# Regularization of the Boundary-Saddle-Node Bifurcation

Authors: Xia Liu
Journal: Advances in Mathematical Physics
 (2018)

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## Abstract

In this paper we treat a particular class of planar Filippov systems which consist of two smooth systems that are separated by a discontinuity boundary. In such systems one vector field undergoes a saddle-node bifurcation while the other vector field is transversal to the boundary. The boundary-saddle-node (BSN) bifurcation occurs at a critical value when the saddle-node point is located on the discontinuity boundary. We derive a local topological normal form for the BSN bifurcation and study its local dynamics by applying the classical Filippov’s convex method and a novel regularization approach. In fact, by the regularization approach a given Filippov system is approximated by a piecewise-smooth continuous system. Moreover, the regularization process produces a singular perturbation problem where the original discontinuous set becomes a center manifold. Thus, the regularization enables us to make use of the established theories for continuous systems and slow-fast systems to study the local behavior around the BSN bifurcation.


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## Body

## 1. Introduction

The study of Filippov system was motivated by its considerable applications in mechanical systems exhibiting dry friction [1–4], biological systems [5–7], and control systems [8–10]. In general, Filippov systems, or differential equations with discontinuous right-hand side, model physical processes which experience abrupt transitions between different modes. Modelling in such way we often idealize the transition to be instantaneous. The consequence of doing so is that the established discontinuous model focuses on the overall dynamics of the entire physical process while slightly ignoring the detailed dynamics in the transition stage. An efficient method to learn the dynamics in the transition stage is modelling this process in a small time scale such that the dynamics of one mode smoothly or continuously changes to the other mode. For a given Filippov system this can be realized by a smooth or continuous approximation which removes the discontinuities. This method is referred to asregularization. As a consequence, this enables us to make use of the established theories for smooth or continuous systems to study the dynamical properties of Filippov systems.Different ways have been proposed to regularize a Filippov system; see [11–13]. In this paper we apply the regularization of [12] to a particular class of planar Filippov systems. Such systems consist of two smooth vector fields that are separated by a smooth discontinuity boundary. The vector field on one side undergoes a standard saddle-node bifurcation, while the vector field on the other side intersects the boundary transversally. Here a “standard saddle-node bifurcation” means a saddle-node bifurcation in smooth systems. We refer for a formal definition of Filippov system and the regularization approach of [12] to Sections 2.1 and 2.2, respectively.Our aim of this work is to describe the local dynamics in the neighborhood of the codimension-2boundary-saddle-node (BSN) bifurcation where two equilibria of one smooth vector field go through a saddle-node bifurcation while they lie on the boundary. We treat this particular class of Filippov systems by applying both Filippov’s convex method and the regularization approach to the BSN point to obtain its local bifurcation diagram, to understand how all the codimension-1 bifurcations interact. As we shall see in Section 3 the BSN point studied by Filippov’s convex method acts as organizing center for three families of codimension-1 bifurcations appearing in its neighborhood, which are standard saddle-node bifurcation,equilibrium transition,and nonsmooth fold bifurcation. However, after regularization, some of thediscontinuity-induced bifurcations disappear, such as the equilibrium transition. The unique bifurcation that occurs in the regularized system is the saddle-node-like bifurcation; see Section 2.2. Here the extension “like” means that this bifurcation is not the standard one as known from smooth systems. We refer to [15] for a detailed description of the generic bifurcations in piecewise-smooth continuous systems.We briefly outline the structure of this paper. In the next section we give an overview of Filippov dynamics and the regularization technique of [12]. Moreover, we explain how to transform the regularized system to a singular perturbation problem. In Section 3 we construct a topological normal form for the BSN bifurcation. Subsequently, we study the BSN bifurcation by Filippov’s convex method and obtain its bifurcation diagram and phase portraits in Section 4. After that, in Section 5 we apply the regularization approach to the topological normal form and investigate its local dynamics as a piecewise-smooth continuous system. Finally, in Section 6 we summarize the main results.


## 2. Preliminaries

In this section we introduce Filippov’s convex method and the regularization approach of [12].


### 2.1. Filippov’s Convex Method

We first introduce the basic concepts in Filippov systems.


#### 2.1.1. Filippov Dynamics

A triplet(X,Y,Σ) defined on a 2-dimensional manifold M is called a Filippov system, denoted by Z. Both X and Y are Cr (r≥1 or r=∞) vector fields, which are extendable over a full neighborhood of the discontinuity boundary Σ. The boundary is given as follows: (1)Σ=x,y∈M:fx,y=0, where f:M→R is a Cr function and has 0 as a regular value. Therefore, Σ is a smooth 1-dimensional submanifold of M. As a consequence, Σ separates M to two open subsets MX and MY: (2)MX=x,y∈M:fx,y<0,MY=x,y∈M:fx,y>0.For any(x,y)∈M\Σ, the dynamics of Z is given by(3)Zx,y=Xx,y,forx,y∈MX,Yx,y,forx,y∈MY.According to Filippov’s convex method [16], Σ can be divided into thecrossing set(4a)Σc=x,y∈Σ:Xfx,yYfx,y>0and thesliding set(4b)Σs=x,y∈Σ:Xfx,yYfx,y≤0,where Xf(x,y) denotes the directional derivative of f with respect to X at the point (x,y). That is, Σ=Σs∪Σc.In particular, the dynamics of a point(x,y) at the sliding set is defined by a unique convex linear combination of X(x,y) and Y(x,y), which is tangent to Σ at (x,y). This dynamics is called thesliding vector field [16], denoted by Zs and given as follows:(5)Zsx,y=Xfx,yYx,y-Yfx,yXx,yXfx,y-Yfx,y,when Xf(x,y)≠Yf(x,y). We define Z(x,y)=0 when Xf(x,y)=Yf(x,y) for (x,y)∈Σs.


