Open access peer-reviewed chapter

# A Perturbation Theory for Nonintegrable Equations with Small Dispersion

Written By

Georgy Omel’yanov

Submitted: May 4th, 2017 Reviewed: September 18th, 2017 Published: December 20th, 2017

DOI: 10.5772/intechopen.71030

From the Edited Volume

## Complexity in Biological and Physical Systems

Edited by Ricardo López-Ruiz

Chapter metrics overview

View Full Metrics

## Abstract

We describe an approach called the “weak asymptotics method” to construct multisoliton asymptotic solutions for essentially nonintegrable equations with small dispersion. This paper contains a detailed review of the method and a perturbation theory to describe the interaction of distorted solitons for equations with small perturbations. All constructions have been realized for the gKdV equation with the nonlinearity uμ, μ ∈ (1, 5).

### Keywords

• generalized Korteweg-de Vries equation
• soliton
• interaction
• perturbation
• weak asymptotics method
• 2010 Mathematics Subject Classification: 35D30
• 35Q53
• 46F10

## 1. Introduction

We consider the problem of propagation and interaction of soliton-type solutions of nonlinear equations. Our basic example is the nonhomogeneous version of the generalized KdV equation

ut+uµx+ε23ux3=fu,εux,xR1,t>0,E1

where μ ∈ (1, 5), ε ≪ 1, f(u, z) is a known smooth function such that f(0, 0) = 0. Note that the restriction on μimplies both the soliton-type solution and the stability of the equation with respect to initial data (see, for example [1, 2]).

In the special case f ≡ 0 and μ = 2 (μ = 3), Eq. (1) is the famous KdV (modified KdV) equation. It is well known that KdV (mKdV) solitons are stable and interact in the elastic manner: after the collision, they preserve the original amplitudes and velocities shifting the trajectories only (see [3] and other bibliographies devoted to the inverse scattering transform (IST) method). In the case of μ = 2 (μ = 3) but with f ≠ 0, Eq. (1) is a nonintegrable one. However, using the smallness of ε(or of ffor other scaling), it is possible to create a perturbation theory that describes the evolution of distorted solitons (see the approaches by Karpman and E. Maslov [4] and Kaup and Newell [5] on the basis of the IST method, and the “direct” method by V. Maslov and Omel’yanov [6]). Moreover, the approach by V. Maslov and Omel’yanov [6] can be easily extended to essentially nonintegrable equations (μ ≠ 2, 3), but for a single soliton only. In fact, it is impossible to use any direct method in the classical sense for the general problem of the wave interaction. To explain this proposition, let us consider the homogeneous gKdV equation

ut+uµx+ε23ux3=0,xR1,t>0.E2

It is easy to find the explicit soliton solution of (2),

uxtε=βxVt/ε,ωη=coshγη/γ,E3
γ=2/μ1,V=β2,Aμ1=Vμ+1/2.E4

Next let us consider two-soliton initial data

ut=0=i=12Aiωβixxi0/ε,E5

where x(1, 0) > x(2, 0) and A2 > A1. Obviously, since (x(2, 0) − x(1, 0))/ε → ∞ as ε → 0, the sum of the waves (3)

u=i=12AiωβixVitxi0/εE6

approximates the problem (2), (5) solution with the precision O(ε) but for t ≪ 1 only. Conversely, the sum (6) does not satisfy the gKdV equation for t ∼ O(1) in view of the trajectories x = Vit + x(i, 0) intersection at a point (x*, t*).

Let us consider shortly how it is possible to analyze the problem (2), (5). There are some different cases:

1. Let A1 ≪ A2. Then, one can construct an asymptotic solution

u=Wxϕ2t/ε,t,x,ε,ν,E7

where ν = A1/A2 ≪ 1 and W((x − ϕ2(t))/ε, t, x, ε, ν) = A2ω(β2(x − V2t − x(2, 0))/ε) + O(ν + ε). Thus, to find the leading term of the asymptotics, we obtain an equation with nonlinear ordinary differential operator; whereas to construct the corrections, it is enough to analyze the linearization of this operator. This construction (with a little bit of other viewpoints) has been realized by Ostrovsky et al. [7].

