Open access peer-reviewed chapter

# Analytical Solutions of Some Strong Nonlinear Oscillators

Written By

Alvaro Humberto Salas and Samir Abd El-Hakim El-Tantawy

Reviewed: 12 April 2021 Published: 28 May 2021

DOI: 10.5772/intechopen.97677

From the Edited Volume

## Engineering Problems - Uncertainties, Constraints and Optimization Techniques

Edited by Marcos S.G. Tsuzuki, Rogério Y. Takimoto, André K. Sato, Tomoki Saka, Ahmad Barari, Rehab O. Abdel Rahman and Yung-Tse Hung

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## Abstract

Oscillators are omnipresent; most of them are inherently nonlinear. Though a nonlinear equation mostly does not yield an exact analytic solution for itself, plethora of elementary yet practical techniques exist for extracting important information about the solution of equation. The purpose of this chapter is to introduce some new techniques for the readers which are carefully illustrated using mainly the examples of Duffing’s oscillator. Using the exact analytical solution to cubic Duffing and cubic-quinbic Duffing oscillators, we describe the way other conservative and some non conservative damped nonlinear oscillators may be studied using analytical techniques described here. We do not make use of perturbation techniques. However, some comparison with such methods are performed. We consider oscillators having the form x¨+fx=0 as well as x¨+2εẋ+fx=Ft, where x=xt and f=fx and Ft are continuous functions. In the present chapter, sometimes we will use f−x=−fx and take the approximation fx≈∑j=1Npjxj, where j=1,3,5,⋯N only odd integer values and x∈−AA. Moreover, we will take the approximation fx≈∑j=0Npjxj, where j=1,2,3,⋯N, and x∈−AA. Arbitrary initial conditions are considered. The main idea is to approximate the function f=fx by means of some suitable cubic or quintic polynomial. The analytical solutions are expressed in terms of the Jacobian and Weierstrass elliptic functions. Applications to plasma physics, electronic circuits, soliton theory, and engineering are provided.

### Keywords

• Nonlinear second-order equation
• Duffing equation
• Cubic-quintic Duffing equation
• Helmholtz oscillator
• Duffing-Helmholtz oscillator
• Mixed parity oscillator
• Damped Duffing equation
• Damped Helmholtz equation
• Forced Duffing equation
• Nonlinear electrical circuit
• Solitons

## 1. Introduction

Both the ordinary and partial differential equations have an important role in explaining many phenomena that occur in nature or in medical engineering, biotechnology, economic, ocean, plasma physics, etc. [1, 2]. Duffing equation is considered one of the most important differential equations due to its ability for demonstrating the scenario and mechanism of various nonlinear phenomena that occur in nonlinear dynamic systems [3, 4, 5, 6, 7, 8, 9, 10, 11]. It is one of the most common models for analyzing and modeling many nonlinear phenomena in various fields of science such as the mechanical engineering [12], electrical engineering [13], plasma physics [14, 15], etc. Mathematically, the Duffing oscillator is a second-order ordinary differential equation with a nonlinear restoring force of odd power

x¨+fx=0,fx=i=1Kix2i1,E1

where fx=fx is a continuous function on some interval AA with f0=0,Ki is a physical coefficient related to the physical problem under study, and i=1,2,3,. It is clear from Eq. (1) that there is no any friction/dissipation (this force arises either as a result of taking viscosity into account or the collisions between the oscillator and any other particle, etc.), and this only occurs in standardized systems such as superfluid (fluid with zero viscosity which it flows without losing any part from its kinetic energy sometimes like Bose–Einstein condensation) or the systems isolated from all the external force that resist the motion of the oscillator. The undamped Duffing equation [9] is considered one of the effective and good models for explaining many nonlinear phenomena that are created and propagated in optical fiber, Ocean, water tank, the laboratory and space collisionless and warm plasma (we will demonstrate this point below). As well known in fluid mechanics and in the fluid theory of plasma physics; the basic fluid equations of any plasma model can be reduced to a diverse series of evolution equations that can describe all phenomena that create and propagate in these physical models. For example, we can mention some of the most famous evolution equations that have been used to explain several phenomena in plasma physics and other fields of sciences; the family of one dimensional (1D) korteweg–de Vries equation (KdV) and it is higher-orders, including the KdV, KdV-Burgers (KdVB), modified KdV (mKdV), mKdV-Burgers (mKdVB), Gardner equation or called Extended KdV (EKdV), EKdV-Burgers (EKdVB), KdV-type equation with higher-order nonlinearity. All the above mentioned equations are partial differential equations and by using an appropriate transformation, we can convert them into ordinary differential equations of the second orders. If the frictional force is neglected, some of these equations can be converted into the undamped Duffing equation with fxPx=K1x+K2x3 like the mKdV equation, the KdV equation can be transformed to the undamped Helmholtz equation with fxPx=K1x+K2x2 [16], the Gardner equation can be converted into the undamped H-D equation for fxPx=K1x+K2x2+K3x3 [17, 18], and so on the other mentioned equations.

