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Motivated by an open problem in the literature “Dynamics of Second Order Rational Difference Equations with Open Problems and Conjecture”, we introduce a difference equation system: x_{n+1}={y_{n-2}+y_{n-3}}/x_{n}, y_{n+1}={x_{n-2}+x_{n-3}}/y_{n} , n=o, 1 ...* Where x_{i, }y_{i }∈（0, ∞）， i=-3, -2, -1, 0. We try to find out some conditions such that the solution of system converges to periodic solution. This model can be applied to the two species competition and population biology.

In the monograph of Dynamics of Second Order Difference Equation [

Open problem 11.4.8:

Determine whether every positive solutions of the following equation converges to a periodic solution of the corresponding equation:

Motivated by the Open Problem, we introduce the difference equation system:

where the initial points

Recently, there has been great interest in studying difference equation systems. One of the reasons for this is the necessity for some techniques that can be used in investigating equations arising in mathematical models describing real life situations in population biology, economics, probability theory, etc. There are many papers related to the difference equations system for example, such as [2-9].

In [

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In this paper, we try to find out some conditions such that the solution of system (2) converges to periodic solution. At the same time, we can get the oscillatory of system (2).

Before giving some results of the system (2), we need some definitions as follows [

Definition 1.1 A pair of sequences of positive real numbers that satisfies system (2) is a positive solution of system (2). If a positive solution of system (2) is a pair of positive constants, that solution is the equilibrium solution.

Definition 1.2 A “string” of consecutive terms (resp.), (,) is said to be a positive semicycle if (resp.), , (resp.), and (resp.). Otherwise, that is said to be a negative semicycle.

A “string” of consecutive terms is said to be a positive(resp.negative) semicycle if, are positive (resp.negative) semicycle.

A solution (resp.) oscillates about (resp.) if for every, there exist, , , such that (resp.). We say that a solution

of system oscillates about if

oscillates about or oscillates about.

Lemma 2.1 The system (2) has a unique positive equilibrium.

The proof of lemma 2.1 is very easy, so we omit it.

Lemma 2.2 If, , , Then ever positive solution of system (2) with prime period two takes the forms

or

is a period-two solution of system (2).

Proof: Let be a period-two solution of system (2).

Then, by system (2) we get

We can see that (7) can be changed to

Form (8), we can obtain

or

and

Therefore, we complete the proof.

Lemma 2.3 Assume that the initial points , and is a positive solution of system (2). Then the following cases are true:

(a) If, ,

;, , , then and

are both increasing.

(b) If, ,

;, , , then and

are both decreasing.

Proof: (a) By system (2), we can get

i.e.

where.

By condition and (9), we get:

By condition, and (9), we get:

By condition and (9), we get:

Equally, we can get:

Hence, by induction and (10)-(15), we proof that

and are both increasing.

Using the same method, we can prove that case (b) holds.

Therefore, we complete the proof.

Lemma 2.4 Assume that

. Then there does not exist a positive solution of system (2) such that and are both increasing or both decreasing.

Proof: By Equation (9), we can get and have the same monotonous.

Firstly, we proof that there does not exist positive solution such that

and are both increasing.

Assume, for the sake of contradiction, that we have the following results:

(i) is increasing;

(ii) is also increasing.

By system (2), we obtain

in Equations (16) and (17), it implies that:

Because of, , we can get

i.e

Also, we can get

Because of the assumptions (i) and (ii), it is easy to see that (18) and (19) do not hold.

This is a contradiction and we proof the case of increasing does not hold.

Next, we proof there does not exist positive solution of system (2) such that and

are both decreasing.

Assume, for the sake of contradiction, that we have the following results:

(i) is decreasing;

(ii) is also decreasing.

By the Limiting Theorem we know that

and are both decreasing into a pair of constants.

We set, , ,

, and, , ,.

By system (2), we know that these constants satisfy the system (2)i.e.

However, if, , , , Equation (20) do not holds, which is contradiction.

Hence, we complete the proof of lemma 10.

Lemma 2.5 Assume that

. Then there does not exist a positive solution of system (2) such that and are both decreasing or both increasing.

Proof: First, we proof there does not exist positive solution of system (2) such that and are both decreasing, the proof of increasing is similar, so we omit it.

Assume, for the sake of contradiction, that we have the following results:

(i) is decreasing;

(ii) is also decreasing.

We set, ,.

By Limit Theorem,we know that and are both decreasing into a pair of constants.

Obviously, the limits of

can not decrease into zero.

By system (2), we can get

where, which can be changed to

However, if, Equation (22) can not hold.

This is a contradiction and we complete the proof.

The the proof of the case of increasing is similar with the proof of the the case of decreasing, so we omit it.

In addition to the method above, we can proof the Lemma 2.5 by the method of Lemma 2.4. Here, we omit it.

Theorem 3.1 Assume that, ,

;, , , and is a positive solution of system (2). Then and

are both decreasing; and converges to a period-two solution as following

where satisfy,

.

Proof: By lemma 2.3(a), we can obtain that

and are both decreasing.

Then by the Limit Theorem, we can get, , , and, all exist and are positive.

We can set

By lemmas 2.4 and 2.5, we know that there does not exist a positive solution or

such that and

are both decreasing.

Hence, there is at least one of satisfy and at least one of satisfy

By system (2), we get

It is to see that is a period-two solution of system (2), and satisfy,.

We complete the proof.

Corollary 3.1 Suppose that is a positive solution of system (2). Then the following statement is true:

If

the solution of system (2)

eventually oscillates about equilibrium

.

Theorem 3.2 Assume that, ,

, and is a positive solution of system (2). Then and

are both increasing; and

converges to a period-two solution as following

where satisfy,

.

Proof: By lemma 2.3(a), we obtain that and are both increasing.

We set, , ,

By Equation (9), we can get

which can be changed into:

By the, we can get

By induction, we can get

From Lemma 10, we know that there at least one. Then by Limiting Theorem, we can get at least one of the limiting of must exist. With no loss generality, we set the limit of exist,we can know.

By limiting Equation (27), we can get

Hence, we can get

i.e.

Next, we try to proof, and.

By system (2), we get

By (30) and (31), we can get

which can be changed into

By the both side of Equation (35), we can get

Assume, by Stolz Theorem we obtain that

Because, then we can get the limit of

However, there exist, such that, which is conduction.

Hence, the assume does not hold. We obtain

.

Use the same method, we can also get

.

By system (2), we get

It is to see that is a period-two solution of system (2), and satisfy,.

Therefore, we complete the proof.

Corollary 3.2 Suppose that is a positive solution of system (2). Then the following statement is true:

If, ,;, , then the solution of system (2) oscillates about equilibrium.

Theorem 3.3 Assume that, ,

;, ,

, and is a positive solution of system (2). Then the system (2) has prime period two solutions, and for

Proof: By the lemma 8, we can complete the proof. Here, we omit it.