1. Let A2 − A1 ≪ 1. We write again the ansatz in the form (7), where ν = A2 − A1 ≪ 1 now, and we assume ν/ε ≪ 1. In fact, this case coincides with the problem considered in [7].

2. The amplitudes A2 > A1 are arbitrary numbers. Then, we should write a two-phase ansatz

ux,t,ε=Wxϕ1t/ε,xϕ2t/ε,t,x,εE8

without any additional parameter. Substituting (8) into equation (2), we obtain for the leading term W0(τ1, τ2, t):

ÂW0+B̂W0µ+B̂3W0=0,Â=i=12ϕ̇iτi,B̂=i=12τi.E9

Since ϕ̇1ϕ̇2, we can pass to new variables, η=τ1τ2/ϕ̇2ϕ̇1, ζ=ϕ̇1τ2ϕ̇2τ1/ϕ̇1ϕ̇2, and transform equation (9) to the gKdV form (2) again

W0η+W0μζ+3W0ζ3=0.E10

Therefore, to construct two-phase asymptotics, we should solve (10) explicitly what is impossible for any essentially nonintegrable case.

This difficulty can be overcome by using the weak asymptotics method. The main point here is that solitons tend to distributions as ε → 0. Thus, it is possible to pass to the weak description of the problem, ignore the actual shape of the multiwave solutions, and find only the main solution characteristics, that is, the time dynamics of wave amplitudes and velocities. The weak asymptotics method has been proposed at first for shock wave type solutions [8] and for soliton-type solutions [9] many years ago. Further generalizations, modifications, and adaptations to other problems can be found in publications by M. Colombeau, Danilov, Mitrovic, Omel’yanov, Shelkovich, and others, see, for example, [10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20] and references therein.

The contents of the paper are the following: in Section 2, we present a detailed survey of the weak asymptotics method application to the problem of multisoliton asymptotics and Section 3 contains new results, namely a perturbation theory to describe the evolution and collision of distorted solitons for equation (1).

## 2. Weak asymptotics method

### 2.1. Main definitions

Let us associate equation (2) with first two conservation laws written in the differential form:

Qjt+Pjx=ε23Rjx3,j=1,2,E11
Q1=u,P1=uμ,Q2=u2,P2=2μuμ+1/μ+13εux2,E12

and R1 = u, R2 = u2. Next, we define smallness in the weak sense:

Definition 1.A function v(t, x, ε) is said to be of the valueOD'εϰif the relationvt,x,εΨxdx=Oεϰholds uniformly in t for any test functionΨRx1. The right-hand side here is a-function for ε = const > 0 and a piecewise continuous function uniformly in ε ≥ 0.

Following [9, 17, 18], we define two-soliton weak asymptotics:

Definition 2.A sequence u(t, x, ε), belonging toC0TCx1for ε = const > 0 and belonging toC0TD'x1uniformly in ε, is called a weak asymptotic modOD'ε2solution of(2)if the relations(11)hold uniformly in t with the accuracyOD'ε2.

Let us consider the interaction of two solitary waves for the model (2) with the initial data (5).

