Open access peer-reviewed chapter

# Periodic Perturbations: Parametric Systems

By Albert Morozov

Submitted: November 21st 2017Reviewed: June 14th 2018Published: October 17th 2018

DOI: 10.5772/intechopen.79513

## Abstract

We are not going to present the classical results on linear parametric systems, since they are widely discussed in literature. Instead, we shall consider nonlinear parametric systems and discuss the conditions of new motion existence in the resonance zones: the regular ones (on an invariant torus) and the irregular ones (on a quasi-attractor). On the basis of the self-oscillatory shortened system which determines the topology of resonance zones, we study the transition from a resonance to a non-resonance case under a change of the detuning. We then apply our results to some concrete examples. It is interesting to study the behavior of a parametric system when the ring-like resonance zone is contracted into a point, i.e., to describe the bifurcations which occur in the course of transition from the plain nonlinear resonance to the parametric one. We are based on article, and we follow a material from the book.

### Keywords

• resonances
• quasi-attractor
• periodic solves
• parametric perturbations

## 1. Introduction

Consider the following system:

dxdt=Hxyy+εgxyνt,dydt=Hxyx+εfxyνt,E1

where ε>0is a small parameter, νis perturbation frequency, and g,fare continuous periodic functions of period 2πwith respect to φ=νt. The Hamiltonian Has well as fand gwill be assumed to be sufficiently smooth in a domain GR2×S1(or GR1×S1×S1=R1×T2). Also, we shall assume that the unperturbed (ε=0) Hamiltonian system is nonlinear and has at least one cell Dfilled with closed phase curves.

We especially emphasize the following condition.

Condition A. gx+fy0.

This implies that system (1) is nonconservative.

Along with (1), we shall consider the autonomous system:

dxdt=Hxyy+εg0xydydt=Hxyx+εf0xy,E2

where g0=<g>φand f0=<f>φ.

We also assume the following condition.

Condition B. System (2) has a finite set of rough limit cycles (LCs) in cell D.

Changing the variables x,yto the action Iand angle θ, we obtain the system in the form

İ=εF1Iθφθ̇=ωI+εF2Iθφφ̇=ν,E3

where

F1fxθgyθ,F2fxI+gyIE4

are periodic of period 2πwith respect to θand φ. System (3) is defined on the direct product Δ×S1×S1=Δ×T2, where T2is two-dimensional torus, Δ=II+,I±=Ih±.

The definition of resonance. We say that in system (3) a resonance takes place if

ωIpq=q/pν,E5

where p,qare relatively prime integer numbers.

The energy level I=Ipq(Hxy=hpq) of the unperturbed system is called the resonance.

The behavior of solutions in the neighborhoods

Uμ=Iθ:Ipq<I<Ipq+0θ2πC=const,μ=ε

of individual resonance levels I=IpqHxy=hpqcan be derived, up to the terms Oμ2, from the pendulum-type equation [1, 2]

d2vdτ2bA0vIpq=μσvIpqdv,E6
b=Ipq/dI,τ=μt,A0vIpq=12πp02πpFIpqv+/pφ,σvIpq=12πp02πpgxyφx+fxyφy,E7

where X=XIpqv+/p,Y=YIpqv+/pis the unperturbed solution on the level I=Ipq. For nondegenerate resonance zones we consider here, it holds that b0. Functions A0vIpq,σvIpqare periodic of period 2π/pwith respect to v.

From Eq. (7) follows.

Theorem 1

If the divergence of the vector field of Eq. (6) depends on v, then the divergence of the vector field of the original system (1) contains terms which depend on both the time t and the spatial coordinates.

In many cases the converse is also true. For example, it holds for the system

dx/dt=y,dy/dt=xx3+P1+P2x2+P3xsinνty+P4sinνt.E8

The terms mentioned in Theorem 1 are called nonlinear parametric terms. Our goal is to study systems of the form (1) with such terms. The existence of those leads to new motions in resonance zones [1, 2, 3]. We shall demonstrate these motions on examples.

