`snssde1d()`

Assume that we want to describe the following SDE:

Ito form³:

\[\begin{equation}\label{eq:05} dX_{t} = \frac{1}{2}\theta^{2} X_{t} dt + \theta X_{t} dW_{t},\qquad X_{0}=x_{0} > 0 \end{equation}\] Stratonovich form: \[\begin{equation}\label{eq:06} dX_{t} = \frac{1}{2}\theta^{2} X_{t} dt +\theta X_{t} \circ dW_{t},\qquad X_{0}=x_{0} > 0 \end{equation}\]

In the above $f(t,x)=\frac{1}{2}\theta^{2} x$ and $g(t,x)= \theta x$ ($\theta > 0$), $W_{t}$ is a standard Wiener process. To simulate this models using snssde1d() function we need to specify:

The drift and diffusion coefficients as R expressions that depend on the state variable x and time variable t.
The number of simulation steps N=1000 (by default: N=1000).
The number of the solution trajectories to be simulated by M=1000 (by default: M=1).
The initial conditions t0=0, x0=10 and end time T=1 (by default: t0=0, x0=0 and T=1).
The integration step size Dt=0.001 (by default: Dt=(T-t0)/N).
The choice of process types by the argument type="ito" for Ito or type="str" for Stratonovich (by default type="ito").
The numerical method to be used by method (by default method="euler").

R> theta = 0.5
R> f <- expression( (0.5*theta^2*x) )
R> g <- expression( theta*x )
R> mod1 <- snssde1d(drift=f,diffusion=g,x0=10,M=1000,type="ito") # Using Ito
R> mod2 <- snssde1d(drift=f,diffusion=g,x0=10,M=1000,type="str") # Using Stratonovich 
R> mod1

Itô Sde 1D:
    | dX(t) = (0.5 * theta^2 * X(t)) * dt + theta * X(t) * dW(t)
Method:
    | Euler scheme with order 0.5
Summary:
    | Size of process   | N  = 1001.
    | Number of simulation  | M  = 1000.
    | Initial value     | x0 = 10.
    | Time of process   | t in [0,1].
    | Discretization    | Dt = 0.001.

R> mod2

Stratonovich Sde 1D:
    | dX(t) = (0.5 * theta^2 * X(t)) * dt + theta * X(t) o dW(t)
Method:
    | Euler scheme with order 0.5
Summary:
    | Size of process   | N  = 1001.
    | Number of simulation  | M  = 1000.
    | Initial value     | x0 = 10.
    | Time of process   | t in [0,1].
    | Discretization    | Dt = 0.001.

Using Monte-Carlo simulations, the following statistical measures (S3 method) for class snssde1d() can be approximated for the $X_{t}$ process at any time $t$:

The expected value $\text{E}(X_{t})$ at time $t$, using the command mean.
The variance $\text{Var}(X_{t})$ at time $t$, using the command moment with order=2 and center=TRUE.
The median $\text{Med}(X_{t})$ at time $t$, using the command Median.
The mode $\text{Mod}(X_{t})$ at time $t$, using the command Mode.
The quartile of $X_{t}$ at time $t$, using the command quantile.
The maximum and minimum of $X_{t}$ at time $t$, using the command min and max.
The skewness and the kurtosis of $X_{t}$ at time $t$, using the command skewness and kurtosis.
The coefficient of variation (relative variability) of $X_{t}$ at time $t$, using the command cv.
The central moments up to order $p$ of $X_{t}$ at time $t$, using the command moment.
The empirical $\alpha \%$ confidence interval of expected value $\text{E}(X_{t})$ at time $t$ (from the $2.5th$ to the $97.5th$ percentile), using the command bconfint.
The result summaries of the results of Monte-Carlo simulation at time $t$, using the command summary.