Following [9, 17, 18] again, we write the asymptotic ansatz in the form:

u=i=12Giτωβixϕit,τ,ε/ε,Giτ=Ai+Siτ.E13

Here ϕi = ϕi0(t) + εϕi1(τ), where ϕi0 = Vit + x(i, 0) are the trajectories of noninteracting solitary waves, τ = ψ0(t)/εdenotes the “fast time”, ψ0(t) = β1(ϕ20(t) − ϕ10(t)), and the phase and amplitude corrections ϕi1, Siare smooth functions such that with exponential rates

ϕi1τ0asτ,ϕi1τϕi1=constiasτ+,E14
Siτ0asτ±.E15

### 2.2. Two-wave asymptotic construction

To construct the asymptotics, we should calculate the weak expansions of the terms from the left-hand sides of the relations (11). It is easy to check that

u=εi=12a1Giβiδxϕi+OD'ε3,E16

where δ(x) is the Dirac delta-function. Here and in what follows, we use the notation

ak=defωηkdη,k>0,a2=defωη2dη.E17

At the same time for any F(u, εu/∂x) ∈ C1, we have

Fi=12Giωβixϕiε,i=12βiGiωβixϕiεψxdx=εi=121βiFAiωη,βiAiωηψϕi+εηβidη+εβ2{Fi=12Giωηi2,i=12βiGiωηi2i=12FAiωηi2,βiAiωηi2}ψϕ2+εηβ2dη,E18

where

η12=θησ,η22=η,σ=β1ϕ1ϕ2/ε,θ=β1/β2.E19

We take into account that the second integrand in the right-hand side of (18) vanishes exponentially fast as ∣ϕ1 − ϕ2∣ grows; thus, its main contribution is at the point x*. We write

ϕi0=x*+Vitt*=x*+εViτ/ψ̇0andϕi=x*+εχi,E20

where ψ̇0=β1V2V1, χi=Viτ/ψ̇0+ϕi1. It remains to apply the formula

fτδxϕi=fτδxx*εχifτδxx*+OD'ε2,E21

which holds for each ϕiof the form (20) with slowly increasing χiand for f(τ) from the Schwartz space. Moreover, the second term in the right-hand side of (21) is OD'ε. Thus, under the assumptions (14) and (15), we obtain the weak asymptotic expansion of F(u, εu/∂x) in the final form:

Fu,εux=εi=12aF,i0βiδxϕiεaF,i1βiδxx*+εβ2RF0δxx*εR¯Fδxx*+OD'ε3,R¯F=χ2RF0+RF1/β2,E22

where

aF,in=ηnFAiωηβiAiωη,E23
Fn=ηnFi=12Giωηi2i=12βiGiωηi2i=12FAiωηi2βiAiωηi2.E24

Here, we take into account that to define u2/tmodOD'ε2, it is necessary to calculate u2 with the precision OD'ε3. Thus, using (22) with F(u) = u2 and transforming (16) with the help of (21), we obtain modulo OD'ε3:

u=εi=12a1Ki01δxϕi+εi=12a1Ki11δxx*εχiδxx*,E25
u2=εi=12a2Ki02δxϕi+εβ2Ru20δxx*εR¯u2δxx*,E26

where

Kin=Gin/βi,Ki0n=Ain/βi,Ki1n=KinKi0n.E27

Calculating weak expansions for other terms from Definition 2 and substituting them into (11), we obtain linear combinations of εδ(x − ϕi), i = 1, 2, δ(x − x*), and εδ(x − x*). Therefore, we pass to the system:

a1ViKi01aP1,i0/βi=0,a2ViKi02aP2,i0/βi=0,i=1,2,E28
i=12Ki11=0,u20=0,i=1,2,E29
ψ̇0ddτi=12Ki01ϕi1+χiKi11=f,ψ̇0ddτi=12a2Ki02ϕi1+R¯u2=F,E30

where

f=1a1β2RP10,F=1β2RP20a1ψ̇0i=12ϕi1dKi12dτ.E31

The first four algebraic equations (28) imply again the relation (4) among Ai, βi, and Vi. Furthermore, there exists a number θ* ∈ (0, 1) such that equations (29), (30) have the required solution Si, ϕi1 with the properties (14) and (15) under the sufficient condition θ ≤ θ* (see [9, 17]). It is obvious that the existence of the weak asymptotics (13) with the properties (14) and (15) implies that the solitary waves interact like the KdV solitons at least in the leading term.