## 2. Investigation of Eq. (6)

The following representations hold:

A0vIpq=AvIpq+BIpq,B=<A0>v,σvIpq=σvIpq+B1Ipq,B1=<σ>v,E9

where BIis the generating function of the autonomous system (2) and B1Iis the derivative of BI. We shall focus on the case when σis sign-alternating. In this case, from Eq. (9) follows the inequality:

B1Ipq<maxvσvIpq.E10

When studying the pendulum Eq. (6), we shall distinguish two cases: (I) BIpq0and (II) BIpq=0.

In case II system (2) has a rough limit cycle (LC) in a neighborhood of the level Hxy=hpq. There is no such cycle in case I.

Case I. Neglecting terms of order μin Eq. (6), we arrive at the integrable equation

d2v/dτ2bA0vIpq=0E11

If BIpq>maxvAvIpq, then Eq. (11) has no equilibrium states. The resonance level I=Ipqis then referred to as passable. Note that the term “passable” has its origin in the topology of the resonance zone, as opposed to the same term used in physics, where “passing” stands for a change in perturbation frequency ν. In the case under consideration, there are no periodic solutions in the vicinity of the resonance level. The most interesting case is when Eq. (11) has equilibrium states, i.e., when the condition

BIpq<maxvAvIpqE12

is satisfied. The resonance level I=Ipqis then said to be partly passable.

Under condition (10), Eq. (6) may have limit cycles. In order to find them, one must construct the Poincaré-Pontryagin generating function.

Figure 1(a) shows the phase portrait of Eq. (6) under conditions (10) and (12), and p=3. On the period 2π/3, there is a single limit cycle (note that on the period 2π(which is the period of the unperturbed solution) there are three limit cycles). If the cycle lies outside the neighborhood of the separatrix loop of Eq. (11), then there is a corresponding two-dimensional invariant torus in the original system. Since the period of the limit cycle of Eq. (11) is of order O1/μ, we then have a long-periodic beating regime in the original system (6) (the generatrices of the torus are of different order).

However, if the limit cycle lies in the neighborhood of the separatrix loop, then the two-dimensional invariant torus in the original system (1) is destroyed. The bifurcation scene in which the cycle is caught into the separatrix loop is shown in Figure 1(b). Taking into account the nonautonomous terms, which were discarded in deriving Eq. (6), leads to the homoclinic structure. Such a structure is shown in Figure 1(c) for the Poincaré map with p=3. Because of the presence of non-compact separatrices, in this case we merely have an irregular transition process.

Case II. Now, Eq. (6) always possesses equilibrium states, and we have the third kind of resonance zone, namely, an impassable zone. In order to better understand the structure of such a zone, we introduce in Eq. (6) the detuning γbetween the level I=Ipqand the level I=I0, near which the autonomous system (2) has a limit cycle:

BIpq=dBI0/dIIpqI0+OIpqI02γμE13

Then, Eq. (6) can be rewritten as

du/=AvIpq+μσvIpqu+γ,dv/=bu.E14

In Eq. (14) we change the variables from uvto the action Jand the angle L(in both the oscillatory and the rotational zones) and average the resulting system over the “fast” angular variable L. As a result, we arrive at the equation

dJ=μbΦJ/2π,

where ΦJis the Poincaré-Pontryagin generating function [2] and it is discontinuous at J=Jcwhen γ0. Here, Jccorresponds to the contour in the “unperturbed” system

du/=AvIpq,dv/=bu.E15

formed by the saddle and two separatrix loops embracing the phase cylinder.

We shall therefore use Melnikov’s formula [4] to determine the relative position of the separatrices which in the shortened system (15) constitute the contour formed by the outer separatrix loops:

Δ=μΔ1+Oμ2Δ1=bσv0Ipq+B1Ipqu022πγ.

Here, v0,u0is the solution of Eq. (15) on the contour consisting of the saddle and the outer separatrix loops. Setting d=maxvσvIpq=σ, a=B1Ipq/dwe find from the formula for Δ1±that Δ1±=dα+βa±2πγ, where

α=b0σ¯v0Ipqu02,β=b0u02,σ¯=σσ.