However, these undamped models (without friction/dissipation) do not exist much in reality except under harsh conditions. In order to describe and simulate the natural phenomena that arise in many realistic physical models and dynamic systems, the friction/dissipation forces must be taken into account, as is the case in many plasma models and electronic systems. Accordingly, the following damped (non-conservative) Duffing equation will be devoted for this purpose

x¨+2εẋ+fx=0.E2

If the frictional force does not neglect, so that all PDEs that have “Burgers x2” term like KdVB-, mKdVB-, EKdVB-, KPB-, mKPB-, EKPB-, ZKB-, mZKB, EZKB-Eq. [1, 2], etc. can be transformed to damped Duffing equation (x¨+2εẋ+K1x+K2x3=0), damped Helmholtz equation (x¨+2εẋ+K1x+K2x2=0), and damped Duffing-Helmholtz equation (x¨+2εẋ+K1x+K2x2+K2x3=0). Eq. (2) without [19] and with [7, 20, 21] including damping term (2εẋ) for fxPx=K1x+K2x3 has been investigated and solved analytically and numerically by many authors using different approaches in order to understand its physical characters [22, 23, 24, 25, 26, 27, 28].

Many authors investigated the (un)damped Duffing equation, (un)damped Helmholtz Eq. [16, 29, 30, 31], and undamped H-D equation. On the contrary, there is a few numbers of published papers about damped Duffing-Helmholtz equation [32, 33]. For example, Zúñiga [32] derived a semi-analytical solution to the damped Duffing-Helmholtz equation in the form of Jacobian elliptic functions, but he putted some restrictions on the coefficient of the linear term, and then obtained a solution that gives good results compared to numerical solutions. Also, it is noticed that Zúñiga solution [32] is very sensitive to the initial conditions. Gusso and Pimentel [33] obtained obtain improved approximate analytical solution to the forced and damped Duffing-Helmholtz in the form of a truncated Fourier series utilizing the harmonic balance method.

In this chapter, we display some novel semi-analytical (approximate analytical) solutions to the strong higher-order nonlinear damped oscillators of the following initial value problem (i.v.p)

x¨+2εẋ+px+qx3+rx5=Ft,x0=x0&x0=ẋ0,E3

and its family (ε=0 or r=0 or ε=r=0).

Our new semi-analytical solution to Eq. (3) is derived in terms of Weierstrass and Jacobian elliptic functions. Also, we will solve Eq. (3) numerically using Runge–Kutta 4th (RK4) and make a comparison between both the semi-analytical and numerical solutions. Moreover, as some realistic physical application to the problem (3) and its family will be investigated.

## 2. Duffing equation

Let us consider the standard (undamping) Duffing equation in the absence both friction (2εẋ) and excitation (Ft) forces [34, 35]

x¨+px+qx3=0,x=xt,E4

which is subjected to the following initial conditions

x0=x0&x0=ẋ0.E5

The general solution of Eq. (4) maybe written in terms of any of the twelve Jacobian elliptic functions.

For example, let us assume

xt=c1cnωt+c2m.E6

By inserting solution (6) in Eq. (4), we get

x¨+px+qx3=c13q2c1cn3+2c1+c1pc1ωcn,E7

where cn = cn ωt+c2m.

Equating to zero the coefficients of cn j gives an algebraic system whose solution gives

ω=p+qc12andm=qc122p+qc12.E8

Thus, the general solution of Eq. (4) reads

xt=cnp+qc12t+c2qc122p+qc12.E9

The values of the constants c1 and c2 could be determined from the initial conditions given in Eq. (5).

Definition 1. The number Δ=p+qx022+2qẋ02 is called the discriminant of the i.v.p. (4) -(5). Below three cases will be discussed depending on the sign of the discriminant Δ.

