Research Article | Open Access

M. R. Ferchichi, I. Djellit, "On Some Properties of Focal Points", *Discrete Dynamics in Nature and Society*, vol. 2009, Article ID 646258, 11 pages, 2009. https://doi.org/10.1155/2009/646258

# On Some Properties of Focal Points

**Academic Editor:**B. Sagar

#### Abstract

We consider some dynamical properties of two-dimensional maps having an inverse with vanishing denominator. We put in evidence a link between a fixed point of a map with fractional inverse and a focal point of this inverse.

#### 1. Introduction

A specific class of maps, defined by , , with at least one of the components or defined by a fractional rational function, has been studied in the work in [1–10], and has very interesting properties. Some peculiar dynamical properties have been evidenced and observed in iterated maps (or in one of the inverses) having a vanishing denominator or having the form 0/0 in a point of . This characteristic has revealed new types of singularities in the phase plane, such as focal points and prefocal curves. The presence of these sets may cause new kinds of bifurcations generated by contacts between them and other singularities, which give rise to new dynamic phenomena and new structures of basins and invariant sets.

In this paper, which is a continuation of a former study [10], we present results on such singularities. We can distinguish typical scenarios between the map and its inverse. We recall definitions and main properties as given in some papers by Bischi et al. (see, e.g., [2–8]). A *prefocal curve* of a map is a set of points, whose image by one inverse is reduced into a single point called *focal point. *

We put in evidence necessary conditions and/or sufficient for a focal point to be a fixed point of the inverse map.

This paper is organized as follows. In Section 2 we present some basic definitions and facts about the key role of focal points and prefocal curves. In Section 3, we give our main results about the link between the property of the basin of attraction of a map being connected (resp., nonconnected) and focal points of being inside (resp., outside) of this attraction basin, and the link existing between fixed point of this map and the focal point of its inverse. Section 4 is devoted to applications to illustrate our results, two examples are considered with one focal point and two focal points.

#### 2. Definitions and Geometric Localization of Focal Point, Prefocal Curve

In this paper, we use the notation introduced in [6–8]. Definitions and generic properties given here concern fractional maps of the form:
where it is assumed that the functions , and are polynomial functions defined from to . The *set of nondefinition* of is given by
In the following, we will suppose that is a smooth curve in the plane. The initial conditions belong to , given by

In order to define the concepts of *focal point* and *prefocal curve*, we consider a smooth arc transverse to and we study the shape of its image under , that is, .

*Definition 2.1 (see [6]). *Consider the map (2.1). A point is a focal point if at least one component of takes the form 0/0 in and there exist smooth simple arcs , with , such that is finite. The set of all these finite values, obtained in different arcs , is called a prefocal curve .

We can compute prefocal curve and the focal point analytically by the following method (see [6]), provided that the inverse map is known explicitly. We search the set for which vanishes, and we calculate the images, under , points of . If contains a curve such that is reduced to a point , thus is a prefocal curve for the map and is then the associated focal point.

The presence of a vanishing denominator induces important effects on the geometrical and dynamical properties of the map. In this case, we need to locate geometrically the focal point in the phase plane. This concept is stated and proved in the following proposition [10].

Proposition 2.2. *Let be a fractional rational map with a unique polynomial inverse . If crosses transversely in a point , where is the set of nondefinition of , then is a focal point of .*

Corollary 2.3. *Let be a focal point of , if and , then belongs to .*

Some of such behaviors can also be observed in maps without a vanishing denominator, but their inverses have a vanishing denominator.

Corollary 2.4. *Let be a polynomial map and its unique fractional inverse. If crosses transversely in a point , where is the set of nondefinition of , then is a focal point of .*

#### 3. Properties of Focal Points and Prefocal Curves

We now describe our main results concerning several focal points in more detail, we confirm and extend some others for the simplest case with one focal point as stated in [10], and similar properties continue to hold.

The following proposition shows the link between the structure of the basin of attraction of an attractor of and the fact that the focal points of are inside or outside the basin.

Proposition 3.1. *Let be a smooth two-dimensional map, with a unique fractional and rational inverse being its set of nondefinition, and , , its focal points. Let be the basin of attraction of an attracting set of of period-1. Suppose that for any , , such that is the branch of containing the focal point . Therefore, the basin is connected, if and only if all focal points belong to .*

*Proof. *Let be the immediate basin of , the boundary of the basin , and the prefocal curves of associated, respectively, with focal points , .

First, we recall the following implications:

(1)(2)(3)(4), and in this case (as restricted to is invertible).Let us assume that all cross the immediate basin of . Otherwise, if a branch crosses an island without crossing , we reason on this island as follows.

We denote by and the parts of located on either side of with , . Since has a unique inverse and that is the set of points at infinity of plus , then and are on either side of .

( 1) We show that if is connected, then all belong to .

Suppose the contrary, in other words there exists , this implies that and therefore, according to the equivalence (3) mentioned above, has no intersection with . Besides, as and are on either side of , this implies that . Therefore, is not connected and then is not connected. This contradicts the hypothesis, *D* is connected, then .

( 2) Conversely, let us prove now that if all belong to *D*, then *D* is connected.

Suppose that all belong to . If a focal point belongs to an island , we reason on this island as follows.

Let , from the equivalence (2), we have . Since and are on either side of , therefore, in order that contains , it is necessary that and merge, this means that . This is true for all prefocal curves , then is connected. Since, by the definition of the attraction basin, contains and taking into account that is the biggest connected component containing the attractor, then contains . This means that . Therefore, and then is connected.

*Remark 3.2. *The proposition does not apply for the case of the periodic attractors of period . Intuitively, some analogies with Proposition 3.1, as considering , would be possible, but more work is needed to precise this fact.