From the condition Δ1±=0, we get

γ=γ±=dα+βa/2π.E16

In system (14) the upper contour exists when γ=γ+, and the lower contour when γ=γ. Eq. (16) defines two straight lines in the aγplane. They intersect each other at a0, where a=α/β. When a>1the function σvIpqis sign-preserving, and when a<1it is sign-alternating.

In virtue of Eq. (10), the second case is the most interesting. The case a<1is somewhat special since system (14) may then have limit cycles in both the oscillatory and rotational domains, which have no generating counterparts in system (2). Limit cycles in Eq. (14) can result from the following phenomena [5]: (a) from a degenerate focus, (b) from a separatrix loop (contour), and (c) from a condensation of trajectories. However, if the number of limit cycles does not matter, it suffices to consider the case when there is no more than one limit cycle in the oscillatory domain. Then, we can make a general conclusion on the change of qualitative dynamics of Eq. (14) under variation of the detuning. However, beforehand, we should study the problem for the case when fand gare trigonometric polynomials of degree Nin φ. Then, Aand σare also trigonometric polynomials of degree N1N:

bAvIpq=i=1N1aicosipv+bisinipvσvIpq=i=1N1dicosipv+cisinipv.E17

From the definition of functions Avand σv(see Eq. (7)), it follows that, in general, different harmonics in the perturbation contribute to Aand σ. This means that different harmonics can dominate in Eq. (17). We count only these main harmonics in Eq. (17) (for A1and σn). We then derive from Eq. (6) the equation

z''+sinz=μcosnz+az'+γ,E18

where z=pv+ψ,ψ=arctanb1/a1.

The generating function ΦJfor Eq. (18) can be presented as [3]

ΦJρ=Φsρ=aFnsρ+F0sρ±δ2s2πγF01ρ=16ρ1K+E,F11ρ=161ρK+2ρ1E/3,F02ρ=8E/ρ,F12ρ=82ρ1K+2ρE/3ρ3/2E19

where s=1corresponds to the oscillatory domain and s=2to the rotational domain. K,Eare the complete elliptic integrals with modulus k(ρ=k2). Note that ρ=(1+h˜)/2in the oscillatory domain and ρ=2/(1+h˜)in the rotational domain, and h˜=h˜Jρis the value of the energy integral of the equation z''+sinz=0. Function Fjsρis the generating function defined by the perturbation term z'cosjz. The plus in Eq. (19) corresponds to the upper half of the cylinder, the minus to the lower half, and δis the Kronecker delta. This enables us to find all the bifurcation sets (except the one corresponding to a contractable separatrix loop) explicitly [6].

We shall first consider the case when γ=0. In this case Eq. (18) is identical to the standard equation [2], and Φρis continuous at ρ=1. Thus, it determines the limit cycles up to the separatrix. This case was considered in Figure 2(a–e) that the rough topological structures are shown for n=1. Note that the limit cycles can “disappear at infinity” only when B1=0. This is impossible when Condition Bis satisfied. Figure 2(e) shows the bifurcation when the limit cycle “clings” to the separatrix contour (Φρhas the simple root ρ=1). Figure 2(f) shows the corresponding behavior of the invariant curves (separatrices) of the Poincaré map for the original system with p=3. The neighborhood of the homoclinic contour is attracting. Moreover, a complicated structure exists in the neighborhood [7], and, consequently, we have a quasi-attractor, i.e., a nontrivial hyperbolic set, and stable points can exist in it.

When γ0the generating function Φρis discontinuous at ρ=1. The bifurcation of the cycle clinging to the separatrix must, therefore, be considered separately.

Using Melnikov’s formula, we compute Δ1±, which measures the split of the unperturbed separatrix for Eq. (18). One can see that equation Δ1±=0is equivalent to Φ21=0. Then, using Eq. (19) and assuming (for concreteness) n=1, we find the bifurcational values γ±=4a+1/3/π. When γ=γ++Oμ, we have a non-contractable separatrix loop lying in the domain z0, and when γ=γ+Oμ, we have a loop in the domain z0. From Eq. (19) we obtain the asymptotic formula, Φ2ρπ8a/ρ+ρ±4γ/2, as ρ0. This implies that the straight line a=0in the plane aγis singular. Furthermore, from Eq. (19) we find in the parametric form the line of the double cycles:

a=a0ρ=F2/F2,γ=γ0ρ=F2Fn2F2F2/2πF2,ρ01,orγ=γ0±a.

