# Positive Decreasing Solutions of Higher-Order Nonlinear Difference Equations

- B Krasznai
^{1}, - I Győri
^{1}and - M Pituk
^{1}Email author

**2010**:973432

**DOI: **10.1155/2010/973432

© B. Krasznai et al. 2010

**Received: **23 December 2009

**Accepted: **13 May 2010

**Published: **13 June 2010

## Abstract

It is shown that the decay rates of the positive, monotone decreasing solutions approaching the zero equilibrium of higher-order nonlinear difference equations are related to the positive characteristic values of the corresponding linearized equation. If the nonlinearity is sufficiently smooth, this result yields an asymptotic formula for the positive, monotone decreasing solutions.

## 1. Introduction and the Main Results

Let , , and be the set of real and complex numbers and the set of integers, respectively. The symbol denotes the set of nonnegative integers.

where , so that and is a Lipschitz continuous function such that in and is identically in . As noted in [1], (1.1) is related to the discretization of traveling wave solutions of the Fisher-Kolmogorov partial differential equation. The main result of [1] is the following theorem about the existence of positive, decreasing solutions of (1.1) (see [1, Theorem ] and its proof).

Theorem 1.1.

Note that in [1], solutions of (1.1) satisfying conditions (1.4) and (1.5) of Theorem 1.1 are called *fast solutions*.

We will show that under appropriate assumptions the decay rates of these solutions are equal to the characteristic values of the corresponding linearized equation belonging to the interval . Our result, combined with asymptotic theorems from [2] or [3], yields asymptotic formulas for the positive, monotone decreasing solutions of (1.6).

*linearization*about the zero equilibrium, namely, the linear homogeneous equation

*characteristic value*of (1.9), we mean a complex root of the

*characteristic polynomial*

where
is a polynomial of degree less than
, the multiplicity of
as a root of
. Such solutions are called *characteristic solutions* corresponding to
. If
is a nonempty set of characteristic values, then by a *characteristic solution corresponding to the set*
, we mean a finite sum of characteristic solutions corresponding to values
.

Now we can formulate our main results. The first theorem applies to the positive, monotone decreasing solutions of (1.6) provided that zero is a hyperbolic equilibrium of (1.6). Recall that the zero equilibrium of (1.6) is *hyperbolic* if the linearized equation (1.9) has no characteristic values on the unit circle
.

Theorem 1.2.

In contrast to Theorem 1.2, the next result applies also in some cases when the zero equilibrium of (1.6) is not hyperbolic.

Theorem 1.3.

Note that conclusion (1.18) of Theorem 1.3 is stronger than (1.14).

For the second-order equation (1.1), we have the following theorem which provides new information about the decreasing fast solutions obtained by Aprahamian et al. [1].

Theorem 1.4.

in the interval .

The proofs of the above theorems are given in Section 3.

## 2. Preliminary Results

In this section, we establish some preliminary results on linear difference equations with asymptotically constant coefficients which will be useful in the proof of our main theorems.

*asymptotically constant*, that is, the (finite) limits

*limiting equation*

Theorem 1.2 will be deduced from the following proposition.

Proposition 2.1.

where is the characteristic polynomial corresponding to (2.4).

Proof.

The boundedness of implies that . By the application of a Perron-type theorem (see [4, Theorem ] or [5, Theorem ]), we conclude that for some characteristic value of (2.4). By virtue of (2.3), the characteristic values of (2.4) are nonzero. Therefore, .

Since the coefficients of the -transform are positive, according to Prinsheim's theorem (see [6, Theorem ] or [7, Theorem , page 262]) has a singularity at . Since and hence is holomorphic in the region , this implies that . Otherwise, (2.16) would imply that can be extended as a holomorphic function to a neighborhood of by . Thus, is a characteristic value of (2.4).

This proves the existence of the limit (2.6) with . Finally, conclusion (2.7) is an immediate consequence of [2, Theorem and Remark ].

