Published: 25 June 2019

Mathematical analysis of transport systems modeled by the stationary Kolmogorov-Feller equation with a nonlinear drift coefficient

Andrei Firsov1
Anton Zhilenkov2
1, 2Peter the Great St. Petersburg Polytechnic University, Saint Petersburg, Russia
Corresponding Author:
Anton Zhilenkov
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Abstract

The paper proposes the formulation of problems modeled with the stationary Kolmogorov-Feller equations with a nonlinear drift coefficient. Mathematical analysis of the model is given. The basis of the proposed method is the application of the Fourier transform to obtain analytical solutions to the problems under consideration.

1. Introduction

Kolmogorov-Feller type equations, which are very popular in solving problems from various fields of natural science (Markov’s process theory, control theory, astronomy, physical and chemical processes), are integro-differential equations with (in general) non-linear coefficients. The theory of such equations (from a mathematical point of view) is sufficiently well developed only for the case of coefficients that depend linearly on the independent variable. In the general case, as far as we know, there are few accurate analytical results, and there are practically no strictly grounded constructive methods suitable for applications. In this paper, we propose an analytically justified constructive method for constructing solutions of the corresponding integro-differential equation of Kolmogorov-Feller type for the case of a quadratic dependence of the drift coefficient on the independent variable. The proposed method is very convenient for numerical implementation and, as far as we know, is not found in the literature.

2. Mathematical model of the problem

Consider a variant of the Kolmogorov-Feller Eq. (1), which occurs in control theory, communication theory, stellar dynamics. In the literature devoted to analytic constructions of solutions of equations of type Eq. (1), the case of a linear dependence (β= 0) of the drift coefficient on the coordinate is usually considered; see, for example, [1-7]. In this paper, we will consider β 0:

1
ddx[(αx+βx2)W(x)]+ν+-p(A)W(x-A)dA-νW(x)=0,-<x<+,

under natural conditions:

2
W(x)0x±,+-W(x)dx=1,
3
p(A)|A|0,+-p(A)dA=1.

Additionally, suppose p(A) – analytical function with |A|<R for R big enough or at least its Fourier transform ˆp(k)=+-p(x)eixkdx exists and is an analytical function in a sufficiently large interval:

4
ˆp(k)=ˆp0+ˆp1k+ˆp2k+...,|k|<k0,k01.

Note that:

5
ˆps=ˆp(s)(0)s!=1s!(i)s+-xsp(x)dx.

In particular, by virtue of Eqs. (2, 3):

6
ˆp0=ˆp(0)=1.

Also, if p(x) – even function, then ˆp2s-1=0, s= 1, 2,... , and ˆp(k) is real.

We now turn in Eq. (1) to the Fourier transform of the function W(x):

ˆW(k)=+-W(x)eixkdx,
k[iβ^W''-α^W'+ν(ˆp-1k)ˆW]=0.

Conditions Eq. (2) at the same time go to:

7
{+-|ˆW(k)|dk<,ˆW(0)=1.

Thus, the problem of solving an Eq. (1) with conditions Eq. (2) can be replaced by a solution of an equation:

8
iβ^W''(k)-α^W'(k)+νρ(k)ˆW(k)=0,

satisfying conditions Eq. (7). Denoted here:

9
ρ(k)=ˆp(k)-1k.

By virtue of Eq. (6), and the conditions imposed on ˆp(k), we have:

10
ρ(0)=ˆp1=+-xp(x)dx.
11
ρ(k)=ˆp1+ˆp2k+ˆp3k2+...,|k|<k0.

Moreover, since ˆp(k)0, |k| (because ˆp is a Fourier transform), then:

12
ρ(k)~-1k,(|k|).

3. Mathematical model analysis

Assume:

13
ˆW(k)=φ(k)e-k0ψ(k)dk.

Notice, that:

14
φ(0)=ˆW(0)=1,
15
φ(-k)=¯φ(k).

(The last equality is a consequence of the choice of ψ(k), which will be done below – see Eq. (16)). Substituting Eq. (12) into Eq. (8), we get:

φ''+(-2ψ+iαβ)φ'+(ψ2-ψ'-iαβψ-iνβρ)φ=0.

Assuming:

16
ψ=iα2β,

yields the following equation for φ(k):

17
φ''-q(k)φ=0,

where:

18
φ(k)=ˆW(k)eiα2βk,
19
q(k)=-α22β2+iνβρ(k).

Let us note some elementary properties of q(k).

1) By virtue of Eq. (12):

20
q(k)+α22β2~-iνβk,(|k|),

and in particular:

21
q(k)-α22β2,(|k|).

2) By virtue of Eq. (11):

22
q(k)=-α22β2+iνβˆp1+iνβ(ˆp2k+ˆp3k2+...),|k|<k0,

or

22a
q(k)=q0+q1k+q2k2+...,|k|<k0,

where:

22b
{q0=-α22β2+iνβˆp1,qn=iνβˆpn+1.

3) Let:

23
δ=δ(k)+α22β2+νβImρ(k).

Then:

24
q(k)=-δ(k)+iνβReρ(k).

At the same time:

– if ρ(k) is real (which will take place, if, for example, p(x) – is an even function), then:

25
δ=const=α22β2>0.

– if Imρ(k)0, then by Eq. (12), for sufficiently large |k|:

26
δ(k)=α24β2+νβImρ(k)>0.

Lemma 1. The branch ofq(k):

27
(q(k))1=|q(k)|{12(1+(1+[Reρ(k)]2ν2δ2β2)-12)}1/2
-i|q(k)|{12(1-(1+[Reρ(k)]2ν2δ2β2)-12)}1/2,

is twice continuously differentiable by k(0,+) and Re(q(k))1>0 for k, which are large enough.

Remarks:

1) |q(k)|=(δ2+ν2β2[Reρ(k)]2)1/4.

2) If ρ(k) - is real, then Re(q(k))1>0 for all k>0.

3) By virtue of property Eq. (15), it suffices to construct a solution to Eq. (17) only for k0 [6-11].

4. Results and conclusions

An algorithm for the analytical solution of the Kolmogorov-Feller Eq. (1) is constructed and substantiated. This algorithm is as follows.

1) Enter the function φ(k)=ˆW(k)eiα2βk, where ˆW(k) is a Fourier transform of the desired function W(x).

2) For function φ(k) the equation holds:

φ''(k)-q(k)φ(k)=0,k>0,

under conditions:

{φ(0)=1,φ(k)0,k+,
q(k)=q0+q1k+q2k2+...,|k|<k0.

3) The coefficients qj are from the relations:

{q0=-α22β2+iνβˆp1,qn=iνβˆpn+1,

where ˆpj are from decomposition coefficients of the Fourier transform ˆp(k)=+-p(x)eixkdx.

4) The sought solution φ(k) can be determined from a system of linear algebraic equations, which will be given in further work.

References

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About this article

Received
12 May 2019
Accepted
18 June 2019
Published
25 June 2019
SUBJECTS
Mathematical models in engineering
Keywords
Kolmogorov-Feller equation
nonlinear drift coefficient
constructive method for solving