It is observed that the transformation of the phase portrait of Eq. (18) for ρ1involves the creation of a contractable separatrix loop. By Condition B, we have a0, which implies that the saddle number is nonzero. The separatrix loop can, therefore, give rise to one limit cycle only [5]. The corresponding bifurcational set γ1±ain the parameter plane can be found numerically.

We thus obtain a partition of the parameter plane aγinto domains corresponding to different topological structures for Eq. (18), as well as the structures themselves (they are shown in Figure 3) for n=1. The structures corresponding to cases 8–12 are not shown in Figure 3, since they can be obtained from structures 5, 6, 3, 2, and 14, respectively, by the directions of the coordinate axes.

Note that, along with a non-contractable separatrix loop, Eq. (18) has either a stable limit cycle, or a stable equilibrium state, or the stable “point at infinity.” This means that no quasi-attractor can exist in the original nonautonomous system when γ0. Remark that the homoclinic structure exists for a small range of γvalues γγ±exp1/μ.

Those limit cycles of Eq. (18) which do not lie in the neighborhood of the unperturbed separatrix contour correspond to the two-dimensional invariant tori in the original system (like in the case B0). Unlike when B0, two kinds of such tori may exist in Eq. (18) corresponding to the limit cycles in the oscillatory and rotational domains. The tori corresponding to the cycles in the rotational domain (with one exception) have no generating “Kolmogorov torus” in the perturbed Hamiltonian system, while the (asymptotically stable) tori corresponding to the limit cycles in the oscillatory domain are images of the tori occupying the next level in the hierarchy of resonances.

Remark The cases of odd and even nshould be considered separately. When nis even, an unstable cycle clings to the separatrix loop. For odd nthe same thing happens to a stable cycle. Only the case of odd nis therefore interesting when one studies the problem of existence of a quasi-attractor.

According to the bifurcation diagram (Figure 3), it is convenient to break the case a<1into three sub-cases: (a) 1<a<a, (b) a<a<0, and (c) 0<a<1, a=1/14n2. Let nbe odd. Then considering the solutions on the original cylinder vmod2πu, we derive the following theorem.

Theorem 2

There are μ, γ±a, γ0±a, γ1±a, and asuch that, if μ<μand nare odd, the following three intervals of a(in Eq. (18)) can be chosen: 1.a1a; 2.aa0; and 3.a01.

1. Let a∈−1a∗. Then, (1) when γ>γ1+>0, Eq. (14) has exactly one stable limit cycle (LC) in the rotational domain and no more than pn−1 LCs in the oscillatory domain (OD); (2) when γ1+<γ<γ+, there are p additional LCs in the OD, which are born from the separatrix loops at γ=γ1+; (3) when γ=γ+, the stable LC in the rotational domain clings to the separatrix contour Γp+ consisting of p saddles and their outer separatrices going from one saddle to another, while the “free” unstable separatrices approach an LC in the OD; (4) when γ−<γ<γ+, there are no LCs in the rotational domain and no more than pn LCs in the OD; (5) when γ=γ−, there appears a separatrix contour Γp− which consists of p saddles and their outer separatrices but has orientation and location different from those of Γp+; (6) when γ1−<γ<γ−, there are no more than pn LCs in the OD and one stable non-contractible LC; and (7) when γ<γ1−, Eq. (14) has one stable non-contractible LC which lies in the lower half-cylinder u<0 and no more than pn−1 in the OD.

2. Let a∈a∗0. Then, in the OD there are pn−1 LCs, and in the rotational domain, (1) when γ>γ−, Eq. (14) has one stable LC for u>0; (2) when γ=γ−, a contour Γp− appears; (3) when γ+<γ<γ−, one stable LC exists on the upper half-cylinder u>0 and one stable LC on the lower half-cylinder u<0; (4) when γ=γ+, a contour Γp+ appears; and (5) when γ<γ+, one stable LC exists for u<0.