The summary of the results of mod1 and mod2 at time $t=1$ of class snssde1d() is given by:

R> summary(mod1, at = 1)


    Monte-Carlo Statistics for X(t) at time t = 1
                              
Mean                  11.19043
Variance              34.77438
Median                 9.80795
Mode                   7.81562
First quartile         7.05936
Third quartile        13.70165
Minimum                2.30727
Maximum               48.28898
Skewness               1.50480
Kurtosis               6.28741
Coef-variation         0.52697
3th-order moment     308.57990
4th-order moment    7603.10190
5th-order moment  167122.80676
6th-order moment 4610438.55545

R> summary(mod2, at = 1)


    Monte-Carlo Statistics for X(t) at time t = 1
                               
Mean                   12.94687
Variance               44.96980
Median                 11.53663
Mode                    9.47033
First quartile          8.21275
Third quartile         16.07308
Minimum                 2.34762
Maximum                56.04992
Skewness                1.47591
Kurtosis                6.41717
Coef-variation          0.51796
3th-order moment      445.08292
4th-order moment    12977.32870
5th-order moment   332826.52899
6th-order moment 10773943.92221

Hence we can just make use of the rsde1d() function to build our random number generator for the conditional density of the $X_{t}|X_{0}$ ($X_{t}^{\text{mod1}}| X_{0}$ and $X_{t}^{\text{mod2}}|X_{0}$) at time $t = 1$.

R> x1 <- rsde1d(object = mod1, at = 1)  # X(t=1) | X(0)=x0 (Ito SDE)
R> x2 <- rsde1d(object = mod2, at = 1)  # X(t=1) | X(0)=x0 (Stratonovich SDE)
R> head(data.frame(x1,x2),n=5)

       x1      x2
1 18.8246 14.6275
2 13.2760  7.1711
3  6.5486  7.8493
4 14.7511  5.3086
5 23.5182 35.2471

The function dsde1d() can be used to show the Approximate transitional density for $X_{t}|X_{0}$ at time $t-s=1$ with log-normal curves:

R> mu1 = log(10); sigma1= sqrt(theta^2)  # log mean and log variance for mod1 
R> mu2 = log(10)-0.5*theta^2 ; sigma2 = sqrt(theta^2) # log mean and log variance for mod2
R> AppdensI <- dsde1d(mod1, at = 1)
R> AppdensS <- dsde1d(mod2, at = 1)
R> plot(AppdensI , dens = function(x) dlnorm(x,meanlog=mu1,sdlog = sigma1))
R> plot(AppdensS , dens = function(x) dlnorm(x,meanlog=mu2,sdlog = sigma2))

$Approximate transitional density for $X_{t}|X_{0}$ at time $t-s=1$ with log-normal curves. mod1: Ito and mod2: Stratonovich$ $Approximate transitional density for $X_{t}|X_{0}$ at time $t-s=1$ with log-normal curves. mod1: Ito and mod2: Stratonovich$

Approximate transitional density for $X_{t}|X_{0}$ at time $t-s=1$ with log-normal curves. mod1: Ito and mod2: Stratonovich

In Figure 2, we present the flow of trajectories, the mean path (red lines) of solution of and , with their empirical $95\%$ confidence bands, that is to say from the $2.5th$ to the $97.5th$ percentile for each observation at time $t$ (blue lines):

R> ## Ito
R> plot(mod1,ylab=expression(X^mod1))
R> lines(time(mod1),apply(mod1$X,1,mean),col=2,lwd=2)
R> lines(time(mod1),apply(mod1$X,1,bconfint,level=0.95)[1,],col=4,lwd=2)
R> lines(time(mod1),apply(mod1$X,1,bconfint,level=0.95)[2,],col=4,lwd=2)
R> legend("topleft",c("mean path",paste("bound of", 95,"% confidence")),inset = .01,col=c(2,4),lwd=2,cex=0.8)
R> ## Stratonovich
R> plot(mod2,ylab=expression(X^mod2))
R> lines(time(mod2),apply(mod2$X,1,mean),col=2,lwd=2)
R> lines(time(mod2),apply(mod2$X,1,bconfint,level=0.95)[1,],col=4,lwd=2)
R> lines(time(mod2),apply(mod2$X,1,bconfint,level=0.95)[2,],col=4,lwd=2)
R> legend("topleft",c("mean path",paste("bound of",95,"% confidence")),col=c(2,4),inset =.01,lwd=2,cex=0.8)

mod1: Ito and mod2: Stratonovich

Return to snssde1d()

