1. Introduction
In 1976, the first version of the 1D logistic map was introduced and modeled by May [
1]. The model was used to study the behavior of some biological species in each generation. However, the mathematical model of this map was simple, but May detected some complex dynamic characteristics about its trajectories. It was described by the following first-order difference equation,
where
x must lie in the interval
otherwise some disadvantages such as population distinction may exist. The parameter
has a significant impact on the dynamics of this equation and has been restricted in the interval
. Some of these dynamics are flip bifurcation, open periodic windows and chaos. These complex dynamics were examined in [
1]. That work opened the gate to several studies on logistic systems that can be used to simulate other complex systems in nature. Indeed, there are many phenomena in different disciplines that have been modeled by logistic systems such as economy, biology, physics and chemistry. For instance, a 2D coupled logistic system was studied in [
2]. In [
3], a symmetrical coupled nonlinear logistic system was investigated and adopted in an encryption process. A simple coupled logistic system was used to examine the influences of spatial heterogeneity on population dynamics [
4]. Heterogeneity and the impact of spatial structure on population dynamics were analyzed in [
5]. The complex dynamics of a kicked logistic map were investigated in [
6]. In [
7], a multimodal family of a logistic map that possesses a single parameter was introduced. Other studies on such systems and their complex dynamic characteristics can be found in the literature ([
8,
9,
10,
11,
12,
13,
14]). Some interesting works that have studied hyperchaotic properties have been reported. For instance, in [
15] a 2D hyperchaotic map with a simple algebraic structure was introduced and studied. In order to enhance chaos complexity in 2D maps, a chaotic 2D map was proposed in [
16]. Other interesting results that give more insights about the analysis carried out in this paper have been reported (see [
17,
18,
19,
20]).
In the current paper, we study the 2D nonsymmetrical logistic map given in [
21]. Even though the mathematical expression of the model is simple, it possesses some periodic cycles and strange attractors with complex attractive basins and some of those attractors have fractal dimensions. They are some transient dynamic behaviors that have not been previously detected in the literature. The map contains two parameters that are restricted in two different intervals, as we show later. Studying the influence of each parameter separately while the other parameter is fixed gives rise to periodic cycles with simple attractive basins; however, it also results in some strange chaotic attractors that have not been reported before. The second parameter seems to have a great influence on the dynamics of the map. The phase plane of the map according to varying this parameter while keeping the first parameter fixed contains several bifurcated points that were not reported in the literature. Interestingly, the influences of those parameters give rise to different attracting behaviors when they take different values in the intervals. Those attracting behaviors include periodic cycles with complex attractive basins and strange attractors whose dimensions are fractals. Furthermore, the phase plane of the map is divided into many regions where preimages coexist, and as a result, the noninvertibility of the map is discussed. Briefly, the main contribution in this paper is the fractal dimension detected for some strange attractors in this simple 2D logistic map. Heteroclinic cycles are detected for some chaotic attractors. There are many approaches to find a numerical approximation of heteroclinic cycle. In this paper, we apply the procedures given in [
22] for that purpose. Furthermore, there are three different types of some periodic cycles found under certain parameter values. For instance, the numerical simulation shown later detects three different types of period 2-cycle. The first one consists of two periodic points lying in a straight line parallel to the
x-axis, the second one lies in a line parallel to
y-axis, and the third one lies in the diagonal. The basins of attraction of the first type seem to be more complex as they contain holes that formed due to points from the divergent and nonconvergent points.
The paper is summarized as follows. In
Section 2, we present the model and the stability of its fixed points.
Section 3 studies the local and global analysis using some numerical simulations by which different periodic cycles with their basins and chaotic attractors are analyzed. In
Section 4, the critical curves for the noninvertible map are obtained. Finally, we present our conclusion.
2. 2D Logistic Map
In this section, we study the complex dynamic characteristics of the following 2D logistic map [
21].
where the parameters
and
are restricted in the intervals
and
, respectively. These parameters are of great importance when studying the complex dynamic features of the map (2). They represent the bifurcation parameters of the map. Biologically,
represents two species that evolve logistically. On varying time the interaction between those two species depends on the growth rate represented by
and
. The interaction between those species represented by the coupled logistic (2) simulates the predator-prey interaction on which we assume
that is constant and
that is an increasing linear growth rate. The map (2) belongs to the group of unidirectional coupled maps; however, these growth rates are simple but they give interesting chaotic behaviors for the map (2). Our restrictions on these parameters differ from those in [
1] on the second parameter
. This restriction on the second parameter comes from numerical simulations performed in laboratory in the next section. The map (2) admits three fixed points,
and
. These points are obtained by setting
and
.