3. Let a∈01. Then, there are at most pn−1 LCs in the OD, and in the rotational domain, (1) when γ>γ− and u<0, Eq. (14) has one stable LC; (2) when γ=γ−, a contour Γp− appears; (3) when γ0−<γ<γ− and u<0, there is a stable LC born from Γp− and an unstable LC; (4) when γ=γ0−, the stable and unstable LCs merge together; (5) when γ0+<γ<γ0−, no LCs exist; (6) when γ=γ0+, a semi-stable LC is formed for u>0; (7) when γ+<γ<γ0+, one stable and one unstable LCs exist for u>0; (8) when γ=γ+, a contour Γp+ is formed; and (9) when γ<γ+, one unstable LC exists for u>0.

## 3. Example 1

Consider system (8) which is equivalent to the equation [3]

x¨+x+x3=P1+P2x2+P3xsinνtẋ+P4sinνt,E20

where Pi,i=1,2,3,4are parameters. Here, we focus only on the effects which are due to the nonlinear parametric term xẋsinνt. Let us assume ν=4. Then, for small Pii=1,2,3,4system (20) can have only two “splittable” resonance levels: Hxy=h11,Hxy=h31and h31<h11. The corresponding autonomous system (P3=P4=0) has at most one LC. The passage of this LC through the resonances under a change of parameter P2was considered in [2]. If this LC lies outside the neighborhoods of resonance levels Hxy=h11,Hxy=h31, then in the original nonautonomous system (20), there is a two-dimensional invariant torus T2corresponding to the cycle. There is a generating “Kolmogorov torus” in the Hamiltonian system P1=P2=P3=0.

A computer program was developed by the author for a simulation of Eq. (20). The results of such simulation are presented in Figures 4, 5, 6. In the numerical integration, the Runge-Kutta-type formulae are used with an error of order Oh6per integration step h. In Figure 4(a) we present the Poincaré map for P1=0.0472, P2=0.008, and P3=0.018, which determines the structure of the main resonance zone p=1q=1. Along with the separatrices of the saddle fixed point S, a closed invariant curve encircling the unstable fixed point Ois shown, which corresponds to a stable LC in the oscillatory domain of Eq. (6). This closed invariant curve appears for P30.014when the fixed point Oloses its stability. As P3increases, so does the size of the closed invariant curve, and for P30.0487the curve clings to the separatrix of the saddle point S, forming a contour (see Figure 4(b)). As P3increases further, two closed invariant curves appear, shown in Figure 5 for P3=0.15. The structural changes of the resonance zone observed in the experiment are in good agreement with the theoretical results for γ=0. The observations for γ0are consistent with the theory, too.

In the case presented in Figure 6, the transversal intersection of the separatrices of Scannot be detected visually. We, therefore, increased P4to obtain a better picture of the homoclinic structure. When P4=8, the structure can be seen clearly (Figure 6(a)). The corresponding quasi-attractor is the only attracting set (Figure 6(b)). Stable periodic points with long periods can exist inside the quasi-attractor itself. However, they are extremely difficult to detect numerically.

## 4. Example 2

As opposed to Example 1, this one pursues a different goal, namely, to study the transition from the classical parametric resonance to the nonlinear resonance. One of the problems for which this can be done is that of the pendulum with a vibrating suspension.

The pendulum with vibrating suspension is a classical example of a problem in which a parametric resonance can be observed. A large number of publications (see, e.g., [8, 9]) are devoted to this problem. Other problems of this sort include the bending oscillations of straight rod under a periodic longitudinal force [10], the motion of a charged particle (electron) in the field of two running waves [11], etc. The parametric resonance in this kind of systems appears when a fixed point of the corresponding Poincaré map loses its stability and is, therefore, usually described by the linearization near this point.

