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

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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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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.


  • 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


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


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)


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]


which is subjected to the following initial conditions


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

For example, let us assume


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


where cn = cn ωt+c2m.

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


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


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


Making use of the additional formula


the solution (10) could be expressed as




Solution (12) is a periodic solution with period


Example 1.

Let us consider the i.v.p.


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


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


and its periodicity is given by


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.

Figure 1.

A comparison between the analytical solution (17) and the approximate numerical RK4.

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


where y=yt is a solution of some Duffing equation


with initial conditions


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


we get


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


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


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




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


Example 2.

Let us assume the following i.v.p.


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


and the periodicity of this solution is given by


Solution (27) is displayed in Figure 2.

Figure 2.

The profile of solution (27) is plotted against t.

2.3 Third case: Δ=0

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




which may be verified by direct computation.

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


is given by


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


is given by


Remark 3. According to the following identity




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






The solution (36) is periodic with period


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




and λ is a root of the cubic


Example 3.

Let us consider the i.v.p.


The approximate solution in trigonometric form is given by


The exact solution reads


with period


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.

Figure 3.

A comparison between the approximate solution (44) and exact solution (45).

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




and λ is a solution of the quintic


Example 4.

The approximate trigonometric solution of




The exact solution is


with period


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.

Figure 4.

A comparison between the approximate solution (50) and exact solution (51).


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

The undamped Duffing-Helmholtz equation reads


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


where BC0.

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




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


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


We have


The number A is a solution to the quartic


Example 5.

The solution of the i.v.p.


according to the relation (54) is given by


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

Figure 5.

A comparison between solution (61) and the approximate numerical solution using RK4.

The periodicity of solution (61) is given by


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.


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


where ζ is a solution to the cubic algebraic equation


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


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


Example 6.

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


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


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

Figure 6.

A comparison between the solution (68) and the RK4 solution.

The periodicity of solution (68) is given by


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

Let us define the following i.v.p.


Suppose that


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


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




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


Let us define the following residual


and by applying the condition R0=0, we obtain


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

Example 7.



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


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


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

Figure 7.

A comparison between solution (78) and RK4 solution.

Remark 5. For the damped and constant forced Helmholtz equation


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


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.


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


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




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


where p˜=122p+3rc12+3rc224ερ+2ρ2.

The value of ρ can be determined from the following equation


Example 8.



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


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


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

Figure 8.

A comparison between solution (90) and RK4 solution.


7. An analytic solution of cubic-quintic Duffing equation

Let us consider the following ordinary differential equation [36]


which is subjected to the following initial conditions


Theorem 1.

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


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


The values of the coefficients p and q are given by


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


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


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


The values of the coefficients p and q are expressed as


and the value of λ is a solution of the cubic


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


and using Eq. (100), we have




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.


We seek approximate analytic solution in the ansatz form




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


Define the residual


then, the condition R0=0 gives


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.



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




The distance error as compared to the RK4 numerical solution reads


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].

Figure 9.

A comparison between RK4 solution and (a) Solution (113) and (b) Zuñiga solution [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


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


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


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


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


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


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