In recent years, piecewise-smooth stochastic systems (governed by piecewise-smooth stochastic differential equations) are usually used to describe biological and physical systems. Although for some simple piecewise-linear stochastic differential equations, analytical solutions of the transition probability distribution can be obtained  , it is difficult to attain analytical expressions for many other cases. Hence, we need to develop some effective numerical methods to deal with the difficulty in order to know more dynamical behaviors of the systems.
In this paper, we attempt to solve numerically a Fokker-Planck equation with discontinuous drift, which results from a so-called Brownian motion with pure dry friction . This dry friction model can be described as the following piecewise linear Langevin equation
Here denotes the sign of the velocity , representing the dry friction force. is the Gaussian white noise with zero mean and delta correlation with . The notation stands for the average overall possible realizations of the noise, and is the Dirac delta function. The transition probability distribution of (1.1) satisfies the following Fokker-Planck equation  ,
The corresponding initial condition is if for (1.1).
Since Equation (1.2) has a discontinuous drift , we must deal with it carefully. The IIM is a sharp interface method which can accurately capture discontinuities in the solution and the flux. This method has been used for many problems, such as elliptic interface problems , parabolic interface problems , moving interface problems  and many other applications    (see   for excellent reviews). To the best of our knowledge, there is no literature about the IIM for solving Fokker-Planck equations with discontinuous drift so far. Hence, our goal is to solve it.
The rest of this paper is organized as follows. In Section 2, we derive the IIM for the Fokker-Planck Equation (1.2). The numerical results are compared with the analytical solutions in Section 3. In addition, the accuracy of the scheme is also obtained. Finally, conclusions are made in Section 4.
2. The Scheme
We set for convenience. At the discontinuous point , we have the matching condition for the solution,
where and stand for the limiting values from the right- and left-hand sides of . Integrating (1.2) across the discontinuity, we find
by replacing with in (2.2).
It follows from (2.1) that , that is
according to Equation (1.2). Then using the relations (2.1)-(2.3) we have
For the numerical scheme, we have first to truncate the computational domain to a finite domain. Without loss of generality, let us assume the finite domain to be , where is a positive constant. Then we assume the probability vanishes at the boundary, i.e.,
A uniform grid with step is chosen here, where is a positive constant.
Therefore, the grid points can be expressed as , with the discontinuous point being between and , .
We hope to develop finite difference scheme of the form
, , (2.7)
where is the time-step size. This means that we need to determine the coefficients and the correction term so that
At a regular grid point , , the coefficients in the explicit difference scheme (2.7) are obtained by the standard approximation as follows
and the correction term
At the irregular grid point , we expand , and in Taylor series at the discontinuous point to obtain
For Equation (2.12), using (2.1), (2.2) and (2.4), we have
Furthermore, substituting (2.10), (2.11) and (2.13) into (2.8) we have
Then by arranging terms we obtain
Comparing both sides of (2.15), one obtains three equations for , and as follows
and the correction term
Therefore, one can solve (2.16) - (2.18) to attain the coefficients of Equation (2.7) for
In a similar way, we can compute the coefficients at the irregular grid point from the equations
and the correction term
3. Numerical results
For the Fokker-Planck Equation (1.2), using spectral decomposition method, one can get the transition probability distribution in closed analytic form  :
is the transition probability distribution in non-dimensional units and
is the error function. In addition, when the Fokker-Planck Equation (1.2) admits a steady stationary state
Let , and the computing interval be . We choose the space-step and the time-step . For simplicity, we take the analytic distribution (3.1) at time as the initial condition for computing. Figure 1 shows the comparison of numerical and analytical results of the probability distribution at different times. It can be seen that the numerical solutions (points) coincide with the exact solutions (solid lines), indicating the effectiveness of the Scheme (2.7).
To see the accuracy of the scheme numerically, we consider the and errors between the numerical solutions and the exact solutions defined by
where is the numerical solution and is the exact solution. Then we calculate the order of accuracy. A small time-step and are chosen and the problem is recalculated from time to . As illustrated in Table 1, the scheme is approximated second order in the velocity direction.
Figure 1. Transition probability distribution of Fokker-Planck Equation (1.2) with solid lines corresponding to the exact solutions, points to the numerical solutions, and dashed line to the stationary solution.
Table 1. Accuracy test in the velocity direction for and .
We have used the IIM to solve a Fokker-Planck equation with discontinuous drift in this paper. The numerical results show that the developed scheme is effective and has second order of accuracy. Moreover, the scheme can be readily extended to other dry friction models and the numerical results obtained are important references to see whether the dry friction effect exists in engineering applications.
This work was supported by the National Natural Science Foundation of China (Grant Nos. 11571366 and 11601517).
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