### 2.1 First case: Δ>0

For Δ>0, the solution of the i.v.p. (4)(5) is given by

xt=ΔpqcnΔ4tsignẋ0cn1qΔpx012p2Δ12p2Δ.E10

Making use of the additional formula

cnx+ym=cnxmcnym+snxmdnxmsnymdnym1msnxmsnym,E11

the solution (10) could be expressed as

xt=x0cnωtm+ẋ0ωdnωtmsnωtm1+p+qx02ω2ΔsnΔ4tm2,E12

where

m=121+pΔandω=Δ4.E13

Solution (12) is a periodic solution with period

T=4Kmω.E14

Example 1.

Let us consider the i.v.p.

xt+xt+x3t=0,x0=1&x0=1.E15

Using formula (10), the exact solution of the i.v.p. (15) reads

xt=61cn64t+cn11611212611266.E16

According to the relation (12)(13), the exact solution of the i.v.p. (15) is also written as

xt=264dn64t11266sn64t1126626cn64t1126662sn64t11266226,E17

and its periodicity is given by

T=4K11266643.27458.

In Figure 1, the comparison between the exact analytical solution (17) and the approximate numerical RK4 solution is presented. Full compatibility between the two analytical and numerical solutions is observed.

### 2.2 Second case: Δ<0

For Δ<0, in this case q<0 and then, δ=p2Δq>0, δ=def2p+qx02x02+2ẋ02>0. Let us introduce the solution in the following form

xt=A2A1+yt,E18

where y=yt is a solution of some Duffing equation

yt+myt+ny3t,E19

with initial conditions

y0=y0=2Aẋ0Ax02y0=ẏ0=A+x0Ax0.E20

Inserting ansatz (18) into Eq. (4) and taking the below relation into account

yt2=ẏ0+my02+n2y04my2tn2y4t,E21

we get

AA2q+4my02+2ny04+p+4ẏ02+A3A2q2mpytA3A2q2mpyt2+AA2q2n+pyt3=0.E22

Equating the coefficients of yjt to zero, gives an algebraic system. A solution to this system gives

m=12p+3A2q,n=12p+A2q,A=2p+qx02x02+2ẋ02q4=δq4.

Note that the i.v.p. (19)(20) has a positive discriminant and it is given by

m+ny022+2nẏ02=δAx042A4ẋ02+δA2+x0224A8x02.

Then the problem reduces to the first case. Accordingly, the solution of the i.v.p. (4)(5) maybe written in the form,

xt=A2A1+Bb0cnωtm+b1snωtmdnωtm1+b2sn2ωtm,E23

where

m=B2A2q+p2A2B2+3q+2B21p,ω=12A2B2+3q+B21p,b0=A+x0ABBx0,b1=2Aẋ0BωAx02,b2=2Ax0x0Ap+qx02+ωA+x0Ax02+4Aẋ022ωAx02A+x0,A=2p+qx02x02+2ẋ02q4=δq4,B=A22qA2qp3Aq+pA2q+p.E24

The solution (23) is unbounded and its periodicity is given by

T=4Kmω=4K1mmω.E25

Example 2.

Let us assume the following i.v.p.

xt+xtx3t=0,x0=1&x0=1.E26

The solution of the i.v.p. (26) according to the relation (23) reads

xt=1.316072.632151+0.13647cn1.75396t1.003530.27976dn1.75396t1.00353sn1.75396t1.003531.1.00463sn1.75396t1.003532,E27

and the periodicity of this solution is given by

T=9.57783.E28

Solution (27) is displayed in Figure 2.

### 2.3 Third case: Δ=0

If the discriminant vanishes Δ=0, then q<0 and the only solution of problem (4) with

x'02=ẋ02=p+qy0222q,E29

xt=pqtanhp2t±tanh1x0qp.E30

which may be verified by direct computation.