Proposition 1. (1) The fixed point is locally stable provided that otherwise it is unstable. (2) The fixed point is locally stable provided that and . (3) The fixed point is locally stable provided that and .
Proof. (1) The Jacobian at this point becomes . It has only two eigenvalues and . It is locally stable if . (2) The Jacobian at this point becomes . The eigenvalues are, and . The condition completes the proof. (3) It is similar to the previous one. □
Proposition 2. The map undergoes the following types of bifurcations at the fixed point :
Proof. A transcritical or fold bifurcation can occur when where and are the trace and determinant of the Jacobian matrix at the point . Solving completes part 1 of the proof. A flip bifurcation can occur when and this completes the proof of the second part. □
3. Local and Global Analysis
Let us assume the following parameters,
with the initial datum
. At this set we have
. The Jacobian matrix at this point becomes
and hence the eigenvalues become
and
. It is clear that
and
, which means that
is a nonhyperbolic point; therefore, the second condition in the stability triangle
where
and
are the trace and determinant of the Jacobian matrix, respectively, equals 0. This means that at this point the period 2-cycle is born as one of the eigenvalues will equal −1 then further increasing in the parameter
gives rise to a series of period-doubling cycles and thus the nonhyperbolic fixed point
loses its stability.
Figure 1a shows the bifurcation diagrams on varying both parameters. The simulation shows that both parameters have a complex impact on each other and on the map’s dynamic that enters chaotic region as confirmed by largest Lyapunov exponent given in
Figure 1b. In order to see that, we separately study the influences of
on the dynamics of the map while the other parameter is fixed. Increasing the bifurcation parameter
to
, a period-2 cycle emerges due to flip bifurcation. Its basins of attraction is plotted in
Figure 1c along with the fixed point
. The fixed point
in all figures in this paper is denoted by a circle whereas the periodic cycle points are denoted by squares. At
the period 4-cycle appears and is plotted in
Figure 1d with its basins of attraction. Increasing this parameter further gives rise to higher periodic cycles as shown in
Figure 1e (period 8-cycle) and
Figure 1f (period 16-cycle).
At
and
, the behavior of the map becomes chaotic as it enters chaos region. This chaotic behavior consists of four disconnected chaotic areas as displayed in
Figure 2a. Increasing the parameter
further makes the four chaotic areas gather into two chaotic areas. The phase plane given in
Figure 2b shows these two disconnected chaotic areas at
and
. These two areas continue to appear until
, where only one piece of chaotic attractor takes place as presented in
Figure 2c. This is because there are heteroclinic cycles connecting those two chaotic areas via the saddle point
. In
Figure 2d, this chaotic attractor becomes more complex as the parameter increases.
On the other hand, we study the influences of the parameter
on the dynamics of the map while fixing the other parameter. Assuming
we get a dynamic situation of the map consisting of two similar forks (or one can say, transient dynamic behaviors) as shown in
Figure 2e. This is because the parameter’s value
is a period-doubling bifurcation point; therefore, the map’s behavior at those parameters’ values is divided into two parts just like the 1D bifurcation diagram. Such transient dynamics are due to the fact that the first variable shows endless damped oscillations because
is a bifurcation point. Oscillations of
through the term
affect the second equation
. At high values of
the dynamics of
tend towards chaotic dynamics while low
makes the dynamics tend toward a stable regime; however, when
becomes constant, there is a steady-state dynamical regime. Increasing
further makes these two forks become larger as displayed in
Figure 2f. This behavior changes into four transient dynamics parts as
increases to
—these are displayed in
Figure 3a. As
increases, the transient dynamics diagrams in the phase plane are doubled.
Figure 3b shows eight parts of the transient dynamics at
. At
, the transient dynamics become large. A further increase in this parameter gives rise to different chaotic behaviors of the map as given in
Figure 3d,e. This numerical simulation shows interesting behaviors of the 2D logistic map that have not been reported in the literature. This leads us to investigate the influence of those parameters together.
Figure 3f shows the 2D bifurcation diagram for both parameters and presents the regions in the phase plane where different periodic cycles coexist when varying both parameters. The shaded region represents the stable region of the point
and the other colors refer to different types of period cycles as shown in the figure and its caption.