#### 2.1.2. Equilibria and Bifurcations

Here we give a brief description of the discontinuity-induced bifurcations (bifurcations where the discontinuity of the Filippov systems plays an essential role) of equilibria appearing in planar Filippov system.Definition 1 (equilibria).
A point(x,y)∈M\Σ is anordinary equilibrium of X (resp. Y) if X(x,y)=0 and (x,y)∈MX (resp., Y(x,y)=0 and (x,y)∈MY). A point (x,y)∈Σ is called aboundary equilibrium of X (resp. Y) if X(x,y)=0 (resp., Y(x,y)=0). A point (x,y)∈M is an ordinary (resp., boundary) equilibrium of Z if it is an ordinary (resp., boundary) equilibrium of X or Y. Equilibria of X in MY and of Y in MX are calledvirtual equilibria.Definition 2 (pseudoequilibria).
A point(x,y)∈Σs is apseudoequilibrium of Z if (x,y) is not a boundary equilibrium and Zs(x,y)=0. If Zs(x,y)=0 for (x,y)∈Σc, then one calls (x,y) avirtual pseudoequilibrium of Z.When an ordinary equilibrium ofZ collides with the discontinuity boundary Σ, it generically gives either anequilibrium transition (ET) or anonsmooth fold (NSF) bifurcation; compare [17]. For the ET an ordinary equilibrium collides with a virtual pseudoequilibrium and then becomes a virtual ordinary equilibrium and a pseudoequilibrium; see Figure 1(a). For the NSF bifurcation an ordinary equilibrium collides with a pseudoequilibrium and then they both become virtual; see Figure 1(b).Figure 1
Equilibrium transition and nonsmooth fold bifurcation. The discontinuity boundaryΣ is displayed as a solid line. The dashed line represents the motion of the equilibrium as a parameter changes. Round points represent ordinary equilibria (filled) or virtual equilibria (hollow). Square points represent pseudoequilibria (filled) or virtual pseudoequilibria (hollow) [14].
(a)
Equilibrium transition
(b)
Nonsmooth foldThe following theorem [14] gives conditions for the occurrence of an equilibrium transition or nonsmooth fold bifurcation in a Filippov system that smoothly depends on a parameter β.Theorem 3 (equilibrium transition and nonsmooth fold bifurcation [14]).
Assume thatX(0,0,0)=0 and that D(x,y)X(0,0,0) is nonsingular. Moreover, assume that for β=0 an equilibrium branch (x(β),y(β)) of the vector field X(x,y,β) transversally crosses the discontinuity manifold Σ at (x,y,β)=(0,0,0); that is, (6)ddββ=0fxβ,yβ,β≠0. Finally, assume that the nondegeneracy condition (7)δ≔Dx,yf0,0,0Dx,yX0,0,0-1Y0,0,0≠0 is satisfied. Then, at β=0, there is an equilibrium transition if δ>0 and there is a nonsmooth fold bifurcation if δ<0.


#### 2.1.3. Equivalence between Filippov Systems

Here we define the equivalence that will be used in this work.Definition 4 (topological equivalence [18]).
Two Filippov systemsZ and Z~ aretopologically equivalent if there is a homeomorphism φ that sends the orbits of Z to Z~ and the sliding set Σs to Σ~s.


### 2.2. Regularization Approach

In this section we introduce the regularization approach of [12] and present its properties about equilibria.According to [12] piecewise-smooth continuous regularization Rε of the Filippov system Z is given in the following way. Let(8)MR=MX∪S∪MYbe theextended manifold, where S=[-ε,ε]×Σ. That is, MR is obtained by cutting the manifold M along Σ and then glueing the two pieces together by adding the “strip” S. The regularized vector field is defined on MR as(9)Rεx,y=Xx,y,forx,y∈MX,Yx,y,forx,y∈MY,1-αxεXx,y+αxεYx,y,forx,y=x,q∈-ε,ε×Σ.Here α is a strictly increasing, smooth function defined in [-1,1] with α(-1)=0, α(1)=1. For simplicity we assume α(0)=1/2.In this paper we are interested in local dynamics of the Filippov system. SinceΣ is a submanifold of M, there are local coordinates (x,y) in a neighborhood of p such that locally Σ={(x,y)∣x=0}. In this case the regularized system can be written as(10)Rεx,y=Xx+ε,y,forx≤-ε,Yx-ε,y,forx≥ε,1-αxεX0,y+αxεY0,y,for-ε≤x≤ε.For the regularized systemRε we have the following theorem about its equilibria and stability type.Theorem 5.
The ordinary equilibria and pseudoequilibria ofZ correspond to equilibria of Rε with the same type of stability. Boundary equilibria of either X or Y (but not both) correspond to equilibria of Rε. Common boundary equilibria of X and Y give rise to a curve of equilibria of Rε in S.


