Research Article | Open Access

# Some Retarded Difference Inequalities of Product Form and Their Application

**Academic Editor:**Weinian Zhang

#### Abstract

A class of new nonlinear retarded difference inequalities is established. An application of the obtained inequalities to the estimation of finite difference equations is given.

#### 1. Introduction

Difference inequalities which give explicit bounds on unknown functions provide a very useful and important tool in the study of many qualitative as well as quantitative properties of solutions of nonlinear difference equations. Various investigators have discovered many useful and new difference inequalities, mainly inspired by their applications in various branches of difference equations; see [1–25] and the references cited therein.

Sugiyama [2] established the most precise and complete discrete analogue of the Gronwall inequality (see [1]) in the following form.

Let and be nonnegative functions defined for , and suppose that for every . If where is the set of points , is a given integer, and is a nonnegative constant, then Pachpatte [4] established a generalized discrete analogue of the Gronwall inequality in the following form.

Let be a positive and monotone nondecreasing function on , and let , be nonnegative functions on . If satisfies then where

Besides the results mentioned above, the following results are closely related to the investigation of the present paper, and, particularly, they will be used as lemmas for the proofs of our main results in Theorems 3 and 4.

Lemma 1 (see [5]). *Let , , , , and be nonnegative functions defined on , for which the inequality
**
holds, where is a nonnegative constant and . If and for all , then
*

Lemma 2 (see [3, 7]). *Let be a real-valued function defined for and monotone nondecreasing with respect to for any fixed . Let be a real-valued function defined for such that
**
Let be a solution of
**
such that . Then
*

Pachpatte [7, 8] also established some difference inequalities of product form as follows.

Let , and be nonnegative functions defined on and let be a nonnegative constant. Let be a nonnegative function defined for , and monotone nondecreasing with respect to for any fixed . If satisfies then where is defined by (5) and is a solution of Let , and be nonnegative functions defined for and let be a nonnegative constant. Let be a nonnegative function defined for , and monotone nondecreasing with respect to for any fixed . If satisfies then where is defined by (5) and is a solution of the difference equation

Motivated by the results given in [5, 7, 8], in this paper, we discuss new nonlinear difference inequalities:

It is important to note that the inequality given above can be used as tools in the study of certain classes of finite difference equations. In Section 3 we provide an application of our results to the estimation of finite difference equations.

#### 2. Main Results

Throughout this paper, let and , . For function , , we define the operator by . Obviously, the linear difference equation with the initial condition has the solution . For convenience, in the sequel we complementarily define that and

Theorem 3. *Let be a nonnegative and monotone nondecreasing function defined on , and let , , and be nonnegative functions defined on . Let be a constant with . If satisfies
**
then
**
where
*

*Proof. *Fix , where is chosen arbitrarily; since is a nonnegative and monotone nondecreasing function, from (18), we have
Now an application of Lemma 1 to (21) yields
Since is arbitrary, from (22), we get the required estimate (19).

Theorem 4. *Let ,and be nonnegative functions defined for and let be a nonnegative constant. Let be a real-valued function defined for , and monotone nondecreasing with respect to for any fixed . Let be a constant with . If satisfies (17), then
**
where is defined by (20) in Theorem 3 and is a solution of the difference equation
*

*Proof. *We first assume that and define a function by the right-hand side of (17). Then is a nonnegative and monotone nondecreasing function defined on . We have
Using the definitions of the operator and , we obtain
From (26), we have
Setting in (27) and substituting , successively, we get
Define a function by
Then and
Using (29), inequality (28) can be written as
since is positive and monotone nondecreasing for and , and satisfy the conditions in Theorem 3. Now an application of Theorem 3 to (31) yields
where is defined by (20) in Theorem 3. Since is monotone nondecreasing with respect to for any fixed , from (30) and (32), we have
Now with a suitable application of Lemma 2, we obtain
where is a solution of (24). Using (25), (32), and (34), we obtain our required estimation (23).

If is nonnegative, we can carry out the above procedure with instead of , where is an arbitrary small number. Letting , we obtain (23).

#### 3. Application to Finite Difference Equations

In this section, we apply our result to the following difference equation: where , and are real-valued functions defined, respectively, on , , and and is as defined in Theorem 4. We assume that where , and are as defined in Theorem 4. From (35), we have From (37), we have Using conditions (36), we obtain Now an application of Theorem 4 to (39) yields the estimation of the difference equation (35) as follows: where is a solution of the difference equation

#### Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

#### Acknowledgments

The present investigation was supported by the National Natural Science Foundation of China (no. 11161018), the Foundation of Scientific Research Project of Fujian Province Education Department (no. JK2012049), the Guangxi Natural Science Foundation (no. 2012GXNSFAA053009), and the Scientific Research Foundation of the Education Department of Guangxi Zhuang Autonomous Region (no. 201106LX599 and no. 201204LX391). The authors are grateful to the anonymous referees for their careful comments and valuable suggestions on this paper.

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#### Copyright

Copyright © 2014 Zongyi Hou et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.