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Abstract and Applied Analysis

Volume 2012 (2012), Article ID 690453, 27 pages

http://dx.doi.org/10.1155/2012/690453

## Hopf Bifurcation of Limit Cycles in Discontinuous Liénard Systems

Department of Mathematics, Shanghai Normal University, Shanghai 200234, China

Received 26 April 2012; Accepted 8 August 2012

Academic Editor: Alberto D'Onofrio

Copyright © 2012 Yanqin Xiong and Maoan Han. 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.

#### Abstract

We consider a class of discontinuous Liénard systems and study the number of limit cycles bifurcated from the origin when parameters vary. We establish a method of studying cyclicity of the system at the origin. As an application, we discuss some discontinuous Liénard systems of special form and study the cyclicity near the origin.

#### 1. Introduction and Main Results

As well known, Liénard systems describe the dynamics of systems of one degree of freedom under existence of a linear restoring fore and a nonlinear dumping. In the first half of the last century models based on the Liénard system were important for the development of radio and vacuum tube technology. Nowadays the system is widely used to describe oscillatory processes arising in various studies of mathematical models of physical, biological, chemical, epidemiological, physiological, economical, and many other phenomena (see Glade et al. [1], Llibre [2] and references therein). Further, quadratic systems and some other systems can be transformed into Liénard systems by suitable changes, see for instance Han et al. [3], Cherkas [4], Gasull [5], Giné, and Llibre [6]. As we have seen, the main concern on Liénard systems is the center and focus problem and the number of limit cycles, see [7–17], and references therein. Here, we briefly list some known results related to our study in this paper. Consider a Liénard system of the form where we suppose that the functions and satisfy two assumptions below.(I)There exists a positive number such that is continuous for with and is continuous for with , and existing.(II)For , , where and . Obviously, if , system (1.1) has a singular point at the origin. By Han and Zhang [12], the origin is a generalized singular point if . For example, the following system: has a generalized center at the origin.

Under the condition (I), the equation has two solutions and for . Let . Then in 1952, Filippov proved the following theorem (see Chapter 5 of Ye [15]).

Theorem 1.1. *Let (I) and (II) be satisfied. Further suppose in (1.1) is continuous at . Then system (1.1) has a stable focus (resp., center, unstable focus) if
*

In 1985, Han [10] obtained the following.

Theorem 1.2. *Consider the system
**
where , and are continuous functions satisfying*(i)* with ,*(ii)* for and
where . Let satisfy for . Then system (1.4) has a stable focus (resp., center, unstable focus) if
for . *

For center conditions, in 1976, Cherkas [7] proved the following.

Theorem 1.3. *Let the functions and in system (1.1) be polynomials in with
**
Then the origin is a center of system (1.1) if and only if the equations
**
have a unique solution for sufficiently small. *

From Cherkas [7], one can see that the above result is also true for analytic system (1.1). In 1998, Gasull and Torregrosa [9] studied the center problem for analytic systems of form (1.4) using the Cherkas’ method, generalized Theorem 1.3 to (1.4) and presented interesting applications to some polynomial systems. Then in 2006, Cherkas and Romanovski [8] gave a necessary and sufficient condition for a Liénard system with nonlinearities of degree six to have a center at the origin. About Theorem 1.2, we have the following two remarks.

*Remark 1.4. *By the variable change
and the scaling of the time , we can obtain from (1.4)
where satisfies , . Thus, Theorem 1.2 is also true if is not continuous at .

*Remark 1.5. *If and are with . Then in (1.6) has the form
In this case, Han [11] proved that if for , then , and the origin is a focus of order if in addition. Further, if a vector parameter appears in , then and
for all and any .

In this paper, we consider the following discontinuous Liénard system: where with being an integer, is a function satisfying and where and are on , and , respectively, with . Further, suppose there exist integers , , , such that for and for .

Let where for and , respectively.

This paper is devoted to studying the local property of system (1.13) at the origin and the number of limit cycles bifurcated from the origin as varies. The authors [13] considered system (1.13) for the case and studied the center focus problem. It is easy to prove that for system (1.13) under (1.15) and (1.16), the condition (ii) of Theorem 1.2 is equivalent to the following: where . For convenience, introduce where are relatively prime numbers, that is, . Then it is easy to see that there must exist an integer such that is an integer, and our main results are the following.