Theorem 1.Let θ ≤ θ*. Then(13)describesmodOD'ε2the elastic scenario of the solitary waves interaction for the μ-gKdVequation (2).

Numerical simulations ([14, 15, 17]) confirm the traced analysis, see Figure 1. Note that a small oscillating tail appears after the soliton collision, see [15] for detail. Obviously, this effect is similar to the “radiation” appearance for the perturbed KdV [21].

### 2.3. Multisoliton interaction

N-wave solutions of the form similar to waves (13) contain 2Nfree functions Si, ϕi1. Thus, to describe an N-soliton collision, we should consider Nconservation laws. However, nonintegrability implies the existence of a finite number of conservation laws only. For this reason, we need to involve into the consideration balance laws. For the gKdV-4 equation, the first conservation and balance laws have the form

Qjt+Pjx+ε1Kj=OD'ε2,E32

where Qj, Pj, j = 1, 2, coincide with (12) for μ = 4, Ki = 0, i = 1, 2, 3,

Q3=εux225u5,P3=16u3εux2u83ε2uxx2,E33
Q4=12ε2uxx2+521u8103u3εux2,K4=εux5,E34
P4=12u3ε2uxx219uεux432ε3uxxx2+160231u111003u6εux2.E35

Note that the nondivergent “production” ε−1K4 has the same value O(ε−1) (in the C-sense and for rapidly varying functions) as the first ones in (32).

The formal scheme of the asymptotic construction is similar to the one described above: we write the ansatz of the form (13) but with Nsummands, found weak representations for all terms in (32), and pass to a system similar to (28)(30). The main obstacle here is the proof that this system admits a solution with the properties of (14), (15). This idea has been realized in [18, 19] for the problem of three soliton collisions for the gKdV-4 equation.

Theorem 2.Let us denote Ai the amplitudes of the original solitons and x(i, 0)their initial positions such that Ai + 1 > Ai, x(i, 0) > x(i + 1, 0), and i = 1, 2. Let all trajectories x = ϕi0(t) have an intersection point(x*, t*). Then, under the assumption

β2/β3=ν3,β1/β3=ν33+α/2,α01E36

with sufficiently smallν< 1, the three-phase asymptotic solution exists and describesmodOD'ε2the elastic scenario of the solitary waves interaction.

Figure 2 depicts the evolution of a three-wave solution [14].

### 2.4. Asymptotic equivalence

Let us come back to the case of two-phase asymptotics and transform the ansatz (13) to the following form:

u˜=i=12Giτωβixϕit,τ,εε+SiτWβixϕit,τ,εε,E37

where Siτ, i = 1, 2 are arbitrary functions from the Schwartz space,

Wη=d2l+1ωη/dη2l+1,E38

and l ≥ 1 is an arbitrary integer. Calculating the weak representations for u˜and u˜2, we obtain

u˜=u+OD'ε2l+2,u˜2=εi=12a2Ki02δxϕi+εβ2Ru˜20δxx*+OD'ε2,E39

where

u˜20=i=12Giωηi2+SiWηi22i=12Aiωηi22,E40

and uin the right-hand side in (39) is the representation (25). Thus, the difference between uof the forms (13) and (37) is arbitrarily small in the sense D'x. At the same time, instead of (29), (30), we obtain

i=12Ki11=0,u˜20=0,i=1,2,E41
ψ̇0ddτi=12Ki01ϕi1+χiKi11=f˜,ψ̇0ddτi=12a2Ki02ϕi1+R¯u˜2=F˜,E42

where f˜, F˜differ from f, Fin the same manner as u˜20differs from u20. The system (41) and (42) have again a solution with the properties (14) and (15) [9, 12]; however, it differs from the solution of Eqs. (29) and (30) with the value O(1) in the C-sense. Moreover, the asymptotic solutions (13) and (37) differ with the precision OD'εin the sense of Definition 1. This implies the principal impossibility to describe explicitly neither the real shape of the waves at the time instant of the collision nor the real ε-size displacements of the trajectories after the interaction. However, the nonuniqueness of the value O(ε) is concentrated within O(ε1 − ν)-neighborhood of the time instant t* of the interaction, ν > 0. Thus, it is small in the D'Rx,t2sense. We set