It is interesting to study the behavior of a parametric system when the ring-like resonance zone is contracted into a point, i.e., to describe the bifurcations which occur in the course of transition from the plain nonlinear resonance to the parametric one. This paragraph is devoted to the solution of this problem in the case of a nonconservative pendulum with a vertically oscillating suspension.

The motion of the pendulum with vertically oscillating suspension (under some simplifying assumptions) is described by the equation [13]

x¨+sinx+p1cosβtsinx+p2ẋ=0,E21

where p1,p2,βare parameters.

Let us now complicate the model even more and consider the equation

x¨+sinx+p1cosβtsinx+p2+p3cosxẋ=0,E22

with the phase space R1×S1×S1. The term p3ẋcosxappears, for example, in the case of the pendulum in which the force of resistance is created by a vertical plate perpendicular to the plane of oscillations. Consider Eq. (22) when it is close to integrable, i.e., for small values of parameters pii=1,2,3. Denote pi=εCi, where εis a small parameter. Then, the original Eq. (22) takes the form

x¨+sinx=εC1cosβtsinx+C2+C3cosxẋ,E23

Eq. (23) in the conservative case, when C2=C3=0, is considered in many publications. For instance, for small angles of the deviation x, the case β2is studied in [8]. The criterion of resonance overlap is applied in [11] to estimating the width of the “ergodic layer.” The existence of homoclinic solutions is discussed in [12] without the assumption on smallness of parameter ε.

Phase curves of the unperturbed mathematical pendulum equation are determined by the integral Hxẋẋ2cosx=h,where h11in the oscillatory domain and h>1in the rotational domain. The peculiarity lies in the way period τdepends on hin the oscillatory domain.

We have

τh=2π/ω=4Kk,k2=1+h/2,1<h<1,τh=2kK,k2=1/1+h,h>1.E24

Here, K=Kkis the complete elliptic integral of the first kind, kbeing its modulus. From Eq. (24) it follows that the period τchanges noticeably only for hclose to 1, i.e., in the neighborhood of the separatrix. Therefore, small intervals of period τ, which determines the width of resonance zones, correspond to fairly large intervals of variable x.

### 4.1. Structure of resonant zones

In the investigation of the perturbed equation, we first focus on the structure of resonance zones in domains G1=xẋ:1<hHxyh+<1and G2={xẋ:Hxyh>1}. The resonance condition τhpq=p/q2π/β, where p,qare relatively prime integers, determines the resonance levels of energy Hxy=hpq.

The structure of individual resonance zones Uμis described (up to the terms Oε3/2) by the pendulum-type Eq. (6). Since functions A0and σhave different forms in the oscillatory and rotational domains, we introduce the notations A0svhpqand σsvhpq, where s=1corresponds to the oscillatory domain and s=2to the rotational one.

In our case the divergence of the vector field of Eq. (23) contains no terms explicitly depending on t; hence, σdoes not depend on v, i.e., σ=const.

The functions A0sand σsin an explicit form were obtained in [13]. It is also found that the width of the resonance zone decreases rapidly with the increase of pwhen q=1.

A computer-generated picture of invariant curves of the Poincaré map for Eq. (22), with β=1.6, is shown in Figure 7. In Eq. 21(a) a case of synchronization of oscillations in the subharmonic with p=2,q=1(p1=0,1, p2=0,07, p3=0,1)is shown, and in Figure 7(b), a partly passable resonance with p=2,q=1(p1=0,1, p2=1/30, p3=0,1)is shown. In the domain G2the synchronization of oscillations on the main resonance (p=q=1) takes place.