`snssde2d()`

The following $2$-dimensional SDE’s with a vector of drift and matrix of diffusion coefficients:

Ito form: \[\begin{equation}\label{eq:09} \begin{cases} dX_t = f_{x}(t,X_{t},Y_{t}) dt + g_{x}(t,X_{t},Y_{t}) dW_{1,t}\\ dY_t = f_{y}(t,X_{t},Y_{t}) dt + g_{y}(t,X_{t},Y_{t}) dW_{2,t} \end{cases} \end{equation}\] Stratonovich form: \[\begin{equation}\label{eq:10} \begin{cases} dX_t = f_{x}(t,X_{t},Y_{t}) dt + g_{x}(t,X_{t},Y_{t}) \circ dW_{1,t}\\ dY_t = f_{y}(t,X_{t},Y_{t}) dt + g_{y}(t,X_{t},Y_{t}) \circ dW_{2,t} \end{cases} \end{equation}\]

where $(W_{1,t}, W_{2,t})$ are a two independent standard Wiener process if corr = NULL. To simulate $2d$ models using snssde2d() function we need to specify:

The drift (2d) and diffusion (2d) coefficients as R expressions that depend on the state variable x, y and time variable t.
corr the correlation structure of two standard Wiener process $(W_{1,t},W_{2,t})$; must be a real symmetric positive-definite square matrix of dimension $2$ (default: corr=NULL).
The number of simulation steps N (default: N=1000).
The number of the solution trajectories to be simulated by M (default: M=1).
The initial conditions t0, x0 and end time T (default: t0=0, x0=c(0,0) and T=1).
The integration step size Dt (default: Dt=(T-t0)/N).
The choice of process types by the argument type="ito" for Ito or type="str" for Stratonovich (default type="ito").
The numerical method to be used by method (default method="euler").

Ornstein-Uhlenbeck process and its integral

The Ornstein-Uhlenbeck (OU) process has a long history in physics. Introduced in essence by Langevin in his famous 1908 paper on Brownian motion, the process received a more thorough mathematical examination several decades later by Uhlenbeck and Ornstein (1930). The OU process is understood here to be the univariate continuous Markov process $X_t$. In mathematical terms, the equation is written as an Ito equation: \[\begin{equation}\label{eq016} dX_t = -\frac{1}{\mu} X_t dt + \sqrt{\sigma} dW_t,\quad X_{0}=x_{0} \end{equation}\] In these equations, $\mu$ and $\sigma$ are positive constants called, respectively, the relaxation time and the diffusion constant. The time integral of the OU process $X_t$ (or indeed of any process $X_t$) is defined to be the process $Y_t$ that satisfies: \[\begin{equation}\label{eq017} Y_{t} = Y_{0}+\int X_{t} dt \Leftrightarrow dY_t = X_{t} dt ,\quad Y_{0}=y_{0} \end{equation}\] $Y_t$ is not itself a Markov process; however, $X_t$ and $Y_t$ together comprise a bivariate continuous Markov process. We wish to find the solutions $X_t$ and $Y_t$ to the coupled time-evolution equations: \[\begin{equation}\label{eq018} \begin{cases} dX_t = -\frac{1}{\mu} X_t dt + \sqrt{\sigma} dW_t\\ dY_t = X_{t} dt \end{cases} \end{equation}\]

We simulate a flow of $1000$ trajectories of $(X_{t},Y_{t})$, with integration step size $\Delta t = 0.01$, and using second Milstein method.

R> x0=5;y0=0
R> mu=3;sigma=0.5
R> fx <- expression(-(x/mu),x)  
R> gx <- expression(sqrt(sigma),0)
R> mod2d <- snssde2d(drift=fx,diffusion=gx,Dt=0.01,M=1000,x0=c(x0,y0),method="smilstein")
R> mod2d

Itô Sde 2D:
    | dX(t) = -(X(t)/mu) * dt + sqrt(sigma) * dW1(t)
    | dY(t) = X(t) * dt + 0 * dW2(t)
Method:
    | Second-order Milstein scheme
Summary:
    | Size of process   | N  = 1001.
    | Number of simulation  | M  = 1000.
    | Initial values    | (x0,y0) = (5,0).
    | Time of process   | t in [0,10].
    | Discretization    | Dt = 0.01.