Theorem 1.6. *Let (1.15), (1.16), (H _{1}), and (H_{2}) hold. Then*(i)

*can be expressed as where satisfies for small and for with .*(ii)

*If there exist and such that and where , then system (1.13) has cyclicity near the origin for all near .*

Theorem 1.7. *Let (1.15), (1.16), (H _{1}), and (H_{2}) hold. If there exists such that
*

*for small and (1.22) is satisfied, then system (1.13) has cyclicity at the origin for small.*

In many cases, the function in (1.13) is linear in . Then the coefficients in (1.21) are linear in . Let there exist and integer such that which implies that the linear equations , of have a unique solution of the form for near . Obviously, is linear in . Further, let for some integer . Then, we can obtain the following.

Theorem 1.8. *Consider system (1.13), where the function is linear in . Suppose (1.15), (1.16), (H _{1}), and (H_{2}) hold. Let there exist integers and points and such that (1.24) and (1.26) hold with
*(i)

*If and then system (1.13) has cyclicity limit cycles near the origin for all near .*(ii)

*If and as , , with then system (1.13) has cyclicity limit cycles at the origin for all near .*

The conclusion (i) of Theorem 1.8 can be proved in a similar manner to the proof of Theorem 2 of [16] and (ii) can be verified by Theorem 1.7. Thus, we do not verify it here. The proof of Theorems 1.6 and 1.7 is presented in Section 2. In Section 3, we give some applications.

#### 2. Proof of Theorems 1.6 and 1.7

In this section, we verify Theorems 1.6 and 1.7. Before proving them, we need to introduce a bifurcation function of system (1.13) and establish some preliminary lemmas, which will be used in our proof. First, we have the following lemma.

Lemma 2.1. *Suppose (1.18) holds. Then the function defined in Theorem 1.6 has the form
**
for with .*

* Proof. * Consider the equation
for and . By (1.18), we can obtain
where . The implicit function theorem implies that the equation has a unique solution
for small with . Let . Then, for sufficiently small is the solution of (2.2). Combining (2.3) and (2.4), the above formula can be represented as
with . Thus, the proof is ended.

Following the idea of Han [11], we have the following lemma.

Lemma 2.2. *Let (1.15) and (1.16) hold. Then system (1.13) is equivalent to
**
where , and
**
with and . *

* Proof. * Let and make the transformation
together with the scaling of the time to system (1.13) so that (1.13) can be changed into
where , and denote the inverse of the transformation (2.8). In fact, by (1.18) and similar to the proof of Lemma 2.1, we can obtain
For , by (1.15) and (2.10), we have
where . Similarly, for , by (1.16) and (2.10), we can obtain
where . This ends the proof.

For a relation between the function in (1.13) and the function in (2.6) we have

Lemma 2.3. *Suppose (1.15) and (1.16) hold. Then we have*(i)* has the form (1.21) for ;*(ii)*let . Then can be expressed as
where
with are positive functions in , and , are as appeared in (1.21). *

* Proof. * By (1.15), (1.16), and Lemma 2.1, we can have
Since is an integer, the above formula can be written as
where . Then we express (2.16) with respect to the power of in ascending order, yielding the form of (1.21). In addition, for defined in Theorem 1.6 there must exist some integers and such that . Hence, we can obtain
This completes the proof of the conclusion (i).

Now we prove the conclusion (ii). For sufficiently small, (2.8) and (2.10) imply that
Then noting that , we can get
By (2.11), (2.12) and the above formula, we have for sufficiently small
Let
Combining (2.10) and (2.21) gives that
Substituting the above formula into (1.21) or (2.16) yields that
Note that
where
with and . Similarly,
where
Combining (2.23), (2.24), and (2.26), we can obtain
Similar to (1.21) and (2.16), the above formula can be rewritten as
where from (2.17)
From (2.25), (2.27), and the above equation, in (2.29) can be rewritten as the form of (2.14). Further, combining (2.17), (2.20), and (2.29) yields (2.13). Thus, the proof is ended.

