#### Abstract

Here we give a very short and elegant proof of an extension of two recent results on global periodicity of two systems of difference equations in a work of Elsayed et al. 2012.

#### 1. Introduction

For a solution , , , of the system of difference equations is said that it is eventually periodic with period , if there is an such that If , then for the solution is said that it is periodic with period . If all well-defined solutions of system (1.1) are eventually periodic with period , it is said that the system is -periodic.

For some periodic difference equations and systems see, for example, [111] and the references therein. Periodic systems are essentially subclasses of the systems which can be solved explicitly, an area that has again attracted attention recently (see, e.g., [1221]). For some classical results in the topic see, for example, [22].

In the recent paper [23] are formulated two results on global periodicity of particular systems of difference equations. Namely, they gave some tedious straightforward calculations which suggest that all the solutions with nonzero real initial values of the following systems of difference equations are periodic with periods and , respectively, without giving complete proofs of these statements.

#### 2. Main Result

Here we prove a result which naturally extends the results on systems (1.3) and (1.4), and give a very short and elegant proof of it.

Theorem 2.1. Assume that , , functions , , satisfy the following condition for every .
Consider the following system of difference equations
Then every solution of system (2.2) whose initial values are arbitrary nonzero real numbers is eventually periodic with period .

Proof. Since for every and , functions , , map the set to and map the set to , from (2.2) with , we see that for every .
Assume we have proved that for and every . Then from this, (2.2) and since functions , , map the set to , and map the set to , we get for every . So, by induction it follows that for every and . Hence, every solution of system (2.2) whose initial values are arbitrary nonzero real numbers is well-defined.
Multiplying all the equations in (2.2), then using condition (2.1) and the above proven fact that , for every , we get from which it follows that that is, the sequence is eventually periodic with period . In particular, the sequences are eventually periodic with period , for every .
This implies that the sequences are eventually periodic with period , which along with (2.2) implies that the sequences are eventually periodic with period , for , from which the theorem follows.

Now we present the main results in paper [23] as two corollaries.

Corollary 2.2. Assume that , , is an arbitrary solution of system (1.3) whose initial values are arbitrary nonzero real numbers. Then the solution is eventually periodic with period .

Proof. Note that for system (1.3), , , and , so that Hence, all the conditions of Theorem 2.1 are fulfilled from which the corollary follows.

Corollary 2.3. Assume that , , is an arbitrary solution of system (1.4) whose initial values are arbitrary nonzero real numbers. Then the solution is eventually periodic with period .

Proof . For system (1.4), we have that , , so that Hence, all the conditions of Theorem 2.1 are fulfilled from which the corollary follows.

The next corollary, which corresponds to case in Theorem 2.1, is obtained similarly, so we omit the proof.

Corollary 2.4. Assume that , , is an arbitrary solution of the following system , whose initial values are arbitrary nonzero real numbers. Then the solution is eventually periodic with period .

#### Acknowledgments

The second author is supported by the Grant P201/10/1032 of the Czech Grant Agency (Prague) and by the Council of Czech Government Grant MSM 00 216 30519. The fourth author is supported by the Grant FEKT-S-11-2-921 of Faculty of Electrical Engineering and Communication, Brno University of Technology. This paper is partially also supported by the Serbian Ministry of Science projects III 41025, III 44006, and OI 171007.