The summary of the results of mod2d at time $t=10$ of class snssde2d() is given by:

R> summary(mod2d, at = 10)

For plotting in time (or in plane) using the command plot (plot2d), the results of the simulation are shown in Figure 3.

R> ## in time
R> plot(mod2d)
R> ## in plane (O,X,Y)
R> plot2d(mod2d,type="n") 
R> points2d(mod2d,col=rgb(0,100,0,50,maxColorValue=255), pch=16)

Ornstein-Uhlenbeck process and its integral

Hence we can just make use of the rsde2d() function to build our random number for $(X_{t},Y_{t})$ at time $t = 10$.

R> out <- rsde2d(object = mod2d, at = 10) 
R> head(out,n=3)

          x       y
1  0.835404 25.1479
2 -0.033169 19.9462
3 -1.169357  9.2241

The density of $X_t$ and $Y_t$ at time $t=10$ are reported using dsde2d() function, see e.g. Figure 4: the marginal density of $X_t$ and $Y_t$ at time $t=10$. For plotted in (x, y)-space with dim = 2. A contour and image plot of density obtained from a realization of system $(X_{t},Y_{t})$ at time t=10, see:

R> ## the marginal density
R> denM <- dsde2d(mod2d,pdf="M",at =10)
R> plot(denM, main="Marginal Density")
R> ## the Joint density
R> denJ <- dsde2d(mod2d, pdf="J", n=100,at =10)
R> plot(denJ,display="contour",main="Bivariate Transition Density at time t=10")

Marginal and Joint density at time t=10

A $3$D plot of the transition density at $t=10$ obtained with:

R> plot(denJ,display="persp",main="Bivariate Transition Density at time t=10")

Marginal and Joint density at time t=10

We approximate the bivariate transition density over the set transition horizons $t\in [1,10]$ by $\Delta t = 0.005$ using the code:

R> for (i in seq(1,10,by=0.005)){ 
+ plot(dsde2d(mod2d, at = i,n=100),display="contour",main=paste0('Transition Density \n t = ',i))
+ }

Return to snssde2d()

The stochastic Van-der-Pol equation

The Van der Pol (1922) equation is an ordinary differential equation that can be derived from the Rayleigh differential equation by differentiating and setting $\dot{x}=y$, see Naess and Hegstad (1994); Leung (1995) and for more complex dynamics in Van-der-Pol equation see Jing et al. (2006). It is an equation describing self-sustaining oscillations in which energy is fed into small oscillations and removed from large oscillations. This equation arises in the study of circuits containing vacuum tubes and is given by: \[\begin{equation}\label{eq:12} \ddot{X}-\mu (1-X^{2}) \dot{X} + X = 0 \end{equation}\] where $x$ is the position coordinate (which is a function of the time $t$), and $\mu$ is a scalar parameter indicating the nonlinearity and the strength of the damping, to simulate the deterministic equation see Grayling (2014) for more details. Consider stochastic perturbations of the Van-der-Pol equation, and random excitation force of such systems by White noise $\xi_{t}$, with delta-type correlation function $\text{E}(\xi_{t}\xi_{t+h})=2\sigma \delta (h)$ \[\begin{equation}\label{eq:13} \ddot{X}-\mu (1-X^{2}) \dot{X} + X = \xi_{t}, \end{equation}\] where $\mu > 0$ . It’s solution cannot be obtained in terms of elementary functions, even in the phase plane. The White noise $\xi_{t}$ is formally derivative of the Wiener process $W_{t}$. The representation of a system of two first order equations follows the same idea as in the deterministic case by letting $\dot{x}=y$, from physical equation we get the above system: \[\begin{equation}\label{eq:14} \begin{cases} \dot{X} = Y \\ \dot{Y} = \mu \left(1-X^{2}\right) Y - X + \xi_{t} \end{cases} \end{equation}\] The system can mathematically explain by a Stratonovitch equations: \[\begin{equation}\label{eq:15} \begin{cases} dX_{t} = Y_{t} dt \\ dY_{t} = \left(\mu (1-X^{2}_{t}) Y_{t} - X_{t}\right) dt + 2 \sigma \circ dW_{2,t} \end{cases} \end{equation}\]

Implemente in R as follows, with integration step size $\Delta t = 0.01$ and using stochastic Runge-Kutta methods 1-stage.

