Proof. Let and be the set of vertices of and , respectively. Given a vertex , we denote by the set of neighbors that has in . The paths of length two in are obtained as follows: (i)paths , , where and , (ii)paths , , where and , (iii)paths , , where and , (iv)paths of length two belonging to , (v)paths of length two belonging to the copies of .
So, we have , where corresponds to the paths type (i), corresponds to the paths type (ii), corresponds to the paths type (iii), corresponds to the paths type (iv), and corresponds to the paths type (v). Thus, the lower bound follows. The upper bound is obtained by analogy.

Corollary 2.4. For , let be a -regular graph of order . Then,

The girth of a graph is the size of its smallest cycle. For instance, the molecular graphs of benzenoid hydrocarbons have girth 6. The molecular graphs of biphenylene and azulene have girth 4 and 5, respectively [11].

The following result, and its proof, was implicitly obtained in the proof of Theorem 1 of [10]. By completeness, here we present it as a separate result.

Lemma 2.5. Let be a graph with girth . If and , then the number of paths of length in is bounded by

Proof. Since , for every , the number of paths of length 2 in of the form is . Therefore, the result follows for .
Suppose now that . Given a vertex , let be the set of paths of length whose second vertex is , that is, paths of the form . We denote by the set of neighbors of an arbitrary vertex . Note that the degree of is . If , then for every and we have . So, for every , there exists a vertex sequence such that , , . If , then the sequence is a path. Conversely, every path of length whose second vertex is can be constructed as above. Hence, the number of paths of length whose second vertex is is bounded by Thus, the result follows.

Now denotes the empty graph of order .

Theorem 2.6. Let be a graph with girth , minimum degree , and maximum degree . If and , then

Proof. The paths of length in contribute to in Moreover, each path of length in leads to paths of length in ; thus, the paths of length in contribute to in Hence, By Lemma 2.5 we obtain the upper bound and the lower bound is obtained by analogy.

Corollary 2.7. Let be a -regular graph of order and girth . If and , then

Acknowledgment

This work was partly supported by the Spanish Government through projects TSI2007-65406-C03-01 “E-AEGIS” and CONSOLIDER INGENIO 2010 CSD2007-00004 “ARES.”