Next, we establish the bifurcation function of system (1.13). For the purpose, let . Then (2.6) can be translated into
In a similar way to the proof of Lemma 2.3 of [13], it is easy to prove that
for small under (H_{1}) and (H_{2}), which shows that in (2.31) is well defined. Introduce
Then by (2.31) and (2.33), we can obtain
where . Clearly, (2.34) is a -periodic equation in . By (2.7), we have
From (2.33) and (2.35), it is easy to see that is in for any given . Let denote the solution of (2.34) satisfying . For convenience, let
Define a function as follows:
It is obvious that system (1.13), (2.6), (2.31), or (2.34) has a periodic orbit near the origin if and only if in (2.37) has a zero in for sufficiently small. See Figure 1.

Hence, the function in (2.37) is called a displacement function or bifurcation function of system (1.13), (2.6), (2.31), or (2.34). About the bifurcation function , we have.

Lemma 2.4. *Let (1.15), (1.16), (H _{1}), and (H_{2}) hold. Then the bifurcation function in (2.37) has the form
*

*where are positive integers satisfying ,*

*with , for and are functions in satisfying .*

* Proof. *Denote by . Then
and it satisfies
Let . Then we can obtain that is a solution of the equation
satisfying . Let . Then and
where
Thus, by (2.43), (2.44) we have easily
by the mean value theorem, where . Further by (2.34) and (2.42), we can obtain
which follows that by Lemma 2.3
where is a function in with
which implies by the above discussion. Then it follows from (2.13), (2.45), and (2.48) that
where are functions in with
which shows that for all . Since and satisfy (2.42) and (2.34), respectively, using (2.46) we have
where and . Then noting (2.40), applying the formula of variation of constants to the above equation yields
Inserting (2.50) into the above formula and taking give by (2.37) that
which implies (2.38) by (2.33). Hence, The proof is completed.

In the following part, we verify Theorems 1.6 and 1.7 using the above lemmas.

*Proof of Theorem 1.6. * One can see that the conclusion (i) is true by Lemmas 2.1 and 2.3. Now, we prove the conclusion (ii). Obviously, it suffices to prove the following two points.(1)There are at most limit cycles near the origin for all near .(2)There can appear limit cycles in any given neighborhood of the origin for some arbitrarily close to .

In fact, noting that
By our assumption, we have from (2.38) and (2.55)
where ,
which implies in (2.56) are in satisfying for sufficiently close to .

We claim that in (2.56) has at most zeros in small for . We can proceed with the proof by induction on .

First, when , from (2.56), we can obtain
Note that
which implies
for and near since by (2.55). Hence, by Rolle’s theorem, the conclusion (1) is true for .

Assume that the conclusion (1) is true for . That is by (2.56)
has at most zeros for and sufficiently small with . Let us prove that the conclusion (1) is also true for . Then from (2.56), we can obtain
where , , . Then,
where , , satisfying and is in with , which implies that has at most zeros in small for near by induction assumption since . Hence, by Rolle’s theorem, it follows that in (2.62) has at most zeros in for near . This ends the proof of conclusion (1).

Now, we verify the conclusion (2). For simplicity, we can assume . Further, by (1.22), without loss of generality, suppose
Then we can fix for since . Consider the equations
By the implicit function theorem, the equations have a unique set of solutions
for near , which means can be taken as free parameters. Hence, by changing the sign of in turn such that
which follows that in (2.56) satisfy
This ensures that the bifurcation function has zeros in for some near , which implies the conclusion (2). This ends the proof.

*Proof of Theorem 1.7. *From the proof of Theorem 1.6, it is easy to see can be taken as free parameters by (1.22), which implies can also be taken as free parameters by the definition of in Lemma 2.4. From (1.23) and Lemma 2.4, we have
for sufficiently small since if and only if , which means that the coefficients in (2.38) satisfy
Since can be taken as free parameters, we can write
where . Inserting (2.71) into (2.38) gives that
where
We can easily prove in (2.72) has at most zeros for near by induction in a similar way to the above proof. Further, we can choose satisfying
which ensures that in (2.72) has zeros for near . The proof is finished.

From the proof of Theorems 1.6 and 1.7, we have immediately the following corollaries.

Corollary 2.5. *Let (1.15), (1.16), (H _{1}), and (H_{2}) hold. If there exist a point and an integer such that
*

*then the origin is a stable (resp., unstable) focus of system (1.13) for and there has at most limit cycles at the origin for all near*