### 2.3. Regularization as a Singular Perturbation Problem

In this section we briefly review singular perturbation theory. Consider a general singular perturbation problem with theslow time system(11)ddtεxy=fx,y,εgx,y,εor equivalently thefast time system, after time rescaling τ=t/ε,(12)ddτxy=fx,y,εεgx,y,ε,where x∈Rk, y∈Rm, 0<ε≪1.Theslow manifold is the set of equilibria of the fast time system for ε=0 and is given by(13)M=x,y:fx,y,0=0. Then M is said to benormally hyperbolic if for every (x,y)∈M the matrix Dxf has no eigenvalues on the imaginary axis.Forε=0 the slow time system (11) is reduced to the following system on M:(14)0dydt=fx,y,0gx,y,0,called thereduced problem.The following result of Fenichel (see [19]) on the preservation of normally hyperbolic invariant manifolds plays a central role in geometric singular perturbation theory.Theorem 6 (Fenichel’s invariant manifold theorem).
IfM is a compact set and normally hyperbolic, then, for ε>0 and sufficiently small, there exists a locally invariant manifold Mε close to M in the C1 topology. The manifold Mε is diffeomorphic to M, and the fast flow on Mε is close to the flow of the reduced equation on M.Now consider the Filippov systemZ=(X,Y,Σ) on R2. Recall that given a point p∈Σ we can find a local coordinate system (x,y) such that Σ is given by x=0 and the regularized system is given by (10). We now focus on the region S=[-ε,ε]×Σ and we perform the scaling transformation u=x/ε. This scaling induces in S′=[-1,1]×Σ the system(15)ddtεuy=1-αuX10,y+αuY10,y1-αuX20,y+αuY20,y.Rescaling time by τ=t/ε we get(16)ddτuy=1-αuX10,y+αuY10,yε1-αuX20,y+αuY20,y.System (15) is theslow time system while (16) is thefast time system. The slow manifold is given by(17)M=u,y:1-αuX10,y+αuY10,y=0.The slow time system (15) for ε=0 defines a dynamical system on M with dynamics given by(18)dydt=1-αuX20,y+αuY20,y. Accordingly the slow time system (15) for ε=0; that is,(19)0dydt=1-αuX10,y+αuY10,y1-αuX20,y+αuY20,yis called thereduced problem of the singularly perturbed system.Comparing the singular perturbation problem (15) and (16) with the original Filippov system Z, we directly derive the following result.Lemma 7.
SupposeX1(x,y)≠Y1(x,y). Then there is a diffeomorphism h:Σs→M, having the form h(0,y)=(a(y),y), where α′(u)>0 at u=a(y), that maps the flow of the sliding vector field Zs to the flow of the reduced problem on M.Remark 8.
The slow manifold and the sliding segment of the boundary manifold are homeomorphic and carry the same dynamics but their geometry is different.


## 2.1. Filippov’s Convex Method

We first introduce the basic concepts in Filippov systems.


### 2.1.1. Filippov Dynamics

A triplet(X,Y,Σ) defined on a 2-dimensional manifold M is called a Filippov system, denoted by Z. Both X and Y are Cr (r≥1 or r=∞) vector fields, which are extendable over a full neighborhood of the discontinuity boundary Σ. The boundary is given as follows: (1)Σ=x,y∈M:fx,y=0, where f:M→R is a Cr function and has 0 as a regular value. Therefore, Σ is a smooth 1-dimensional submanifold of M. As a consequence, Σ separates M to two open subsets MX and MY: (2)MX=x,y∈M:fx,y<0,MY=x,y∈M:fx,y>0.For any(x,y)∈M\Σ, the dynamics of Z is given by(3)Zx,y=Xx,y,forx,y∈MX,Yx,y,forx,y∈MY.According to Filippov’s convex method [16], Σ can be divided into thecrossing set(4a)Σc=x,y∈Σ:Xfx,yYfx,y>0and thesliding set(4b)Σs=x,y∈Σ:Xfx,yYfx,y≤0,where Xf(x,y) denotes the directional derivative of f with respect to X at the point (x,y). That is, Σ=Σs∪Σc.In particular, the dynamics of a point(x,y) at the sliding set is defined by a unique convex linear combination of X(x,y) and Y(x,y), which is tangent to Σ at (x,y). This dynamics is called thesliding vector field [16], denoted by Zs and given as follows:(5)Zsx,y=Xfx,yYx,y-Yfx,yXx,yXfx,y-Yfx,y,when Xf(x,y)≠Yf(x,y). We define Z(x,y)=0 when Xf(x,y)=Yf(x,y) for (x,y)∈Σs.