Remark 1. The solution of the i.v.p.

x¨+px+qx3=0,x0=x0&x0=0,E31

is given by

xt=x0cnp+qx02qx022p+qx02.E32

Remark 2. For p+p2+2q x_0 2>0, then the solution of the i.v.p.

x¨+px+qx3=0,x0=0&x0=ẋ0.E33

is given by

xt=2ẋ0p2+2qẋ02+psnp+p2+2qẋ022tp2+qẋ02p2+2qẋ02pqẋ02.E34

Remark 3. According to the following identity

cnωtm=1S01+S1tg2g3,E35

with

S0=64m+1,S1=124m+1ω,g2=11216m216m+1ω2,g3=12162m132m232m1ω3,

the solution of the i.v.p. (4)(5) could be written in terms of the Weierstrass elliptic function tg2g3. More precisely, if Δ>0 then

xt=AA4p3A2q+p+21+123A2q+pt+t0g2g3,E36

with

t0=13A3q+3A2qx0+5Ap+px012Ax0g2g3,g2=1123A4q26A2pq+p2,g3=p2169A4q2+18A2pq+p2,E37

and

A=p±p+qx022+2qẋ02q=±p±Δq.E38

The solution (36) is periodic with period

T=2ρ+dx4x3g2xg3,E39

where ρ is the greatest real root of the cubic 4x3g2xg3=0.

Remark 4. An approximate analytic solution of the i.v.p. (31) is given by

xt=x01+λcoswt1+λcos2wt,E40

where

w=125p+qx02λ2+12p+11qx02λ+8p+6qx023λ+2E41

and λ is a root of the cubic

25p+qx02λ3+58p+59qx02λ2+216p+21qx02λ+8qx02=0.E42

Example 3.

Let us consider the i.v.p.

x¨+x+10x3=0,x0=4,x0=0.E43

The approximate solution in trigonometric form is given by

xappt=3.34603cos10.7542t10.300255cos210.7542t.E44

xt=4cn161t80161,E45

with period

T=4K80161161.

The error on the interval 0tT equals 0.025.

The comparison between the approximate analytic solution (44) and the exact analytic solution (45) is illustrated in Figure 3.

Remark 5. An approximate analytical solution of the i.v.p. (33) is given by

xappt=ẋ0sinωtω1+λsin2ωt,E46

where

ω=λ264p2160qẋ02+25p2λ4+80p2λ3128qẋ02λ+5λ+816λE47

and λ is a solution of the quintic

125p2λ5+1079p2+125qẋ02λ4+4043p2+85qẋ02λ3+8196p2+389qẋ02λ2+648p2+17qẋ02λ+128qẋ02=0E48

Example 4.

The approximate trigonometric solution of

x¨+3x+5x3=0,x0=0,x0=1.E49

xappt=0.499502sin2.00199t10.0817025sin22.00199t.E50

The exact solution is

xt=23+19sn123+19t1514+319,E51

with period

T=415193K1514+319=3.1383.E52

The error on the interval 0tT equals 0.00018291.

Figure 4 demonstrates the comparison between the approximate analytic solution (50) and the exact analytic solution.

## 3. An analytical solution of the undamped Duffing-Helmholtz Equation

x¨+px+qx2+rx3=0,x0=x0andx0=ẋ0.E53

We will give a solution to the i.v.p. (53) in terms of Weierstrass elliptic functions. For solving this problem the following ansatz is considered

xt=A+B1+Ct+t0g2g3,E54

where BC0.

Substituting the ansatz (54) into the ordinary differential equation (ode) Rx¨+px+qx2+rx3=0, gives

121+C3j=03Kjj=0,E55

with

K3=2C2A3Cr+A2Cq+ACp+2B,K2=2C3A3Cr+3A2BCr+3A2Cq+2ABCq+3ACp+BCp6B,K1=C6A3r+12A2Br+6A2q+6AB2r+8ABq+6Ap+2B2q3BCg2+4Bp,K0=A3r+6A2Br+2A2q+6AB2r+4ABq+2Ap+2B3r+2B2q4BC2g3+BCg2+2Bp.

Equating the coefficients Kj to zero will give us an algebraic system. Solving this system, we finally get

B=6AA2r+Aq+p3A2r+2Aq+p,C=123A2r+2Aq+p,g2=1123r2A4+4qrA3+6prA2p2,g3=12169pr23q2rA4+12pqr4q3A3+18p2r6pq2A2+p3,E56

The values of t0 and A could be determined from the initial conditions x0=x0 and x0=ẋ0 and

x¨0+px0+qx20+rx30=0.E57

We have

t0=±1x0ABCAx0g2g3.E58

The number A is a solution to the quartic

3rA4+4qA2+6pA3rx04+4qx03+6px02+6ẋ02=0.E59

Example 5.