### 4.2. Neighborhood of the origin

Denote Un=xy:0HxyCε2/nand substitute in Eq. (23):

x=ε1/nξ,y=ẋ=ε1/nη

As a result, we arrive at the system

ξ̇=η,η̇=ξ+εC1ξcosβt+C2+C3η+ε2/nξ3/6ε1+2/nC1ξ3cosβt/6+ξ2η+E25

System (25) is defined in D×S1where Dis a certain domain in R2. In the neighborhood U1n=1, system (25) assumes the form

ξ̇=η,η̇=ξ+εC1ξcosβt+C2+C3η+Oε2.E26

By discarding in Eq. (26) the terms Oε2, we arrive at the Mathieu equation with the extra term resulting from the viscous friction. It is clear that in the framework of a linear equation one cannot observe the (nonlinear) effects which accompany the transition from the nonlinear resonance to the parametric one. So, let us consider a wider neighborhood U2n=2of the origin. In Eq. (25) we discard the terms Oε2and, for the resulting system, consider the resonance cases when ω=1=/p(pand qbeing relatively prime integers). We then study the bifurcations pertaining to the transition from the parametric resonance to the ordinary one. We once again introduce the detuning 1/p=γ1ε. As a result, the system in question will be rewritten as

ξ̇=/pη+γ1εη̇=/pξ+εC1ξcosβt+C2+C3ηγ1ξ+ξ3/6.E27

Now, we introduce the action (I) – angle (ϑ) variables. Since the unperturbed system is linear, the substitution has the simple form ξ=2Isinϑand η=2Icosϑ. In terms of this variables, system (27) will be written as

İ=εFIϑφ,ϑ̇=/pεRIϑφ,φ̇=β,E28

where F=2IGcosϑγ12Isinϑ, R=Gsinϑ+γ1cosϑ/2I

G=C1sinϑcosφ+C2+C3cosϑγ1sinϑ+I/3sin3ϑ.

Let us introduce in Eq. (28) the “resonance phase” ψ=ϑ/pand average the resulting system over the “fast” variable φ. As a result, we arrive at the two-dimensional autonomous system

u̇=εC1/2usin2v+C2+C3u]v̇=εC1/4cos2vu/8γ1/2E29

when p=2and q=1and to the system

u̇=εC2+C3v̇=εu/8γ1/2E30

when p2and/or q>1. As we know, u=I+Oε,v=ψ+Oε2. From Eq. (29) and (30), it follows that (in our approximation) only one resonance with p=2,q=1appears in the neighborhood U2.

The investigation of system (29) when C22+C320for different values of detuning γ1presents no difficulty, because, according to the Bendixson criterion, there are no limit cycles. The most typical rough phase portraits are presented in Figure 8 where, parallel with the phase portraits in the uvplane, the corresponding phase portraits in Cartesian coordinates xy=ẋare shown. Figure 8(a) corresponds to the case when we have γ1>γ>0, γ=C124C2+C32/2, Figure 8(b) when γ1γ, and Figure 8(c) when γ1>γand γ1<0. In addition, in all three cases, we assume C2+C3<0.

### 4.3. Conclusion

The number of splittable resonances is bounded, when C22+C320. For the actual pendulum (Eq. (22)), when the small nonconservative forces are present, we, most likely, have one resonance regime with p=2,q=1in the oscillatory domain and the one with p=1,q=1in the rotational domain.

In conclusion we make the following remarks on Eqs. (22) and (23).

1. The transition from Figure 8(a)–(c) corresponds to two period-doubling bifurcations, while the passage from the parametric resonance (Figure 8(b)) to the ordinary nonlinear resonance (Figure 8(c)) corresponds to the birth of two periodic (of period 2) saddle points and a node (focus) from a multiple saddle fixed point.

2. The bifurcation which involves the birth of a quasi-attractor (Figure 7(b)) in the neighborhood of the unperturbed separatrix is the most interesting one. It may take place at any magnitude of the external force (parameter C1). It suffices to have Bs1=0,C2=C3/3,εC2C3<0, for example, C2=1/30,C3=0.1,ε>0..

3. In the quasi-integrable nonconservative case, there appear no resonances with q>1and odd pin the oscillatory domain and no resonances with q>1and even pin the rotational domain.

## Acknowledgments

This work was supported in part by the Russian Foundation for Basic Research under grant no. 18-01-00306, by the Russian Science Foundation under grant no. 14-41-00044.

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Albert Morozov (October 17th 2018). Periodic Perturbations: Parametric Systems, Perturbation Methods with Applications in Science and Engineering, İlkay Bakırtaş, IntechOpen, DOI: 10.5772/intechopen.79513. Available from:

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