### 2.1.2. Equilibria and Bifurcations

Here we give a brief description of the discontinuity-induced bifurcations (bifurcations where the discontinuity of the Filippov systems plays an essential role) of equilibria appearing in planar Filippov system.Definition 1 (equilibria).
A point(x,y)∈M\Σ is anordinary equilibrium of X (resp. Y) if X(x,y)=0 and (x,y)∈MX (resp., Y(x,y)=0 and (x,y)∈MY). A point (x,y)∈Σ is called aboundary equilibrium of X (resp. Y) if X(x,y)=0 (resp., Y(x,y)=0). A point (x,y)∈M is an ordinary (resp., boundary) equilibrium of Z if it is an ordinary (resp., boundary) equilibrium of X or Y. Equilibria of X in MY and of Y in MX are calledvirtual equilibria.Definition 2 (pseudoequilibria).
A point(x,y)∈Σs is apseudoequilibrium of Z if (x,y) is not a boundary equilibrium and Zs(x,y)=0. If Zs(x,y)=0 for (x,y)∈Σc, then one calls (x,y) avirtual pseudoequilibrium of Z.When an ordinary equilibrium ofZ collides with the discontinuity boundary Σ, it generically gives either anequilibrium transition (ET) or anonsmooth fold (NSF) bifurcation; compare [17]. For the ET an ordinary equilibrium collides with a virtual pseudoequilibrium and then becomes a virtual ordinary equilibrium and a pseudoequilibrium; see Figure 1(a). For the NSF bifurcation an ordinary equilibrium collides with a pseudoequilibrium and then they both become virtual; see Figure 1(b).Figure 1
Equilibrium transition and nonsmooth fold bifurcation. The discontinuity boundaryΣ is displayed as a solid line. The dashed line represents the motion of the equilibrium as a parameter changes. Round points represent ordinary equilibria (filled) or virtual equilibria (hollow). Square points represent pseudoequilibria (filled) or virtual pseudoequilibria (hollow) [14].
(a)
Equilibrium transition
(b)
Nonsmooth foldThe following theorem [14] gives conditions for the occurrence of an equilibrium transition or nonsmooth fold bifurcation in a Filippov system that smoothly depends on a parameter β.Theorem 3 (equilibrium transition and nonsmooth fold bifurcation [14]).
Assume thatX(0,0,0)=0 and that D(x,y)X(0,0,0) is nonsingular. Moreover, assume that for β=0 an equilibrium branch (x(β),y(β)) of the vector field X(x,y,β) transversally crosses the discontinuity manifold Σ at (x,y,β)=(0,0,0); that is, (6)ddββ=0fxβ,yβ,β≠0. Finally, assume that the nondegeneracy condition (7)δ≔Dx,yf0,0,0Dx,yX0,0,0-1Y0,0,0≠0 is satisfied. Then, at β=0, there is an equilibrium transition if δ>0 and there is a nonsmooth fold bifurcation if δ<0.


### 2.1.3. Equivalence between Filippov Systems

Here we define the equivalence that will be used in this work.Definition 4 (topological equivalence [18]).
Two Filippov systemsZ and Z~ aretopologically equivalent if there is a homeomorphism φ that sends the orbits of Z to Z~ and the sliding set Σs to Σ~s.


## 2.1.1. Filippov Dynamics

A triplet(X,Y,Σ) defined on a 2-dimensional manifold M is called a Filippov system, denoted by Z. Both X and Y are Cr (r≥1 or r=∞) vector fields, which are extendable over a full neighborhood of the discontinuity boundary Σ. The boundary is given as follows: (1)Σ=x,y∈M:fx,y=0, where f:M→R is a Cr function and has 0 as a regular value. Therefore, Σ is a smooth 1-dimensional submanifold of M. As a consequence, Σ separates M to two open subsets MX and MY: (2)MX=x,y∈M:fx,y<0,MY=x,y∈M:fx,y>0.For any(x,y)∈M\Σ, the dynamics of Z is given by(3)Zx,y=Xx,y,forx,y∈MX,Yx,y,forx,y∈MY.According to Filippov’s convex method [16], Σ can be divided into thecrossing set(4a)Σc=x,y∈Σ:Xfx,yYfx,y>0and thesliding set(4b)Σs=x,y∈Σ:Xfx,yYfx,y≤0,where Xf(x,y) denotes the directional derivative of f with respect to X at the point (x,y). That is, Σ=Σs∪Σc.In particular, the dynamics of a point(x,y) at the sliding set is defined by a unique convex linear combination of X(x,y) and Y(x,y), which is tangent to Σ at (x,y). This dynamics is called thesliding vector field [16], denoted by Zs and given as follows:(5)Zsx,y=Xfx,yYx,y-Yfx,yXx,yXfx,y-Yfx,y,when Xf(x,y)≠Yf(x,y). We define Z(x,y)=0 when Xf(x,y)=Yf(x,y) for (x,y)∈Σs.