The proposition which follows, localizes a focal point of , when it is a fixed point of .

Proposition 3.3. *Let be a smooth planar map, let be, its unique and fractional inverse and let be a focal point of . Then, is a fixed point of if and only if .*

*Proof. * fixed point of .

The following proposition made the link between prefocal curves of and of associated with the point focal and the fact that this last is a fixed point of .

Proposition 3.4. *Let be a smooth planar map and its unique and fractional inverse. Let and be the prefoal curves of and of associated with the focal point . Then we can claim that:
*

*Proof. *( 1) Let us show that is a necessary condition for to be a fixed point of . Let , then the Jacobian , thus . Besides, as is a fixed point of , for all we have

Thus, one showed that .

( 2) Now let us show that is a sufficient condition for to be a fixed point of . Let then we obtain

Consequently, is a fixed point of .

We can give another necessary condition so that a focal point of is a fixed point of , given by this proposition.

Proposition 3.5. *Let be a smooth planar map and its unique and fractional inverse, and let be a focal point of . If is a fixed point of , then an eigenvalue of the Jacobian matrix of in is null.*

*Proof. *Let be a focal point of , this implies that on . As is a fixed point of , consequently, and then . This last () is necessary and sufficient to have a null eigenvalue, being the determinant of a matrix equal to the product of their eigenvalues.

#### 4. Applications

*Example 4.1. *Case of a polynomial mapping, having a unique fractional inverse with a unique focal point.

We consider this map

where , and are real parameters. Its unique inverse is given by
The nondefinition set of is given by
However, has a prefocal curve of equation
associated with the focal point .

First let us fix , and we vary the parameter . We have the following situations.

( 1) For the map has a saddle fixed point of coordinates (1.469,1.469), whose invariant stable manifold delimits the basin of attraction of a stable focus of coordinates (1.021,1.021). We can see in Figure 1 that the prefocal curve and the focal point are outside the basin that this last is not connected and is constituted of an immediate basin containing and of islands with a shape of crescents.

( 2) For the value , the focal point is on the prefocal curve . From Proposition 3.3, is then a fixed point of . It is merged with the saddle fixed point , whose invariant stable manifold delimits the basin of attraction of the stable focus . in Figure 2, we can see that there exists a contact between the boundaries of the immediate basin and islands, so the closure of the total basin is connected. We can see also that the prefocal curve delimits a part of the basin of attraction, which implies that is on the boundary of the basin.

( 3) For the value , the iterates of focal point converge toward the attractor, thus the focal point and prefocal curve are inside the basin. We can see, in Figure 3, that the attractor is now an invariant closed curve, resulting from a Neimark-Hopf bifurcation for the value . We see that islands aggregate to the immediate basin. Therefore, the basin is connected. This value of the parameter is at the same time, a basin-bifurcation value “connected nonconnected” (as established in Proposition 3.1) and Neimark-Hopf bifurcation value.

*Example 4.2. *Case of polynomial mapping, having a unique fractional inverse with two focal points is considered.

We consider the following example:

where *a* and *b* are real parameters. Its unique inverse is given by
The set of nondefinition of is constituted of two branches and :
has two prefocal curves:
associated, respectively, with the two focal points:

( I) Let us check that , , if and only if is not a fixed point of (Proposition 3.4). We have this implies that

The images of these curves, by , give us
We deduce that is a prefocal curve of associated with the focal point , and hence if is not fixed point of , is equal to
with . When is a fixed point of , is equal to
Thus .

The result is quite similar for the focal point .

(II) Study of the basin bifurcation “connected nonconnected”

Fix and we vary the parameter .(1)For the value , we see in Figure 4 that the basin of attraction is connected, and the two prefocal curves and the two focal points are inside the basin.(2)For the value , we see in Figure 5 that the basin of attraction is not more connected. and are outside of the basin (Proposition 3.1).(3)For the value , we can see in Figure 6 that the basin is not connected and the two prefocal curves do not belong to the basin.

(III) We can obtain basin bifurcations curves, when the focal points and are fixed points of on the boundary of the basin.

has three fixed points , , of coordinates
According to Proposition 3.3, is a fixed point of , if and only if , then , which gives the curve for and .

For , is an attractive fixed point (with eigenvalues ), hence is not on the boundary of a basin. For *a* = 1, is not a fixed point of .

By identification of with the fixed points , for , we obtain

Similarly, is a fixed point of if and only if , then , which gives for . For *a* =0, is an attractive fixed point.

By identification of with fixed points , for , we obtain

In the parameter plane (), the curves and can be bifurcation curves “connected nonconnected” (when an attractor exists and that the fixed points , are on the boundary of the basin of this attractor), if between these two curves, a focal point cannot reach the boundary of the attraction basin without being a fixed point. Otherwise, it will be basin bifurcations curves, in the sense that the number of connected components of the basin changes while crossing them.

#### 5. Conclusion

Fractional maps have suggested interesting analysis of the dynamic behavior and showed that they have their own characteristics, and have revealed new types of singularities considered as theoretical tools for analyzing the bifurcation phenomena. In this paper, we have presented complementary results that appear not to have been given in the works cited therein. We have proved some results, the first permits to localize geometrically the focal point in the phase plane. The second result makes the link between the property of the basin of attraction of an attractor of a map with a unique inverse being connected (resp., nonconnected) and focal points of being inside (resp., outside) of this attraction basin. The third and last result gives the link between fixed point and focal point of and . It would be interesting to study these same properties for maps with several inverses.

#### Acknowledgment

This work has been performed under the activity of the National Research Project B01120060034.

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#### Copyright

Copyright © 2009 M. R. Ferchichi and I. Djellit. 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.