R> mu = 4; sigma=0.1
R> fx <- expression( y ,  (mu*( 1-x^2 )* y - x)) 
R> gx <- expression( 0 ,2*sigma)
R> mod2d <- snssde2d(drift=fx,diffusion=gx,N=10000,Dt=0.01,type="str",method="rk1")

For plotting (back in time) using the command plot, and plot2d in plane the results of the simulation are shown in Figure 6.

R> plot(mod2d,ylim=c(-8,8))   ## back in time
R> plot2d(mod2d)              ## in plane (O,X,Y)

The stochastic Van-der-Pol equation

Return to snssde2d()

The Heston Model

Consider a system of stochastic differential equations:

\[\begin{equation}\label{eq:115} \begin{cases} dX_{t} = \mu X_{t} dt + X_{t}\sqrt{Y_{t}} dB_{1,t}\\ dY_{t} = \nu (\theta-Y_{t}) dt + \sigma \sqrt{Y_{t}} dB_{2,t} \end{cases} \end{equation}\]

Conditions to ensure positiveness of the volatility process are that $2\nu \theta > \sigma^2$, and the two Brownian motions $(B_{1,t},B_{2,t})$ are correlated. $\Sigma$ to describe the correlation structure, for example: \[ \Sigma= \begin{pmatrix} 1 & 0.3 \\ 0.3 & 2 \end{pmatrix} \]

R> mu = 1.2; sigma=0.1;nu=2;theta=0.5
R> fx <- expression( mu*x ,nu*(theta-y)) 
R> gx <- expression( x*sqrt(y) ,sigma*sqrt(y))
R> Sigma <- matrix(c(1,0.3,0.3,2),nrow=2,ncol=2) # correlation matrix
R> HM <- snssde2d(drift=fx,diffusion=gx,Dt=0.001,x0=c(100,1),corr=Sigma,M=1000)
R> HM

Itô Sde 2D:
    | dX(t) = mu * X(t) * dt + X(t) * sqrt(Y(t)) * dB1(t)
    | dY(t) = nu * (theta - Y(t)) * dt + sigma * sqrt(Y(t)) * dB2(t)
    | Correlation structure:                    
             1.0 0.3
             0.3 2.0
Method:
    | Euler scheme with order 0.5
Summary:
    | Size of process   | N  = 1001.
    | Number of simulation  | M  = 1000.
    | Initial values    | (x0,y0) = (100,1).
    | Time of process   | t in [0,1].
    | Discretization    | Dt = 0.001.

Hence we can just make use of the rsde2d() function to build our random number for $(X_{t},Y_{t})$ at time $t = 1$.

R> out <- rsde2d(object = HM, at = 1) 
R> head(out,n=3)

       x       y
1 667.47 0.59944
2 152.76 0.59491
3 414.38 0.58505

The density of $X_t$ and $Y_t$ at time $t=1$ are reported using dsde2d() function. See:

R> denJ <- dsde2d(HM,pdf="J",at =1,lims=c(-100,900,0.4,0.75))
R> plot(denJ,display="contour",main="Bivariate Transition Density at time t=10")
R> plot(denJ,display="persp",main="Bivariate Transition Density at time t=10")

Return to snssde2d()

`snssde3d()`

The following $3$-dimensional SDE’s with a vector of drift and matrix of diffusion coefficients:

Ito form: \[\begin{equation}\label{eq17} \begin{cases} dX_t = f_{x}(t,X_{t},Y_{t},Z_{t}) dt + g_{x}(t,X_{t},Y_{t},Z_{t}) dW_{1,t}\\ dY_t = f_{y}(t,X_{t},Y_{t},Z_{t}) dt + g_{y}(t,X_{t},Y_{t},Z_{t}) dW_{2,t}\\ dZ_t = f_{z}(t,X_{t},Y_{t},Z_{t}) dt + g_{z}(t,X_{t},Y_{t},Z_{t}) dW_{3,t} \end{cases} \end{equation}\] Stratonovich form: \[\begin{equation}\label{eq18} \begin{cases} dX_t = f_{x}(t,X_{t},Y_{t},Z_{t}) dt + g_{x}(t,X_{t},Y_{t},Z_{t}) \circ dW_{1,t}\\ dY_t = f_{y}(t,X_{t},Y_{t},Z_{t}) dt + g_{y}(t,X_{t},Y_{t},Z_{t}) \circ dW_{2,t}\\ dZ_t = f_{z}(t,X_{t},Y_{t},Z_{t}) dt + g_{z}(t,X_{t},Y_{t},Z_{t}) \circ dW_{3,t} \end{cases} \end{equation}\]

$(W_{1,t},W_{2,t},W_{3,t})$ are three independents standard Wiener process if corr = NULL. To simulate this system using snssde3d() function we need to specify:

The drift (3d) and diffusion (3d) coefficients as R expressions that depend on the state variables x, y , z and time variable t.
corr the correlation structure of three standard Wiener process $(W_{1,t},W_{2,t},W_{2,t})$; must be a real symmetric positive-definite square matrix of dimension $3$ (default: corr=NULL).
The number of simulation steps N (default: N=1000).
The number of the solution trajectories to be simulated by M (default: M=1).
The initial conditions t0, x0 and end time T (default: t0=0, x0=c(0,0,0) and T=1).
The integration step size Dt (default: Dt=(T-t0)/N).
The choice of process types by the argument type="ito" for Ito or type="str" for Stratonovich (default type="ito").
The numerical method to be used by method (default method="euler").

Basic example

Assume that we want to describe the following SDE’s (3D) in Ito form: \[\begin{equation}\label{eq0166} \begin{cases} dX_t = 4 (-1-X_{t}) Y_{t} dt + 0.2 dW_{1,t}\\ dY_t = 4 (1-Y_{t}) X_{t} dt + 0.2 dW_{2,t}\\ dZ_t = 4 (1-Z_{t}) Y_{t} dt + 0.2 dW_{3,t} \end{cases} \end{equation}\]

with $(W_{1,t},W_{2,t},W_{3,t})$ are three indpendant standard Wiener process.

We simulate a flow of $1000$ trajectories, with integration step size $\Delta t = 0.001$.

R> fx <- expression(4*(-1-x)*y , 4*(1-y)*x , 4*(1-z)*y) 
R> gx <- rep(expression(0.2),3)
R> mod3d <- snssde3d(x0=c(x=2,y=-2,z=-2),drift=fx,diffusion=gx,M=1000)
R> mod3d

Itô Sde 3D:
    | dX(t) = 4 * (-1 - X(t)) * Y(t) * dt + 0.2 * dW1(t)
    | dY(t) = 4 * (1 - Y(t)) * X(t) * dt + 0.2 * dW2(t)
    | dZ(t) = 4 * (1 - Z(t)) * Y(t) * dt + 0.2 * dW3(t)
Method:
    | Euler scheme with order 0.5
Summary:
    | Size of process   | N  = 1001.
    | Number of simulation  | M  = 1000.
    | Initial values    | (x0,y0,z0) = (2,-2,-2).
    | Time of process   | t in [0,1].
    | Discretization    | Dt = 0.001.

The following statistical measures (S3 method) for class snssde3d() can be approximated for the $(X_{t},Y_{t},Z_{t})$ process at any time $t$, for example at=1:

R> s = 1
R> mean(mod3d, at = s)
R> moment(mod3d, at = s , center = TRUE , order = 2) ## variance
R> Median(mod3d, at = s)
R> Mode(mod3d, at = s)
R> quantile(mod3d , at = s)
R> kurtosis(mod3d , at = s)
R> skewness(mod3d , at = s)
R> cv(mod3d , at = s )
R> min(mod3d , at = s)
R> max(mod3d , at = s)
R> moment(mod3d, at = s , center= TRUE , order = 4)
R> moment(mod3d, at = s , center= FALSE , order = 4)

The summary of the results of mod3d at time $t=1$ of class snssde3d() is given by:

R> summary(mod3d, at = t)

For plotting (back in time) using the command plot, and plot3D in space the results of the simulation are shown in Figure 7.