Definition 3.Functions u1(x, t, ε) and u2(x, t, ε) are said to be asymptotically equivalent if for any test functionψDR2

u1xtεu2xtεψxtdxdt=Oε2.E43

In this sense, the solutions (13) and (37) are asymptotically equivalent.

We now focus attention on another question: how to choose, from the set of all possible conservation and balance laws, those that allow to construct a multiphase asymptotic solution? It seems that there is not any rule and it is possible to use arbitrary combination of the laws. Thus, there appears the next question: what is the difference between such solutions? This problem has been discussed in [20] for two-phase asymptotic solutions of the gKdV-4 equation. Let us define two-phase asymptotics in the following manner:

Definition 4.Let1 ≤ k0 < k1 ≤ 4 and let a sequence uk0, k1 = uk0, k1(t, x, ε) belong to the same functional space as u(t, x, ε) inDefinition 2. Then, uk0, k1is called a weak asymptotic modOD'ε2solution of(2)if therelations (32)hold for j = k0and j = k1uniformly in t.

A detailed analysis implies the assertion [20].

Theorem 3.Let θ be sufficiently small. Then, the weak asymptotic solutions u1, k1andu1,k1of the problem(2),(5)exist and they are asymptotically equivalent for allk1,k1234.

## 3. Collision of distorted solitons

We consider now the nonhomogeneous version of the gKdV equation (1). It is easy to verify that, in the case of rapidly varying solutions, the right-hand side fcan be treated as a “small perturbation.”

An approach to construct one-phase self-similar asymptotic solutions for (1) had been created in [6] (see also [17]). Let us generalize this approach to the multiphase case. From the beginning, we state that equation (1) is associated with balance laws, the first two of which are

Qjt+Pjx+Kj=OD'ε2,j=1,2,E44

where Qjand Pjcoincide with ones described in (12),

K1=fuεux,K2=ufuεux.E45

Note that, in contrast to Kjin (32), productions here are regularly degenerating functions with the value O(1) in the C-sense.

Let us first construct a two-phase version of self-similar asymptotics, which assumes a special initial data for (1) and discuss afterward how to treat it for more realistic initial data. By analogy with Definition 2, we write:

Definition 5.Let a sequence u = u(t, x, ε) belong to the same functional space as in Definition 2. Then u is called a weak asymptotic modOD'εq¯solution of(1)if therelation (44)hold uniformly in t ∈ (0, T), q¯=minμ2.

Generalizing one-phase asymptotics, we write the ansatz as

u=i=12Giτ,tωηi+εzix,tHηi+Giτωηi,E46
Giτ,t=Ait+Siτ,ηi=βitxϕi/ε,ϕit,τ,ε=ϕi0+εφi1.E47

Here Ai(t), ϕi0 = ϕi0(t), βi2t=γAμ1t, ω(η), Si(τ), ϕi1(τ) are the same as in (13); τ = ψ0(t)/εwith ψ0(t) = ϕ20(t) − ϕ10(t) denotes the “fast time” again; zixtC; and Gi, ℌ are smooth functions such that