Now, we study the situation when both parameters are different and give rise to periodic cycles with basins of attraction that include holes from divergent or nonconvergent points in the phase plane. The numerical simulation shows that there are three types of period 2-cycles. The first one consists of two points located vertically on the same line; the second one lies horizontally; the third one is located on a line parallel to the diagonal. In
Figure 4a we display the period 2-cycle with its basins of attraction. The yellow color refers to the divergent points in the phase plane, the dark and light gray denote the period-2 basins of attraction. This cycle is born at the parameters’ values,
and
. In
Figure 4b we give the basins of attraction of the period 4-cycle around the fixed point at the parameters’ values,
and
. It is obvious that the attractive basins are more complex than period 2-cycle. The basins of attraction become more complex due to the increasing numbers of divergent holes when the period 5-cycle coexists. Furthermore, the region of divergent points in the phase plane increased. It is clear in
Figure 4c that the points of the period 5-cycle and the fixed point
lie in a perpendicular line. Another interesting periodic cycle is period-6 which has six periodic points distributed on two sides around the fixed point
.
Figure 4d displays this cycle with its basins of attraction. The period 7-cycle emerges at the parameters’ values,
and
. It has complex basins like the period 5-cycle, as shown in
Figure 4e. It has seven periodic points located on the same line where
exists. It is worth mentioning here that the numerical experiments carried out have shown three different types of period 7-cycle. We have focused only on the one with the complex basins of attraction that have more yellow holes from the divergent points. In
Figure 4f, the basins of attraction for period 8-cycle are given. In addition, all the types of period-3 and period 9-cycles have no complex basins of attraction. They have the same basins as those plotted in
Figure 1. In
Figure 4g, we depict the basins of period 10-cycles, which also have complex basins. It should be noted that there are some chaotic attractors of the map, which are born for different values of the parameters
and
. In
Figure 4h,I we give two different chaotic attractors that take place at
and
, respectively. Interestingly, the results obtained so far have made us to detect the fractal dimension for some strange attractors for the map. For the strange attractor given in
Figure 2c, the two Lyapunov exponents are
and
and according to [
23], it has the fractal dimension
, whereas for the strange attractor given in
Figure 4h one gets
and
and then
. The above numerical experiments for the basins of attraction for different periodic cycles and chaotic attractors lead us to investigate some of the geometric properties of the map. Even though the map has a simple form, it has complex basins of attraction due to the coexistence of holes of divergent points and some strange chaotic attractors that deserve to be studied. The next section analyzes the noninvertibility and critical curves of the current 2D logistic map.
4. Noninvertible Map and Critical Curves
It is clear that the map (2) is trapped to the origin point
. This means that setting
or
makes
or
, respectively. This point can be used to identify the boundaries of the basins of attraction of any attracting set for the map (2). In order to discuss the structure of the basins we put
and
in (2). It means that the time evolution of the map is attained by the iteration of the map
as follows
The map
T is called invertible [
24] if there exists an inverse
such that
is unique in each point in the range. The point
is called a rank-1 image while the set
is called the rank-1 preimages where
. Now, the map
T is called a noninvertible map if an image
has at least two rank-1 preimages. To investigate that, the following proposition is raised.
Proposition 3. (1) The origin point has four real rank-1 preimages. (2) Any point in the form that lies in the invariant x-axis has four real rank-1 preimages if otherwise it has no real rank-1 preimages. (3) Any point in the form that lies in the invariant y-axis has two real rank-1 preimages if otherwise it has no real rank-1 preimages. (4) Any point in the form has four real rank-1 preimages provided that and otherwise it has no real preimages.
Proof. (1) Setting
in (3) gives the four points
and
. (2) Setting
and
in (3) then solving it algebraically completes the proof. (3) The proof is the same as the previous one. (4) Solving (3) algebraically with respect to
x and
y we get,
These points are real if and otherwise they are complex. The proof is completed. □
Now we conclude that the map T may have or 4 real rank-1 preimages and hence it is a noninvertible map. Furthermore, the boundaries of any attracting set of the map (3) will be represented by the line segments and their preimages and , respectively.
Proposition 4. Let and thenforms the set of all rank-n preimages. Using (3), one can get
:
or
and
or
. Hence, any attracting set will have basins of attraction that are bounded by these line segments and their preimages. The phase plane of the map
T can be divided into many regions denoted by
where the index
i refers to the number of real rank-1 preimages under the map
T. In order to identify these regions we should calculate the critical curve
that requires to first get
and hence
. Since
T is continuous and differentiable then
is obtained by vanishing the determinant of the Jacobian of
T. It is represented by the following union
where,
It is clear that
does not depend on
and
. Using (3) and (5)
is represented by the union
where,
Both
and
are plotted in
Figure 5. It is clear that
separates the region
whose points have four real rank-1 preimages from the region
whose points have two distinct rank-1 preimages.
separates at some intervals the region
from the
region [
24]. In addition, all the corner points
and
have no preimages and hence they belong to
region. Any points in the form
and
belong to
region.