The solution of the i.v.p.

x¨+x+2x2+3x3=0,x0=1andx0=1,E60

according to the relation (54) is given by

xt=1.076272.720781+0.762858t0.1483177.16667,0.675926.E61

In Figure 5, the comparison with the approximate analytic solution (61) and the approximate numerical solution using RK4 is investigated.

The periodicity of solution (61) is given by

T=20.093853814x3+7.16667x0.675926dx=3.12129.

## 4. The solution of the forced undamped Duffing-Helmholtz equation

Suppose that the physical system to be studied is under the influence of some constant external/excitation force, so the standard Duffing-Helmholtz equation can be reformulated to the following constant forced Duffing-Helmholtz i.v.p.

x¨+px+qx2+rx3=F,x0=x0andx0=ẋ0.E62

For solving the i.v.p. (62), the following assumption is introduced

xt=yt+ζ,E63

where ζ is a solution to the cubic algebraic equation

rζ3+qζ2+F=0.E64

Substituting Eq. (63) into the i.v.p. (62), we have

y''t+p+2+3rζ2yt+q+3yt2+ryt3=0.E65

Note that the constant forced Duffing-Helmholtz Eq. (62) has been reduced to the standard Duffing-Helmholtz Eq. (65) with the following new initial conditions

y0=x0ζ&y0=ẋ0.E66

Example 6.

Suppose that we have the following i.v.p. and we want to solve it

v¨+2v12v2+v3=4,v0=1&v0=1.E67

It is clear that the i.v.p. (67) is a constant forced Duffing-Helmholtz equation. The solution of this problem is given by

vt=15.804615.77141+0.03225390.761045t9.41667,47.287.E68

The comparison between the solution (68) and the RK4 solution is introduced in Figure 6.

The periodicity of solution (68) is given by

T=21.9365714x3+9.41667x47.287dx=1.68202.

## 5. An approximate analytic solution of the forced damped Duffing-Helmholtz equation

Let us define the following i.v.p.

x¨+2εẋ+px+qx2+rx3=F,x0=x0&x0=ẋ0.E69

Suppose that

limx+xt=d,ε>0,E70

then the first equation in system (69) can be written as

pd+qd2+rd3=F.E71

For solving the i.v.p. (69), the following ansatz is assumed

xt=expρtyft,E72

with

ft=1exp2ερt2ερ,E73

where the function yyt represents the exact solution to the following i.v.p.

yt+3d2r+2dq2ερ+p+ρ2yt+3dr+qyt2+ryt3=0,y0=x0d&y0=ẋ0+ρx0d.E74

Let us define the following residual

Rtx¨t+2εẋt+pxt+qx2t+rx3tF,E75

and by applying the condition R0=0, we obtain

4ρ312ερ2+3d2r+3dq+3drx0+8ε2+4p+5qx0+6rx02ρ4εd2r+dq+drx0+p+qx0+rx02=0.E76

By solving this equation we can get the value of ρ.

Example 7.

Let

x¨+0.02ẋ+5x+2x2+x3=1/2,x0=0.1&x0=0.1.E77

The approximate analytic solution of the i.v.p. (77) reads

xappt=0.0961263+e0.0099959t0.04291350.2528081+2.137490.631364121944.1e8.2×106t2.43588,0.737005.E78

The distance error as compared to the RK4 numerical solution is given by

max0t5xapptxRK4t=0.000944148.E79

Also, the comparison between solution (78) and RK4 solution is presented in Figure 7.

Remark 5. For the damped and constant forced Helmholtz equation

x¨+2εẋ+px+qx2=F,x0=x0&x0=ẋ0.E80

The value of d can be determined from: pd+qd2=F. However, if this equation has no real solutions we can choose d=0.

Remark 6. Letting q=0, we obtain the damped and constant forced Duffing equation

x¨+2εẋ+px+rx3=F,x0=x0&x0=ẋ0.E81

In this case, the number d must be a root to the cubic pd+rd3=F.