Giτ0asτ,GiτGi=constiasτ+,E48
η1asη,η0asη+E49

with exponential rates. We assume also the intersection of the trajectories x = ϕi0(t), i = 1, 2 at a point x* = ϕi0(t*) namely,

t*>0such thatϕ10t*=ϕ20t*,ψ̇0=defddtϕ20tϕ10tt=t*0.E50

It is easy to verify the weak representations with the precision OD'ε2:

u=εi=12a1Ki01δxϕi+a1Ki11*δxx*+zix,tHϕix,E51
ut=a1ψ̇0τi=12Ki11*δxx*+εi=12zitHϕix+εa1i=12dKi11dt+ziϕi,tdϕi0dtδxϕiεa1i=12Ki01dϕi0dtδxϕi+εi=12ψ̇0ta1Siβi+zix*,tϕi1a1βi2Sidβidtt=t*δxx*εa1i=12Ki11dϕi0dt+ψ̇0Ki1ϕi1τ+χiβiKi1τt=t*δxx*,E52

where H(x) is the Heaviside function, H(x) = 0 for x < 0 and H(x) = 1 for x > 0; g*=defgτ,tt=t*, χi=defϕiΦετ,t*,τ,εx*, and Φ(ετ, t*) is the solution of the equation ϕ20(t* + Φ) − ϕ10(t* + Φ) = ετ, which exists in accordance with (50).

Next, the existence of nonsoliton summands in (51) implying a correction of formula (22), namely

Fu,εux=εi=12aF,i0βi1δxϕi+εβ21RF0δxx*+εFu0,0i=12zix,tHϕix+OD'εq¯,E53

where aF,i0and F0are defined in (23), (24), Fu0,0=F(u,0)/uu=0.

Repeating the same calculations as above, we obtain linear combinations of εδ(x − ϕi), εδ(x − ϕi), εH(ϕi − x), i = 1, 2; δ(x − x*), εδ(x − x*), and εδ(x − x*). Equating zero, the coefficients of εδ(x − ϕi) and εδ(x − ϕi) yield

a1Aidϕi0dt=aP1,i0,a2ddtAi2βi=aK2,i0βi,E54
a2Ai2dϕi0dt=aP2,i0,a1ddtAiβi+ziϕi,tdϕi0dt=af,i0βi.E55

Equation (54) forms the closed system to define Ai(t) and ϕi0(t). To simplify it, let us use the equalities (4) and rewrite the model equation for ω(η) as follows:

dω+μ+12ωμ+d2ωdη2=0.E56

Simple manipulations with (56) allow us to find relations between structural constants:

a1=μ+1aμ/2,a2=μ+3aμ+1/4,a2=μ1aμ+1/4.E57

Next, we use (57), the equality βi2=γAiμ1, add the initial conditions, and obtain from (54) the Cauchy problem

dAidt=c1aK2,i0Ai1,dϕi0dt=aµa1Aiµ1,t>0,E58
Ait=0=Ai0,ϕi0t=0=xi,0,E59

where c1 = 2/(a2(5 − μ)); Ai0>0and x(i, 0) are arbitrary numbers; and i = 1, 2. Note also that the first equalities in equations (54) and (55) are equivalent.

Next, equating zero the coefficients of the Heaviside functions, we obtain the equations

zit=fu0,0zi,x<ϕi0t,t>0,i=1,2.E60

In view of (58)i0/dt > 0, so we use the second equality in (55) to state the correct initial condition for (60)

zix,tx=ϕi0t=γaf,i0Ai34µ/2t+c2aK2,i0Ai13µ/2t,t>0,E61
zixtt=0=zi0x,xxi0,E62

where c2 = a1(3 − μ)(1 + μ)/(2a2(5 − μ)), zi0xis an arbitrary smooth function, which satisfies the consistency condition

zi0xi0=γaf,i0Ai34μ/2+c2aK2,i0Ai13μ/2t=0.E63

We should note that the nonlinearity uμin (1) can require the inequality u ≥ 0. To this end, we will assume

Ait>0,zi(x,t)x=ϕi0t0fort0.E64

Furthermore, equating zero the coefficients of δ(x − x*) and εδ(x − x*) yield (29), (30) again. Consequently, the condition θ ≤ θ* guaranties the existence of Si, ϕi1 with the properties of (14), (15). In particular

