Research Article | Open Access

Stevo Stević, Josef Diblík, Bratislav Iričanin, Zdeněk Šmarda, "On the Difference Equation ", *Abstract and Applied Analysis*, vol. 2012, Article ID 409237, 20 pages, 2012. https://doi.org/10.1155/2012/409237

# On the Difference Equation

**Academic Editor:**Jean Pierre Gossez

#### Abstract

The behavior of well-defined solutions of the difference equation , where is fixed, the sequences , and are real, , , and the initial values are real numbers, is described.

#### 1. Introduction

Recently there has been a huge interest in studying nonlinear difference equations and systems (see, e.g., [1–33] and the references therein). Here we study the difference equation where is fixed, the sequences and are real, , , and the initial values are real numbers. Equation (1.1) is a particular case of the equation with real sequences , and . For , , the equation is trivial, and, for , it is reduced to equation (1.1) with and .

Equation where , which was treated in [32], is a particular case of equation (1.1).

As in [32], here, we employ our idea of using a change of variables in equation (1.1) which extends the one in our paper [21] and is later also used, for example, in [4]. For similar methods see also [22, 25]. Equation (1.3) in the case was also studied in [1, 2], in a different way. The case when the sequences and are two-periodic was studied in [31] (some related results are also announced in talk [3]). For related symmetric systems of difference equations, see [27, 29]. For some other recent results on difference equations and systems which can be solved, see, for example, [6, 7, 20–22, 30, 31, 33]. Some classical results can be found, for example, in [11].

Equation (1.1) is a particular case of the equation where is a continuous function. Numerous particular cases of (1.4) have been investigated, for example, in [9, 21, 23]. In this paper we adopt the customary notation and .

#### 2. Case

Here we consider the case . In this case equation (1.1) becomes ,, from which it follows that for each Using formula (2.2) the following theorem can be easily proved.

Theorem 2.1. * Consider equation (1.1) with , ,. Then the following statements are true: ** if
**for some , then as ;** if for each , the limits in (2.3) are greater than , then as ;** if , for every and for some , then , ; ** if , for every and for some , then , ;** if
**and , for some , then , as ;** if for each , the limits in (2.4) belong to the interval and , then as . *

#### 3. Case

In this section we consider the case . Note that in this case equation (1.1) becomes where . If is a well-defined solution of equation (3.1) (i.e., a solution with initial values , , which implies , ), then

Hence for each

Using formula (3.3) we easily prove the next theorem.

Theorem 3.1. * Consider equation (1.1) with , , . Then the following statements are true: ** if
**for some , then as ;** if, for each , the limits in (3.4) are greater than , then as ;** if , for every and for some , then , ; ** if , for every and for some , then , ;** if
**and for some , then , as ;** if, for each , and the limits in (3.5) belong to the interval , then as . *

#### 4. Case and

The case when and for every is considered in this section.

If for some , then from (1.1) we have that From (4.1) and (1.1) we have that for each From (4.1) we see that, for , (4.2) also holds. Hence Theorem 2.1 can be applied in this case. Note that if for some , then (1.1) implies that there is an such that , and by the previous consideration we have that (4.2) also holds.

If , for each , then for every well-defined solution we have for (note that there are solutions which are not well defined, that is, those for which , for some ).

Multiplying equation (1.1) by and using the transformation we obtain equation Note that from (4.3), for every well-defined solution of equation (1.1) such that , for each , it follows that , .

Since , , we have that

From (4.3) and (4.5) we have that for every .

Hence, from (4.6), we obtain that for every and each , where

#### 5. Case

Here we consider the case , . In this case, from (4.7) we have that for each Note that this formula includes also the case when for some .

Now we formulate and prove a result in this case by using formula (5.1).

Theorem 5.1. * Consider equation (1.1) with , , , , , and
**
Then the following statements hold: ** if for some **then as ;** if (5.3) and (5.4) hold for every , then as ;** if for some the sum
**converges, then the sequence is also convergent;** if the sum in (5.5) is finite for every , then the sequences are convergent. *

* Proof. *Let be a solution of equation (1.1). Using condition , , it is easy to see that if (5.4) holds for some , there is an such that for the terms in the product in (5.1) are positive and that the following asymptotic formula
can be used with being the fraction in the limit (5.4). From (5.1) and (5.6) we have that
where

Using formula (5.7), the assumptions regarding the sum and the comparison test for the series whose terms are of eventually the same sign, the results in the theorem easily follow.

#### 6. Case

Here we consider the case , . In this case from (4.7) we have for every and each , where is defined by (4.8).