## 6. Approximate analytic solution of the damped and trigonometric forced Duffing-Helmholtz equation

Let us define the following new i.v.p.

x¨+2εẋ+px+qx2+rx3=Fcosωt,x0=x0&x0=ẋ0.E82

We suppose that q24pr<0, and the following residual is defined

Rtx¨t+2εẋt+pxt+qx2t+rx3tFcosωt.E83

Let us define the solution of i.v.p. (82) as follows

xt=expρtyt+c1cosωt+c2sinωt,E84

where

9F2r2c13+96ε2Frω2c12+464ε4ω4+16ε2ω6+3F2pr3F2rω2+16ε2p2ω232ε2pω4c14F16ε2ω4+3F2r+16ε2pω2=0.E85
6144ε3Frω3+432F2r3c23+2304εFr2ωpω2c22+3072ε2rω24ε2ω2+p22pω2+ω4c2=0.E86

The function yyt is a solution to the i.v.p.

yt+2εyt+p˜yt+qyt2+ryt3=0,y0=x0c1&y0=ẋ0ωc2.E87

where p˜=122p+3rc12+3rc224ερ+2ρ2.

The value of ρ can be determined from the following equation

4ρ312ερ2+4p+8ε25qc1+12rc12+6rc22+5qx012rc1x0+6rx02ρ2ε2p2qc1+5rc12+3rc22+2qx04rc1x0+2rx02=0.E88

Example 8.

Let

x¨+0.2ẋ+13x+x2+x3=0.25cos0.5t,x0=0&x0=0.2.E89

The approximate analytic solution of the i.v.p. (89) is given by

xappt=e0.100036t0.05877640.3508931+0.9147390.52914413766.91e0.0000726379t14.0404,10.1967+0.000153771sin0.5t+0.0196062cos0.5tE90

The distance error according to the RK4 numerical solution is calculated as

max0t60xapptxRK4t=0.000671928.E91

Moreover, solution (90) is compared with RK4 solution as shown in Figure 8.

## 7. An analytic solution of cubic-quintic Duffing equation

Let us consider the following ordinary differential equation [36]

x¨+αx+βx3+γx5=0,x=xt,E92

which is subjected to the following initial conditions

x0=x0andx0=ẋ0.E93

Theorem 1.

a. Suppose that x00, then the solution of the i.v.p. (92)(93) is given by

xt=x01+λvt1+λv2t,E94

where the function vvt is the solution to the following Duffing equation

v¨+pv+qv3=0,v0v0=1,v0v̇0=ẋ0x01+λ.E95

The values of the coefficients p and q are given by

p=λ24α+3βx02+2γx04+λ3βx02+4γx04+2γx042λ2,E96
q=2λ2α+βx02+γx04+λβx02+2γx04+γx04λ,E97

and the value of the quantity λ is a solution of the cubic

6ẋ02λ36αx02+6βx04+6γx06λ23βx04+6γx06λ2γx06=0.E98

The solution to the the i.v.p. (95) is obtained from the formulas in the first section.

b. Suppose that x0=0, in this case, the solution of the i.v.p. (92)(93) is given by

xt=1+λvt1+λv2t,E99

where the function vvt is the solution of the following Duffing equation

v¨+pv+qv3=0,v0v0=0,v0v̇0=ẋ01+λ.E100

The values of the coefficients p and q are expressed as

p=2γ+3β+4γλ+4α+3β+2γλ22λ2,E101
q=2γ+β+2γλ+α+β+γλ2λ,E102

and the value of λ is a solution of the cubic

6α+3β+2γ6ẋ02λ3+6α+6β+6γλ2+3β+6γλ+2γ=0.E103

Note that the solution of the i.v.p. (100) could be obtained from the formulas in the first section.

• Proof: case (a)

Inserting ansatz (94) into Eq. (92) taking the following equation into consideration

v̇2=v̇02+pv02+q2v04pv2q2v4,E104

and using Eq. (100), we have

j=15Hjvtj=0,E105

with

H1=6px02λ3qx02λ+2x02α6ẋ02λ312ẋ02λ26ẋ02λ2x022x02,H3=3+qx02βλx02β2αλ+λ,H5=12+2x02βλ2+2x02βλ+2x04γλ2+4x04γλ+2x04γ+2αλ2,

where j=1,3,5.

Equating the coefficients Hj to zero gives an algebraic system: H1=0, H3=0, and H5=0. Solving H1=0 and H3=0 will give the values of p and q that are given in Eqs. (101)(102). Finally, by inserting the values of p and q into H1=0, we obtain the cubic Eq. (103). Likewise, the case (b) can be proved.