S1=θS2S1=θβ1γλσ1+Oθq,E65

where

q=min1γ,λσ=a21ωηωη12.E66

The last step of the construction is the determination of Giτ, i = 1, 2. By setting the coefficients of εδ(x − x*) zero, we obtain

τi=12Giβi*=F1,τi=12Giβi+Gi¯β2λσt=t*Gi=F2,E67

where i¯=2for i = 1 and i¯=1for i = 2,

F1=i=12zix*,ta1dϕi1dτ+ψ̇01tSiβit=t*+1β2Rf0t=t*,E68
F2=τi=12aHzix*,tSiβi+zi¯x*,tGiβ2λH,iσt=t*+RK20cHt=t*,E69
aH=a1/2a2,cH=2a2ψ̇0β2,λH,iσ=a21ωηi2ηi¯2.

Calculating the determinant Δof the matrix in the left-hand part of (67) and using (65), we conclude

Δ=G2G1+λG1θG2t=t*=β2γ1θγλθθγλ2θγ1+Oθqt=t*.E70

Obviously, Δ ≠ 0 for sufficiently small θ. Since the right-hand sides Fibelong to the Schwartz space, the functions Giexist and satisfy the assumption (48).

Henceforth, we pass to the final result:

Theorem 4.Let θ be sufficiently small and let the assumptions(50),(63), and(64), if it is necessary, be fulfilled. Then, the self-similar two-wave weak asymptotic modOεq¯solution of theequation (1)exists and has the form(46).

Let us finally stress that the self-similarity implies a special choice of the initial data: for the classical asymptotics in the C-sense, there appears a very restrictive condition for small correction of the soliton A(0)ω((x − x0)/ε) (see [6, 17]), and for weak asymptotics, there appears the restriction (63). If it is violated, then the perturbed soliton generates a rapidly oscillating tail of the amplitude o(1) (“radiation”) instead of the smooth tails εu(x, t) (see [21] and numerical results [14, 15, 17]). Nowadays, this radiation phenomenon can be described analytically only for integrable equations, so that we should use self-similar approximation for essentially nonintegrable equations. However, the smooth tail εu(x, t), which can be treated as an average of the radiation, describes sufficiently well the tendency of the radiation amplitude behavior, see graphics depicted in Figures 3 and 4, and other numerical results in [15, 17]).

## Acknowledgments

The research was supported by SEP-CONACYT under grant no. 178690 (Mexico).