## 2.1.2. Equilibria and Bifurcations

Here we give a brief description of the discontinuity-induced bifurcations (bifurcations where the discontinuity of the Filippov systems plays an essential role) of equilibria appearing in planar Filippov system.Definition 1 (equilibria).
A point(x,y)∈M\Σ is anordinary equilibrium of X (resp. Y) if X(x,y)=0 and (x,y)∈MX (resp., Y(x,y)=0 and (x,y)∈MY). A point (x,y)∈Σ is called aboundary equilibrium of X (resp. Y) if X(x,y)=0 (resp., Y(x,y)=0). A point (x,y)∈M is an ordinary (resp., boundary) equilibrium of Z if it is an ordinary (resp., boundary) equilibrium of X or Y. Equilibria of X in MY and of Y in MX are calledvirtual equilibria.Definition 2 (pseudoequilibria).
A point(x,y)∈Σs is apseudoequilibrium of Z if (x,y) is not a boundary equilibrium and Zs(x,y)=0. If Zs(x,y)=0 for (x,y)∈Σc, then one calls (x,y) avirtual pseudoequilibrium of Z.When an ordinary equilibrium ofZ collides with the discontinuity boundary Σ, it generically gives either anequilibrium transition (ET) or anonsmooth fold (NSF) bifurcation; compare [17]. For the ET an ordinary equilibrium collides with a virtual pseudoequilibrium and then becomes a virtual ordinary equilibrium and a pseudoequilibrium; see Figure 1(a). For the NSF bifurcation an ordinary equilibrium collides with a pseudoequilibrium and then they both become virtual; see Figure 1(b).Figure 1
Equilibrium transition and nonsmooth fold bifurcation. The discontinuity boundaryΣ is displayed as a solid line. The dashed line represents the motion of the equilibrium as a parameter changes. Round points represent ordinary equilibria (filled) or virtual equilibria (hollow). Square points represent pseudoequilibria (filled) or virtual pseudoequilibria (hollow) [14].
(a)
Equilibrium transition
(b)
Nonsmooth foldThe following theorem [14] gives conditions for the occurrence of an equilibrium transition or nonsmooth fold bifurcation in a Filippov system that smoothly depends on a parameter β.Theorem 3 (equilibrium transition and nonsmooth fold bifurcation [14]).
Assume thatX(0,0,0)=0 and that D(x,y)X(0,0,0) is nonsingular. Moreover, assume that for β=0 an equilibrium branch (x(β),y(β)) of the vector field X(x,y,β) transversally crosses the discontinuity manifold Σ at (x,y,β)=(0,0,0); that is, (6)ddββ=0fxβ,yβ,β≠0. Finally, assume that the nondegeneracy condition (7)δ≔Dx,yf0,0,0Dx,yX0,0,0-1Y0,0,0≠0 is satisfied. Then, at β=0, there is an equilibrium transition if δ>0 and there is a nonsmooth fold bifurcation if δ<0.


## 2.1.3. Equivalence between Filippov Systems

Here we define the equivalence that will be used in this work.Definition 4 (topological equivalence [18]).
Two Filippov systemsZ and Z~ aretopologically equivalent if there is a homeomorphism φ that sends the orbits of Z to Z~ and the sliding set Σs to Σ~s.


## 2.2. Regularization Approach

In this section we introduce the regularization approach of [12] and present its properties about equilibria.According to [12] piecewise-smooth continuous regularization Rε of the Filippov system Z is given in the following way. Let(8)MR=MX∪S∪MYbe theextended manifold, where S=[-ε,ε]×Σ. That is, MR is obtained by cutting the manifold M along Σ and then glueing the two pieces together by adding the “strip” S. The regularized vector field is defined on MR as(9)Rεx,y=Xx,y,forx,y∈MX,Yx,y,forx,y∈MY,1-αxεXx,y+αxεYx,y,forx,y=x,q∈-ε,ε×Σ.Here α is a strictly increasing, smooth function defined in [-1,1] with α(-1)=0, α(1)=1. For simplicity we assume α(0)=1/2.In this paper we are interested in local dynamics of the Filippov system. SinceΣ is a submanifold of M, there are local coordinates (x,y) in a neighborhood of p such that locally Σ={(x,y)∣x=0}. In this case the regularized system can be written as(10)Rεx,y=Xx+ε,y,forx≤-ε,Yx-ε,y,forx≥ε,1-αxεX0,y+αxεY0,y,for-ε≤x≤ε.For the regularized systemRε we have the following theorem about its equilibria and stability type.Theorem 5.
The ordinary equilibria and pseudoequilibria ofZ correspond to equilibria of Rε with the same type of stability. Boundary equilibria of either X or Y (but not both) correspond to equilibria of Rε. Common boundary equilibria of X and Y give rise to a curve of equilibria of Rε in S.