Theorem 6.1. * Consider equation (1.1) with , , , and
**
Then the following statements hold: ** if for some **then as ;** if for every , (6.3), (6.4), and (6.5) hold, then as ;** if for some **conditions (6.4) and (6.5) hold, and , then as ;** if, for every , conditions (6.4), (6.5), and (6.6) hold, and , , then as ;** if for some the sum
**converges and condition (6.5) holds, then the sequences and are also convergent;** if, for every the sum in (6.7) converges and condition (6.5) holds, then the sequences , are convergent.*

* Proof. * Let be a solution of equation (1.1). By (6.4) we see that irrespectively on , there is an such that for the terms in the product in (6.1) belong to the interval and that asymptotic formulae
can be used with being the fraction in (6.4). From this and (6.1) we have that
where

Using formula (6.9), the assumptions of the theorem and some well-known convergence tests for series, the results in (a)–(f) easily follow.

#### 7. Case

In this section we consider equation (1.1) for the case , , , that is, when the sequences and are -periodic.

First we show the existence of -periodic solutions of equation (4.4). If is such a solution, then we have that By successive elimination, or by Kronecker theorem (note that system (7.2) is linear), we get if , where is the permutation defined by and , where Id denotes the identity.

It is easy to see that (4.4) along with periodicity of sequences and implies for every and , such that .

Since (7.5) is a linear first-order difference equation, we have that when , its general solution is

By letting in (7.6) we obtain the following corollary.

Corollary 7.1. * Consider equation (4.4) with , , . Assume that
**
Then for every solution of the equation we have that
**
for every , that is, converges to the -periodic solution in formula (7.3). *

Let

From now on we will use the following convention: if , then we regard that , if . Also if a sequence is defined by the relation , where is a real function, then we will assume that , if .

Using (7.6) and notation (7.9) in the relation (see (4.6)), for the case , we have that for every and each , and

Now we present some results, which are applications of formulae (7.10) and (7.11).

##### 7.1. Case

If , then by (7.5) we get for ; that is, is two-periodic for each . Hence is a -periodic solution of equation (4.4), in this case.

Hence from the relation (see (4.6)), for each , we have for .

From (7.13) and by periodicity of , we get for each .

From (7.14), the behavior of solutions of equation (1.1), in this case, easily follows. For example, if for each , then the solution of (1.1) is -periodic.

##### 7.2. Case

If and , then from (7.5) we obtain when , from which along with (4.6), it follows that

Corollary 7.2. * Consider equation (1.1). Let , , and , . Then the following statements hold true. ** If , for some , then as . ** If , or and , for some , then as , if . ** If , for some , and , then as , if . ** If , for some , and , then as . ** If , for some , and , then the sequence is convergent. ** If , and , then as , if . ** If , and , then as . ** If , and , then the sequence is convergent. *

* Proof. * The statements in (a) and (b) follow from the facts that
if ,
if and ,
if , and
if and .

Now assume that and let be a solution of equation (1.1). It is easy to see that there is an such that for the terms in the products in (7.17) are positive and that the following asymptotic formulae
can be applied with , whenor with . Using these formulae, for the case , we have that
where

Letting in (7.23), using the facts that
and that the series converges, we get statements (c)–(e).

If , that is , then by using (7.22) we get
where
Letting in (7.26), using (7.25) and the fact that the series converges, we get statements (f)–(h), as desired.

##### 7.3. Case

If , then from (7.6) we get where from (4.6) it follows that for , and Note that , if .

Corollary 7.3. * If , , and , for every and , then the following statements hold true.** If , for some , we have that as . ** If , and if for some , we have that as , if . ** If , for some , then is convergent. ** If , and for some , then as . ** If , and , then as . ** If , and for some , then as , if . ** If , and , then as , if . ** If , and for some , then is constant. ** If , and , then is constant. ** If , and for some , then . ** If , and , then . ** If , for some , then the subsequences and are convergent. *

* Proof. *Since we have that
when ,
when and ,
when , and
when and , the statements in (a) and (b) easily follow from (7.30)–(7.35).

(c) If , then
for , and if , then
from which the statement in (c) easily follows.

(d)–(k) If , then
for , and if , then
from which the statements in (d)–(k) easily follow.

(l) If , then we have that
for , and if , then
from which the statement in (l) easily follows.

Corollary 7.4. * If and , and , for every and , then the following statements hold true. ** If , for some , then as . ** If , then as . ** If , or , and , for some , then as , if . ** If , or if , and , then as , if . ** If , for some *