R> plot(mod3d,union = TRUE)         ## back in time
R> plot3D(mod3d,display="persp")    ## in space (O,X,Y,Z)

Flow of $1000$ trajectories of $(X_t ,Y_t ,Z_t)$

Hence we can just make use of the rsde3d() function to build our random number for $(X_{t},Y_{t},Z_{t})$ at time $t = 1$.

R> out <- rsde3d(object = mod3d, at = 1) 
R> head(out,n=3)

         x        y       z
1 -0.83626 1.005921 0.85364
2 -0.76947 0.768522 0.77156
3 -0.59477 0.018633 0.58222

For each SDE type and for each numerical scheme, the marginal density of $X_t$, $Y_t$ and $Z_t$ at time $t=1$ are reported using dsde3d() function, see e.g. Figure 8.

R> den <- dsde3d(mod3d,pdf="M",at =1)
R> plot(den, main="Marginal Density")

Marginal density of $X_t$, $Y_t$ and $Z_t$ at time $t=1$

For an approximate joint transition density for $(X_t,Y_t,Z_t)$ (for more details, see package sm or ks.)

R> denJ <- dsde3d(mod3d,pdf="J")
R> plot(denJ,display="rgl")

Return to snssde3d()

Attractive model for 3D diffusion processes

If we assume that $U_w( x , y , z , t )$, $V_w( x , y , z , t )$ and $S_w( x , y , z , t )$ are neglected and the dispersion coefficient $D( x , y , z )$ is constant. A system becomes (see Boukhetala,1996): \[\begin{eqnarray}\label{eq19} % \nonumber to remove numbering (before each equation) \begin{cases} dX_t = \left(\frac{-K X_{t}}{\sqrt{X^{2}_{t} + Y^{2}_{t} + Z^{2}_{t}}}\right) dt + \sigma dW_{1,t} \nonumber\\ dY_t = \left(\frac{-K Y_{t}}{\sqrt{X^{2}_{t} + Y^{2}_{t} + Z^{2}_{t}}}\right) dt + \sigma dW_{2,t} \\ dZ_t = \left(\frac{-K Z_{t}}{\sqrt{X^{2}_{t} + Y^{2}_{t} + Z^{2}_{t}}}\right) dt + \sigma dW_{3,t} \nonumber \end{cases} \end{eqnarray}\]

with initial conditions $(X_{0},Y_{0},Z_{0})=(1,1,1)$, by specifying the drift and diffusion coefficients of three processes $X_{t}$, $Y_{t}$ and $Z_{t}$ as R expressions which depends on the three state variables (x,y,z) and time variable t, with integration step size Dt=0.0001.

R> K = 4; s = 1; sigma = 0.2
R> fx <- expression( (-K*x/sqrt(x^2+y^2+z^2)) , (-K*y/sqrt(x^2+y^2+z^2)) , (-K*z/sqrt(x^2+y^2+z^2)) ) 
R> gx <- rep(expression(sigma),3)
R> mod3d <- snssde3d(drift=fx,diffusion=gx,N=10000,x0=c(x=1,y=1,z=1))

The results of simulation (3D) are shown in Figure 9:

R> plot3D(mod3d,display="persp",col="blue")

Attractive model for 3D diffusion processes

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Transformation of an SDE one-dimensional

Next is an example of one-dimensional SDE driven by three correlated Wiener process ($B_{1,t}$,$B_{2,t}$,$B_{3,t}$), as follows: \[\begin{equation}\label{eq20} dX_{t} = B_{1,t} dt + B_{2,t} dB_{3,t} \end{equation}\] with: \[ \Sigma= \begin{pmatrix} 1 & 0.2 &0.5\\ 0.2 & 1 & -0.7 \\ 0.5 &-0.7&1 \end{pmatrix} \] To simulate the solution of the process $X_t$, we make a transformation to a system of three equations as follows: \[\begin{eqnarray}\label{eq21} \begin{cases} % \nonumber to remove numbering (before each equation) dX_t = Y_{t} dt + Z_{t} dB_{3,t} \nonumber\\ dY_t = dB_{1,t} \\ dZ_t = dB_{2,t} \nonumber \end{cases} \end{eqnarray}\]

run by calling the function snssde3d() to produce a simulation of the solution, with $\mu = 1$ and $\sigma = 1$.