## 2.3. Regularization as a Singular Perturbation Problem

In this section we briefly review singular perturbation theory. Consider a general singular perturbation problem with theslow time system(11)ddtεxy=fx,y,εgx,y,εor equivalently thefast time system, after time rescaling τ=t/ε,(12)ddτxy=fx,y,εεgx,y,ε,where x∈Rk, y∈Rm, 0<ε≪1.Theslow manifold is the set of equilibria of the fast time system for ε=0 and is given by(13)M=x,y:fx,y,0=0. Then M is said to benormally hyperbolic if for every (x,y)∈M the matrix Dxf has no eigenvalues on the imaginary axis.Forε=0 the slow time system (11) is reduced to the following system on M:(14)0dydt=fx,y,0gx,y,0,called thereduced problem.The following result of Fenichel (see [19]) on the preservation of normally hyperbolic invariant manifolds plays a central role in geometric singular perturbation theory.Theorem 6 (Fenichel’s invariant manifold theorem).
IfM is a compact set and normally hyperbolic, then, for ε>0 and sufficiently small, there exists a locally invariant manifold Mε close to M in the C1 topology. The manifold Mε is diffeomorphic to M, and the fast flow on Mε is close to the flow of the reduced equation on M.Now consider the Filippov systemZ=(X,Y,Σ) on R2. Recall that given a point p∈Σ we can find a local coordinate system (x,y) such that Σ is given by x=0 and the regularized system is given by (10). We now focus on the region S=[-ε,ε]×Σ and we perform the scaling transformation u=x/ε. This scaling induces in S′=[-1,1]×Σ the system(15)ddtεuy=1-αuX10,y+αuY10,y1-αuX20,y+αuY20,y.Rescaling time by τ=t/ε we get(16)ddτuy=1-αuX10,y+αuY10,yε1-αuX20,y+αuY20,y.System (15) is theslow time system while (16) is thefast time system. The slow manifold is given by(17)M=u,y:1-αuX10,y+αuY10,y=0.The slow time system (15) for ε=0 defines a dynamical system on M with dynamics given by(18)dydt=1-αuX20,y+αuY20,y. Accordingly the slow time system (15) for ε=0; that is,(19)0dydt=1-αuX10,y+αuY10,y1-αuX20,y+αuY20,yis called thereduced problem of the singularly perturbed system.Comparing the singular perturbation problem (15) and (16) with the original Filippov system Z, we directly derive the following result.Lemma 7.
SupposeX1(x,y)≠Y1(x,y). Then there is a diffeomorphism h:Σs→M, having the form h(0,y)=(a(y),y), where α′(u)>0 at u=a(y), that maps the flow of the sliding vector field Zs to the flow of the reduced problem on M.Remark 8.
The slow manifold and the sliding segment of the boundary manifold are homeomorphic and carry the same dynamics but their geometry is different.


## 3. The Boundary-Saddle-Node Bifurcation

Here we first define the boundary-saddle-node (BSN) bifurcation.Definition 9 (boundary-saddle-node bifurcation).
Consider a planar Filippov systemZ=(X,Y,Σ) which smoothly depends on parameters μ∈R2. In particular, for μ=μ0 the vector field X has an equilibrium (x0,y0). Then the Filippov system Z undergoes aboundary-saddle-node (BSN) bifurcation at μ=μ0 when the following conditions hold true: (1)
the equilibrium(x0,y0) is located on the boundary; that is, f(x0,y0;μ0)=0;(2)
DxX1(x0,y0;μ0)=0, DxxX1(x0,y0;μ0)≠0, and DμX1(x0,y0;μ0)≠0, where X1 is the first component of X;(3)
Y(x0,y0;μ0) is transversal to Σμ0 at (x0,y0); that is, Yf(x0,y0;μ0)≠0;(4)
δ(μ0)=D(x,y)f(x0,y0;μ0)(D(x,y)X(x0,y0;μ0))-1Y(x0,y0;μ0) = 0.Remark 10.
Condition(2) implies that the vector field X goes through a saddle-node bifurcation at μ=μ0; condition (4) defines a degenerate situation for the boundary equilibrium (x0,y0) at μ=μ0. From Theorem 3 we know that the system goes through a nonsmooth fold bifurcation for δ<0, while it goes through an equilibrium transition for δ>0.Theorem 11 (topological normal form).
Consider a general Filippov systemZ which undergoes a BSN bifurcation at μ0. Then at the equilibrium point (x0,y0) there exist a local homeomorphism φ depending on parameters μ and a smooth invertible reparameterization μ↦λ=(ρ,ν) such that Z istopologically equivalent to the following system:(20a)Zx,y;λ=Xx,y;λ,forfx,y;λ<0,Yx,y;λ,forfx,y;λ>0,with(20b)Xx,y;λ=ν+κx2-y,(20c)Yx,y;λ=σ0,(20d)fx,y;λ=x-ρ, where κ=±1 and σ=sgn(Yf(x0,y0;μ0))=±1. All higher order terms in X, Y, and f smoothly depend on λ.Note that the vector filedX undergoes a saddle-node bifurcation at ν=0. The parameter ρ determines the passage of the equilibrium of X through the boundary Σ.Proof.
The method of proving this theorem is similar to [5]. Thus we omit it here.