Theorem 1.3 can be regarded as a corollary of the following result.

Proposition 2.2.

Before we give a proof of Proposition 2.2, we establish two lemmas.

Lemma 2.3.

Proof.

a contradiction. Thus, (2.25) holds for some .

The following lemma will play a key role in the proof of Proposition 2.2.

Lemma 2.4.

Proof.

Relation (2.34), combined with Pringsheim's theorem [6, Theorem ], implies that . (Otherwise, can be extended as a holomorphic function to a neighborhood of by .) Since and the only root of in is , we have that .

From this, taking into account that for all , we see that . Therefore, (2.57) reduces to (2.31).

Now we are in a position to give a proof of Proposition 2.2.

Proof of Proposition 2.2.

## 3. Proofs of the Main Theorems

Proof of Theorem 1.2.

From this, (1.10), (3.2) and the Lipschitz continuity of the partial derivatives , , it is easily shown that hypothesis (2.5) of Proposition 2.1 also holds. The conclusions of Theorem 1.2 follow from Proposition 2.1.

Proof of Theorem 1.3.

As noted in the proof of Theorem 1.2, is a solution of (2.1) with the asymptotically constant coefficients given by (3.2) and the limiting equation of (2.1) is the linearized equation (1.9). Further, by virtue of (1.7), condition (2.3) holds. Thus, all hypotheses of Proposition 2.2 are satisfied and the result follows from Proposition 2.2.

Proof of Theorem 1.4.

Since , we have that and (1.19) holds.

This shows that the convergence of the coefficients of (3.6) to their limits is exponentially fast. Thus, Proposition 2.1 applies and the limit relation (1.21) follows from the asymptotic formula (2.7).

## Declarations

### Acknowledgment

This research was supported in part by the Hungarian National Foundation for Scientific Research (OTKA) Grant no. K 732724.

## Authors’ Affiliations

## References

- Aprahamian M, Souroujon D, Tersian S:
**Decreasing and fast solutions for a second-order difference equation related to Fisher-Kolmogorov's equation.***Journal of Mathematical Analysis and Applications*2010,**363**(1):97-110. 10.1016/j.jmaa.2009.08.009MATHMathSciNetView ArticleGoogle Scholar - Agarwal RP, Pituk M:
**Asymptotic expansions for higher-order scalar difference equations.***Advances in Difference Equations*2007,**2007:**-12.Google Scholar - Bodine S, Lutz DA:
**Exponentially asymptotically constant systems of difference equations with an application to hyperbolic equilibria.***Journal of Difference Equations and Applications*2009,**15**(8-9):821-832. 10.1080/10236190802357602MATHMathSciNetView ArticleGoogle Scholar - Pituk M:
**More on Poincaré's and Perron's theorems for difference equations.***Journal of Difference Equations and Applications*2002,**8**(3):201-216. 10.1080/10236190211954MATHMathSciNetView ArticleGoogle Scholar - Elaydi S:
*An Introduction to Difference Equations, Undergraduate Texts in Mathematics*. 3rd edition. Springer, New York, NY, USA; 2005:xxii+539.Google Scholar - Bak J, Newman DJ:
*Complex Analysis, Undergraduate Texts in Mathematics*. Springer, New York, NY, USA; 1982:x+244.Google Scholar - Schaefer HH:
*Topological Vector Spaces, Graduate Texts in Mathematics*.*Volume 3*. 3rd edition. Springer, New York, NY, USA; 1971:xi+294.View ArticleGoogle Scholar - Pituk M:
**Nonnegative iterations with asymptotically constant coefficients.***Linear Algebra and Its Applications*2009,**431**(10):1815-1824. 10.1016/j.laa.2009.06.020MATHMathSciNetView ArticleGoogle Scholar - Agarwal RP:
*Difference Equations and Inequalities, Monographs and Textbooks in Pure and Applied Mathematics*.*Volume 155*. Marcel Dekker, New York, NY, USA; 1992:xiv+777.Google Scholar

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