## References

1. 1. Bona J, Souganidis P, Strauss W. Stability and instability of solitary waves of Korteweg-de Vries type. Proceedings of the Royal Society of London A. 1987;411(1841):395-412. DOI: 10.1098/rspa.1987.0073
2. 2. Faminskii A. Cauchy problem for the Korteweg-de Vries equation and its generalizations. Journal of Soviet Mathematics. 1990;50(1):1381-1420. Available from:https://link.springer.com/article/10.1007/BF01097030
3. 3. Ablowitz M, Segur H. Solitons and the Inverse Scattering Transform. Philadelphia: SIAM; 1981. Available from:http://dx.doi.org/10.1137/1.9781611970883.bm
4. 4. Karpman V, Maslov E. Perturbation theory for solitons. Journal of Experimental and Theoretical Physics. 1977;46(2):281-291. Available from:http://jetp.ac.ru/cgi-bin/dn
5. 5. Kaup D, Newell A. Solitons as particles, oscillators and in slowly changing media: A singular perturbation theory. Proceedings of the Royal Society of London Series A. 1978;361:413-446. DOI: 10.1098/rspa.1978.0110
6. 6. Maslov V, Omel’yanov G. Asymptotic soliton-form solutions of equations with small dispersion. Russian Mathematical Surveys. 1981;36:73-149. Available from:http://iopscience.iop.org/0036-0279/36/3/R02
7. 7. Gorshkov K, Ostrovsky L. Interaction of solitons in nonintegrable systems: Direct perturbation method and applications. Physica D. 1981;3:428-438. Available from:https://doi.org/10.1016/0167-2789(81)90146-9
8. 8. Danilov V, Shelkovich V. Generalized solutions of nonlinear differential equations and the Maslov algebras of distributions. Integral Transformations and Special Functions. 1997;6:137-146. Available from:http://dx.doi.org/10.1080/10652469808819161
9. 9. Danilov V, Omel’yanov G. Weak asymptotics method and the interaction of infinitely narrow delta-solitons. Nonlinear Analysis: Theory Methods & Applications. 2003;54:773-799 Available from:https://doi.org/10.1016/S0362-546X(03)00104-4
10. 10. Abreu E, Colombeau M, Panov E. Weak asymptotic methods for scalar equations and systems. Journal of Mathematical Analysis and Applications. 2016;444(2):1203-1232. Available from:https://doi.org/10.1016/j.jmaa.2016.06.047
11. 11. Danilov V, Mitrovic D. Shock wave formation process for a multidimensional scalar conservation law. Quarterly of Applied Mathematics. 2011;69(4):613-634. Available from: doi: 10.1090/S0033-569X-2011-01234-9
12. 12. Danilov V, Omel’yanov G, Shelkovich V. Weak asymptotics method and interaction of nonlinear waves. In: Karasev M, editor. Asymptotic Methods for Wave and Quantum Problems. Trans. Ser. 2. Vol. 208. Providence, RI: American Mathematical Society; 2003. p. 33-164. Available from:https://goo.gl/aPPhqG
13. 13. Danilov V, Shelkovich V. Dynamics of propagation and interaction of delta-shock waves in conservation law systems. Journal of Differential Equations. 2005;211(2):333-381. Available from:https://goo.gl/Y6g4vv
14. 14. Garcia M, Omel’yanov G. Interaction of solitary waves for the generalized KdV equation. Communications in Nonlinear Science and Numerical Simulation. 2012;17(8):3204-3218. Available from:https://doi.org/10.1016/j.cnsns.2011.12.001
15. 15. Garcia M, Omel’yanov G. Interaction of solitons and the effect of radiation for the generalized KdV equation. Communications in Nonlinear Science and Numerical Simulation. 2014;19(8):2724-2733. Available from:https://doi.org/10.1016/j.cnsns.2013.06.024
16. 16. Kalisch H, Mitrovic D. Singular solutions of a fully nonlinear 2 × 2 system of conservation laws. Proceedings of the Edinburgh Mathematical Society II. 2012;55:711-729. DOI:https://doi.org/10.1017/S0013091512000065
17. 17. Omel’yanov G, Valdez-Grijalva M. Asymptotics for a C1-version of the KdV equation. Nonlinear Phenomena in Complex Systems. 2014;17(2):106-115. Available from:http://elib.bsu.by/handle/123456789/116849
18. 18. Omel’yanov G. Soliton-type asymptotics for non-integrable equations: A survey. Mathematical Methods in The Applied Sciences. 2015;38(10):2062-2071. DOI: 10.1002/mma.3198 2014
19. 19. Omel’yanov G. Propagation and interaction of solitons for nonintegrable equations. Russian Journal of Mathematical Physics. 2016;23(2):225-243. DOI: 10.1134/S1061920816020084
20. 20. Omel’yanov G. Multi-soliton Collision for essentially nonintegrable equations. In: Oberguggenberger M et al., editors. Generalized Functions and Fourier Analysis, Series: Operator Theory: Advances and Applications. Birkhäuser, Cham. 2017;260:153-170. DOI:https//doi.org/10.1007/978-3-319-51911-1_9
21. 21. Kalyakin L. Perturbation of the Korteweg-de Vries soliton. Theoretical and Mathematical Physics. 1992;92:736-747. Available from:https://goo.gl/aPPhqG

Written By

Georgy Omel’yanov

Submitted: May 4th, 2017 Reviewed: September 18th, 2017 Published: December 20th, 2017