## 4. Bifurcation Diagram and Phase Portraits

System (20a), (20b), (20c), and (20d) undergoes 3 different codimension-1 bifurcations: saddle-node bifurcation, equilibrium transition, and nonsmooth fold bifurcation. Next we will only describe these three bifurcations in detail for the case κ=σ=-1. The other cases follow in the same way.The vector fieldX undergoes a saddle-node bifurcation at ν=0. It has a saddle at (-ν,0) and a stable node at (ν,0) for ν>0, while it has no equilibrium for ν<0. The sliding set is given by (21)Σs=x,y:x=ρ,ρ2-ν≤0. Therefore, the sliding set is empty for ν<ρ2, while Σs=Σ for ν≥ρ2. According to (5), at the sliding set Σs, the sliding vector field Zs is defined as(22)Zsρ,y=1ρ2-ν-10y.It is direct to check that (ρ,0) is the unique attractive pseudoequilibrium.Now we check that system (20a), (20b), (20c), and (20d) satisfies all the conditions of Theorem 3. For (20a), (20b), (20c), and (20d) it is direct to check that the equilibria (±ν,0) transversally meets Σ at ρ=±ν or equivalently ν=ρ2. Moreover, (23)δλ=Dx,yf0,0,0Dx,yX0,0;λ-1Y0,0;λ=±12ν. By Theorem 3, this implies that the equilibrium (ν,0) goes through an equilibrium transition for ρ>0, while the equilibrium (-ν,0) goes through a nonsmooth fold bifurcation for ρ<0. Apart from these two boundary bifurcations, the line ν=0 for ρ>0 in the parameter plane corresponds to a standard saddle-node bifurcation; see Figure 2. The phase portraits corresponding to the open regions I, II, and III labelled in Figure 2 are given in Figure 3.Figure 2
Bifurcation diagram ofZ (20a), (20b), (20c), and (20d) in the (ρ,ν)-plane when δ=σ=-1.Figure 3
From left to right phase portraits corresponding to the open regions I, II, and III in Figure2, respectively.
(a)
ρ
=
1, ν=-0.5
(b)
ρ
=
1, ν=0.5
(c)
ρ
=
1, ν=2


## 5. Regularization of the BSN Bifurcation

In this section we apply the regularization approach (9) to the normal form of the BSN bifurcation and study the bifurcations of the regularized system.Following (9) the regularization of (20a), (20b), (20c), and (20d) for ε>0 is defined as follows:(24)Rεx,y=Xx+ε,y,forx≤ρ-ε,Yx-ε,y,forx≥ρ+ε,1-αxεXρ,y+αxεYρ,y,forρ-ε≤x≤ρ+ε,where the smooth function α is defined in [ρ-ε,ρ+ε] with α(ρ-ε)=0, α(ρ+ε)=1. For our computations we have chosen α(x)=(x-ρ+1)/2.Now we consider the bifurcations that occur in the regularized system (24). Before that we first define the following bifurcation for piecewise-smooth continuous system.Saddle-Node-Like Bifurcation (SNL). A “saddle” and a “node” collide at a certain parameter value and then both disappear.We note that the “saddle” and “node” in the definition are not restricted to a standard saddle and a standard node. They can also be pseudosaddle and pseudonode. The extension “like” means that these bifurcations are not the standard ones as known from smooth systems. A detailed description of bifurcations in piecewise-smooth continuous systems is referred to in [15].


### 5.1. Equilibrium Transition

Recall that(ν,0) is an ordinary stable node for ν<ρ2,ρ>0. It hits Σ for ν=ρ2 and becomes a boundary node. Then it is replaced by a stable pseudonode on Σ for ν>ρ2. By Theorem 5, this equilibrium is still an equilibrium of Rε after regularization with different position. It is located in MX for ν<ρ2, on the line x=-ε for ν=ρ2, and in the region (-ε,ε) for ν>ρ2. Thus, the regularized system Rε does not experience any qualitative change as ν varies. This type of discontinuity-induced bifurcation disappears after regularization, see Figure 4.Figure 4
(a) Equilibrium transition between an ordinary node and a pseudonode. (b) The corresponding regularization. (c) The corresponding slow-fast flow.
(a)
(b)
(c)


### 5.2. Nonsmooth Fold

Recall that forν>ρ2,ρ<0, an ordinary saddle (-ν,0) and a stable pseudonode (ρ,0) coexist, colliding on Σ for ν=ρ2 and then both disappear for ν<ρ2,ρ<0. By Theorem 5 these two equilibria both preserve in their regularization Rε with the same type of stability. To be precise, the saddle of Rε is located in MX, while the stable node is located in the region (-ε,ε) for ν>ρ2. The unique equilibrium of Rε is located on the line x=-ε for ν=ρ2. However, there is no equilibrium in the regularized system for ν<ρ2. Notice that Rε is a piecewise-smooth continuous system; then a saddle-node-like bifurcation occurs in Rε; see Figure 5.Figure 5
(a) Nonsmooth fold bifurcation in the case of boundary node. (b) The corresponding regularization. (c) The corresponding slow-fast flow.
(a)
(b)
(c)


