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Runge–Kutta method (SDE) : ウィキペディア英語版
Runge–Kutta method (SDE)
In mathematics of stochastic systems, the Runge–Kutta method is a technique for the approximate numerical solution of a stochastic differential equation. It is a generalization of the Runge–Kutta method for ordinary differential equations to stochastic differential equations (SDEs). Importantly, the method does not involve knowing derivatives the coefficient functions in the SDEs.
==Most basic scheme==

Consider the Itō diffusion X satisfying the following Itō stochastic differential equation
: = a(X_) \, t + b(X_) \, W_,
with initial condition X_0=x_0, where W_t stands for the Wiener process, and suppose that we wish to solve this SDE on some interval of time (). Then the basic Runge–Kutta approximation to the true solution X is the Markov chain Y defined as follows:〔P. E. Kloeden and E. Platen. ''Numerical solution of stochastic differential equations'', volume 23 of Applications of Mathematics. Springer--Verlag, 1992.〕
* partition the interval () into N subintervals of width \delta=T/N>0:
:0 = \tau_ < \tau_ < \dots < \tau_ = T;
* set Y_0:=x_0;
* recursively compute Y_n for 1\leq n\leq N by
:Y_ := Y_ + a(Y_) \delta + b(Y_) \Delta W_ + \frac \left( b(\hat_) - b(Y_) \right) \left( (\Delta W_)^ - \delta \right) \delta^,
where \Delta W_ = W_}
and \hat_ = Y_ + a(Y_n) \delta + b(Y_) \delta^.
The random variables \Delta W_ are independent and identically distributed normal random variables with expected value zero and variance \delta.
This scheme has strong order 1, meaning that the approximation error of the actual solution at a fixed time scales with the time step \delta. It has also weak order 1, meaning that the error on the statistics of the solution scales with the time step \delta. See the references for complete and exact statements.
The functions a and b can be time-varying without any complication. The method can be generalized to the case of several coupled equations; the principle is the same but the equations become longer.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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