R> fx <- expression(y,0,0) 
R> gx <- expression(z,1,1)
R> Sigma <-matrix(c(1,0.2,0.5,0.2,1,-0.7,0.5,-0.7,1),nrow=3,ncol=3)
R> modtra <- snssde3d(drift=fx,diffusion=gx,M=1000,corr=Sigma)
R> modtra

Itô Sde 3D:
    | dX(t) = Y(t) * dt + Z(t) * dB1(t)
    | dY(t) = 0 * dt + 1 * dB2(t)
    | dZ(t) = 0 * dt + 1 * dB3(t)
    | Correlation structure:                          
             1.0  0.2  0.5
             0.2  1.0 -0.7
             0.5 -0.7  1.0
Method:
    | Euler scheme with order 0.5
Summary:
    | Size of process   | N  = 1001.
    | Number of simulation  | M  = 1000.
    | Initial values    | (x0,y0,z0) = (0,0,0).
    | Time of process   | t in [0,1].
    | Discretization    | Dt = 0.001.

The histogram and kernel density of $X_t$ at time $t=1$ are reported using rsde3d() function, and we calculate emprical variance-covariance matrix $C(s,t)=\text{Cov}(X_{s},X_{t})$, see e.g. Figure 10.

R> X <- rsde3d(modtra,at=1)$x
R> MASS::truehist(X,xlab = expression(X[t==1]));box()
R> lines(density(X),col="red",lwd=2)
R> legend("topleft",c("Distribution histogram","Kernel Density"),inset =.01,pch=c(15,NA),lty=c(NA,1),col=c("cyan","red"), lwd=2,cex=0.8)
R> ## Cov-Matrix
R> color.palette=colorRampPalette(c('white','green','blue','red'))
R> filled.contour(time(modtra), time(modtra), cov(t(modtra$X)), color.palette=color.palette,plot.title = title(main = expression(paste("Covariance empirique:",cov(X[s],X[t]))),xlab = "time", ylab = "time"),key.title = title(main = ""))

The histogram and kernel density of $X_t$ at time $t=1$. Emprical variance-covariance matrix

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Department of Probabilities & Statistics, Faculty of Mathematics, University of Science and Technology Houari Boumediene, BP 32 El-Alia, U.S.T.H.B, Algeria, E-mail (acguidoum@usthb.dz)↩
Faculty of Mathematics, University of Science and Technology Houari Boumediene, BP 32 El-Alia, U.S.T.H.B, Algeria, E-mail (kboukhetala@usthb.dz)↩
The equivalently of $X_{t}^{\text{mod1}}$ the following Stratonovich SDE: $dX_{t} = \theta X_{t} \circ dW_{t}$.↩

Monte-Carlo Simulations and Analysis of Stochastic Differential Equations

A.C. Guidoum¹ and K. Boukhetala²

2020-05-04

`snssde1d()`

`snssde2d()`

Ornstein-Uhlenbeck process and its integral

The stochastic Van-der-Pol equation

The Heston Model

`snssde3d()`

Basic example

Attractive model for 3D diffusion processes

Transformation of an SDE one-dimensional

Further reading

References

Monte-Carlo Simulations and Analysis of Stochastic Differential Equations

A.C. Guidoum1 and K. Boukhetala2

2020-05-04

snssde1d()

snssde2d()

Ornstein-Uhlenbeck process and its integral

The stochastic Van-der-Pol equation

The Heston Model

snssde3d()

Basic example

Attractive model for 3D diffusion processes

Transformation of an SDE one-dimensional

Further reading

References

A.C. Guidoum¹ and K. Boukhetala²

`snssde1d()`

`snssde2d()`

`snssde3d()`