## 8. Damped Cubic-Quintic Oscillator

Let us define the following i.v.p.

x¨++2εẋ+px+qx3+rx5=0,x0=x0&x0=ẋ0.E106

We seek approximate analytic solution in the ansatz form

xt=expρtyft,E107

with

ft=1exp2ερt2ερ,E108

where yyt is the exact solution to the i.v.p.

y¨+pερ+ρ2y+qy3+ry5=0,y0=x0&y0=ẋ0+x0ρ.E109

Define the residual

Rt=x¨+2εẋ+px+qx3+rx5,E110

then, the condition R0=0 gives

2ρ36ερ2+2p+3qx02+4rx0+4ε2ρ22qx02ε2rx04ε=0.E111

Some real roots of Eq. (111) give the value of ρ. For x0=0, the default value of ρ could be chosen as ρ=2/3ε.

Example 9.

Let

x¨+0.05ẋ+96.6289x3.5x30.8x5=0,x0=1&x0=0.E112

The approximate analytical solution of the i.v.p. is given by

xappt=1.00575e0.0257101tcnftm0.00261773dnftmsnftm1+5.14×109snftm21+0.0107805cnftm0.00261773dnftmsnftm21+5.14×109snftm22+0.000750033cnftm0.00261773dnftmsnftm41+5.14×109snftm24,E113

where

ft=6807.396807.39e0.00142017t&m=0.000750607.E114

The distance error as compared to the RK4 numerical solution reads

max0t5xapptxRK4t=0.0561216.E115

In Figure 9, we make a comparison between our solution, RK4 solution, and Zuñiga solution given in Ref. [17]. It is clear that the accuracy of our solution is better than the solution of Zuñiga [17].

## 9. Realistic physical applications

The above solutions could be applied to various fields of physics and engineering such as they could be used for describing the behavior of oscillations in RLC electronic circuits, plasma physics etc. In the below section, the above solution will be devoted for studying oscillations in various plasma models.

### 9.1 Nonlinear oscillations in RLC series circuits with external source

In the RLC series circuits consisting of a linear resistor with resistance R in Ohm unit, a linear inductor with inductance L in Henry unit, and nonlinear capacitor with capacitance C in Farady unit as well as external applied voltage E in voltage unit, the Kirchhoff’s voltage law (KVL) could be written as

Lti't+itR+sq+aq2=E,E116

where the relation between the current the charge is given by i=tqq̇, iti, the coefficients as are related to the nonlinear capacitor, and E represents the voltage of the battery which is constant. By reorganizing Eq. (116), the following constant forced and damped Helmholtz equation could be obtained as

q¨+2γq̇+αq+βq2=F,E117

with γ=R/2L,α=1/LC, β=1/Cq0L, and F=E/L where q0=qt=0 is the initial charge value at t=0, q¨t2q, and q̇tq.

The solution of Eq. (117) can be devoted for interpreting and analyzing the oscillations that can generated in the RLC circuit.

### 9.2 Duffing-Helmholtz equation for modeling the oscillations in a plasma

For studying the plasma oscillations using fluid theory, the basic equations of plasma particles using the reductive perturbation method (RPM) will be reduced to some evolution equations such as KdV equation and its family [37, 38, 39, 40, 41]. Let us consider a collisionaless and unmagnetized electronegative complex plasma, consisting of inertialess cold positive and negative ion species, inertia non-Maxwellian electrons in addition to stationary negative dust impurities [42]. Thus, the quasi-neutrality condition reads: n20+ne0=n10 where ns,e0 donates the unperturbed number density of the plasma particles (here, the index “s= “1” and “2” point out the positive ion and negative ion, and “e” refers to the electron, respectively). It is assumed that the plasma oscillations take place only in xdirectional which means that the fluid equations of the plasma particles become perturbed only in xdirectional. If the effect of the ionic kinematic viscosities ηs for both positive η1 and negative η2 ions are included in the present investigation, as a source of damping/dissipation, in this case we will get a new evolution equation governs the dynamics of damping pulses. The dynamics of plasma oscillations are governed by the following fluid equations: xns+tnsus=0,tus+usxus+δ/Qsxϕηsx2us=0, and x2ϕnd0n2ne+n1=0, where ne=μ1βϕ+βϕ2expϕ. Here, ns donates the normalized number density of positive and negative ions, and us represents the normalized fluid velocity of positive and negative ions, and ϕ is the normalized electrostatic wave potential. The mass ratio is defined as: Qs=m1/ms (note that Q1=m1/m1=1 and Q2Q=m1/m2), where ms is the ionic mass, δ=11 for positive (negative) ion, and β illustrates nonthermality parameter. The quasi-neutrality condition in the normalized form reads: μ=1α, where α=n20/n10 gives the the negative ion concentration and μ=ne0/n10 is the electron concentration.