### 5.3. Standard Saddle-Node Bifurcation

Finally, the discontinuous system (20a), (20b), (20c), and (20d) has a standard saddle at (-ν,0) and a standard stable node at (ν,0) for any ν>0, colliding for ν=0, and then both disappear for ν<0. By Theorem 5 the equilibria for different ν persist after regularization. Thus a saddle-node-like bifurcation occurs in the regularized system Rε; see Figure 6.Figure 6
(a) A standard saddle-node bifurcation. (b) The corresponding regularization. (c) The corresponding slow-fast flow.
(a)
(b)
(c)From the above results we find that the saddle-node-like bifurcation is the unique bifurcation occurring in the regularized system, which has codimension-1; see Figure7. The phase portraits corresponding to the open regions I and II are presented in Figure 8.Figure 7
Bifurcation diagram of the regularization of system (20a), (20b), (20c), and (20d) in (ρ,ν) plane when δ=σ=-1.Figure 8
From left to right phase portraits corresponding to the open regions I and II in Figure7, respectively.
(a)
(b)


## 5.1. Equilibrium Transition

Recall that(ν,0) is an ordinary stable node for ν<ρ2,ρ>0. It hits Σ for ν=ρ2 and becomes a boundary node. Then it is replaced by a stable pseudonode on Σ for ν>ρ2. By Theorem 5, this equilibrium is still an equilibrium of Rε after regularization with different position. It is located in MX for ν<ρ2, on the line x=-ε for ν=ρ2, and in the region (-ε,ε) for ν>ρ2. Thus, the regularized system Rε does not experience any qualitative change as ν varies. This type of discontinuity-induced bifurcation disappears after regularization, see Figure 4.Figure 4
(a) Equilibrium transition between an ordinary node and a pseudonode. (b) The corresponding regularization. (c) The corresponding slow-fast flow.
(a)
(b)
(c)


## 5.2. Nonsmooth Fold

Recall that forν>ρ2,ρ<0, an ordinary saddle (-ν,0) and a stable pseudonode (ρ,0) coexist, colliding on Σ for ν=ρ2 and then both disappear for ν<ρ2,ρ<0. By Theorem 5 these two equilibria both preserve in their regularization Rε with the same type of stability. To be precise, the saddle of Rε is located in MX, while the stable node is located in the region (-ε,ε) for ν>ρ2. The unique equilibrium of Rε is located on the line x=-ε for ν=ρ2. However, there is no equilibrium in the regularized system for ν<ρ2. Notice that Rε is a piecewise-smooth continuous system; then a saddle-node-like bifurcation occurs in Rε; see Figure 5.Figure 5
(a) Nonsmooth fold bifurcation in the case of boundary node. (b) The corresponding regularization. (c) The corresponding slow-fast flow.
(a)
(b)
(c)


## 5.3. Standard Saddle-Node Bifurcation

Finally, the discontinuous system (20a), (20b), (20c), and (20d) has a standard saddle at (-ν,0) and a standard stable node at (ν,0) for any ν>0, colliding for ν=0, and then both disappear for ν<0. By Theorem 5 the equilibria for different ν persist after regularization. Thus a saddle-node-like bifurcation occurs in the regularized system Rε; see Figure 6.Figure 6
(a) A standard saddle-node bifurcation. (b) The corresponding regularization. (c) The corresponding slow-fast flow.
(a)
(b)
(c)From the above results we find that the saddle-node-like bifurcation is the unique bifurcation occurring in the regularized system, which has codimension-1; see Figure7. The phase portraits corresponding to the open regions I and II are presented in Figure 8.Figure 7
Bifurcation diagram of the regularization of system (20a), (20b), (20c), and (20d) in (ρ,ν) plane when δ=σ=-1.Figure 8
From left to right phase portraits corresponding to the open regions I and II in Figure7, respectively.
(a)
(b)


## 6. Conclusions

This paper investigated the boundary-saddle-node (BSN) bifurcation by Filippov’s convex method and the regularization approach. This is a codimension-2 bifurcation of Filippov systems. We derived the topological normal form of the BSN bifurcation and gave a detailed description of its bifurcations and phase portraits. After regularization the Filippov system becomes a piecewise-smooth continuous system. Some of the discontinuity-induced bifurcations disappear, such as equilibrium transition. The unique bifurcation that occurs in the regularized system is the saddle-node-like bifurcation. The BSN point becomes a codimension-1 bifurcation in the piecewise-smooth continuous system, which makes our discussion easier. The regularization approach enables us to make use of the established theories for continuous systems and slow-fast systems to study the local behavior around the BSN bifurcation. We will apply the regularization approach to more complicated Filippov systems involving limit cycles in the future work, where more advantages of our approach in proving the existence and uniqueness of limit cycles will be presented.



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*Source: 5094878-2018-01-04.xml*