Now, the RPM is introduced to reduce the fluid plasma equations to the evolution equation. According to the RPM, the independent quantities xt are stretched as: ξ=εxVpht, τ=ε3t, and ηs=εηs0 where Vph is the wave phase velocity of the ion-acoustic waves and ε is a real and small parameter (0<ε<<1). The dependent perturbed quantities Πxtn1n2u1u2ϕT are expanded as: Π=Π0+j=1εjΠξτj, where Π0=1α,0,0,0T and T represents the transpose of the matrix. Inserting both the stretching and expansions of the independent and dependent quantities into the basic fluid equations and after boring but straightforward calculations, the Gardner-Burgers/EKdVB equation is obtained

τφ+P1φ+P2φ2ξφ+P3ξ3φ+P4ξ2φ=0,E118

with the coefficients of the quadratic nonlinear, cubic nonlinear, dispersion, and dissipation terms P1,P2, P3, and P4, respectively,

P1=32P31Vph43αVph4Q22h23,P2=34P35Vph65αVph6Q32h3,P3=Vph3Q2Q+α,P4=P3η10Vph4+αη20QVph3&Vph=Q+αQh1,

where φϕ1, h1=μ1β, h2=μ/2, and h3=μ1+3β/6.

It is shown that the coefficients P1,P2, P3, and P4, are functions in the physical plasma parameters namely, negative ion concentration α, the mass ratio Q, and the electron nonthermal parameter β. It is known that at the critical plasma compositions say βc or αc (critical value of negative ion concentration), the coefficient P1 vanishes and in this case Eq. (118) will be reduced to the following mKdVB equation which is used to describe the damped wave dynamics at critical plasma compositions

τφ+P2φ2ξφ+P3ξ3φ+P4ξ2φ=0,E119

To convert EKdVB Eq. (118) to the damped H-D Eq. (4), the traveling wave transformation φξτφX with X=ξ+λτ should be inserted into Eq. (118) and integrate once over η, and by applying the boundary conditions: φφφ0 as X, the constant forced damped following constant forced damped Duffing-Helmholtz equation is obtained

φ+2εφ++qφ2+rφ3+D=0,E120

where λ represents the reference frame speed, φ' and φ'' denote the first and second ordinary derivative of regarding X, ε=P4/2P3, p=λ/P3, q=P1/2P3, r=P2/3P3, and D=C/P3.

Note that the coefficient q may be positive or negative according to the values of plasma parameters and for studying oscillations using (120), solution (72) can be devoted for this purpose. In the absence of the ionic kinematic viscosity (P4=0 or ε=0), then Eq. (120) reduces to the constant forced undamping Duffing-Helmholtz equation and in this case the solution (63) can be applied for investigating the undamped oscillations in the present plasma model. Also, for q=0, Eq. (120) reduces to the constant forced damped Helmholtz equation. Moreover, the constant forced damped Duffing equation can be obtained for p=0.

## 10. Conclusion

The analytical and semi-analytical solutions for nonlinear oscillator integrable and non-integrable equations have been investiagted. First, the standard integrable Duffing equation has been analyized and its solutions have been obtained depending on the sign of its discriminant Δ. Accordingly, three cases Δ>0Δ<0andΔ=0 have been discussed in details and the solutions of each case has be obtained. Second, the analytical and semi-analytical solutions of the integrable Duffing-Helmholtz equation and its non-integrable family including the damped Duffing-Helmholtz equation, forced undamped Duffing-Helmholtz equation, forced damped Duffing-Helmholtz equation, and the damped and trigonometric forced Duffing-Helmholtz equation have been obtained and discussed in details. Third, the solutions to the intgrable cubic-quintic Duffing equation and the non-intgrable damped cubic-quintic Duffing equation have been investigated. Moreover, some realistic applications reaslted to the RLC circuits and physics of plasmas have been introduced and discussed depending on the solutions of the mentioned evolution equations.

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Written By

Alvaro Humberto Salas and Samir Abd El-Hakim El-Tantawy

Reviewed: 12 April 2021 Published: 28 May 2021