Examples
FPT for 1-Dim SDE
Consider the following SDE and linear boundary:
\begin{align*} dX_{t}= & \frac{1}{2} \alpha^2 X_{t} dt + \alpha X_{t} dW_{t},~x_{0} \neq 0.\\ S(t)= & a+bt \end{align*}The analytical solution of this model is: \[ X_t = x_{0}\exp\left(\alpha W_{t}\right) \] generating the first passage time (FPT) of this model through this boundary: \[ \tau_{S(t)}= \inf \left\{t: X_{t} \geq S(t) |X_{t_{0}}=x_{0} \right\} ~~ \text{if} \quad x_{0} \leq S(t_{0}) \]
Set the model \(X_t\):
alpha=2
f <- expression( alpha^2 * x )
g <- expression( alpha * x )
mod1d <- snssde1d(drift=f,diffusion=g,x0=0.5,M=1000)Generate the first-passage-time \(\tau_{S(t)}\), with fptsde1d() function:
St <- expression( -5*t+1 )
fpt1d <- fptsde1d(mod1d, boundary = St)
names(fpt1d) ## names of output## [1] "SDE" "boundary" "fpt"summary(fpt1d)##
## Monte-Carlo Statistics for the F.P.T of X(t)
## | T(S,X) = inf{t >= 0 : X(t) >= -5 * t + 1}
## T(S,X)
## NA's 0
## Mean 0.07697
## Variance 0.00174
## Median 0.06983
## First quartile 0.04269
## Third quartile 0.10641
## Skewness 0.58381
## Kurtosis 2.46553
## Moment of order 3 4e-05
## Moment of order 4 1e-05
## Moment of order 5 0
## Int.conf Inf (95%) 0.01884
## Int.conf Sup (95%) 0.1666
The histograms of \(\tau_{S(t)}\) with boundary \(S(t) = -5t+1\), see e.g. Figure 2: the empirical distribution and kernel density of \(\tau_{S(t)}\)
require(MASS)
truehist(fpt1d$fpt,xlab = expression(tau[S(t)]) );box()
lines(density(fpt1d$fpt),col="red",lwd=2)
legend("topright",c("Distribution histogram","Kernel Density"),inset =.01,pch=c(15,NA),lty=c(NA,1),col=c("cyan","red"),lwd=2,cex=0.8)
FPT for 2-Dim SDE’s
The following \(2\)-dimensional SDE’s with a vector of drift and a diagonal matrix of diffusion coefficients:
\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}\(W_{1,t}\) and \(W_{2,t}\) is a two independent standard Wiener process. First passage time (2D) \((\tau_{(S(t),X_{t})},\tau_{(S(t),Y_{t})})\) is defined as:
\[ \left\{ \begin{array}{ll} \tau_{S(t),X_{t}}=\inf \left\{t: X_{t} \geq S(t)|X_{t_{0}}=x_{0} \right\} & \hbox{if} \quad x_{0} \leq S(t_{0}) \\ \tau_{S(t),Y_{t}}=\inf \left\{t: Y_{t} \geq S(t)|Y_{t_{0}}=y_{0} \right\} & \hbox{if} \quad y_{0} \leq S(t_{0}) \end{array} \right. \] and \[ \left\{ \begin{array}{ll} \tau_{S(t),X_{t}}= \inf \left\{t: X_{t} \leq S(t)|X_{t_{0}}=x_{0} \right\} & \hbox{if} \quad x_{0} \geq S(t_{0}) \\ \tau_{S(t),Y_{t}}= \inf \left\{t: Y_{t} \leq S(t)|Y_{t_{0}}=y_{0} \right\} & \hbox{if} \quad y_{0} \geq S(t_{0}) \end{array} \right. \]
Assume that we want to describe the following SDE’s (2D):
\begin{equation}\label{eq016} \begin{cases} dX_t = 5 (-1-Y_{t}) X_{t} dt + 0.5 dW_{1,t}\\ dY_t = 5 (-1-X_{t}) Y_{t} dt + 0.5 dW_{2,t} \end{cases} \end{equation}and \[ S(t)=-3+5t \]
Set the system \((X_t , Y_t)\):
fx <- expression(5*(-1-y)*x) ; gx <- expression(0.5)
fy <- expression(5*(-1-x)*y) ; gy <- expression(0.5)
mod2d <- snssde2d(driftx=fx,diffx=gx,drifty=fy,diffy=gy,x0=2,y0=-2,M=1000)Generate the couple \((\tau_{(S(t),X_{t})},\tau_{(S(t),Y_{t})})\), with fptsde2d() function::
St <- expression(-3+5*t)
fpt2d <- fptsde2d(mod2d, boundary = St)
names(fpt2d) ## names of output## [1] "SDE" "boundary" "fptx" "fpty"summary(fpt2d)##
## Monte-Carlo Statistics for the F.P.T of (X(t),Y(t))
## | T(S,X) = inf{t >= 0 : X(t) <= -3 + 5 * t }
## | T(S,Y) = inf{t >= 0 : Y(t) <= -3 + 5 * t }
## T(S,X) T(S,Y)
## NA's 0 0
## Mean 0.63154 0.59644
## Variance 0.00077 0.00079
## Median 0.63216 0.59602
## First quartile 0.61208 0.57843
## Third quartile 0.6503 0.6164
## Skewness -0.01551 -0.08085
## Kurtosis 3.2649 2.81178
## Moment of order 3 0 0
## Moment of order 4 0 0
## Moment of order 5 0 0
## Int.conf Inf (95%) 0.57888 0.54179
## Int.conf Sup (95%) 0.68668 0.65066
Created using ggplot2 package plotted in (x, y)-space with dim = 2. A contour plot of density obtained from a realization of \((\tau_{(S(t),X_{t})},\tau_{(S(t),Y_{t})})\).
out <- data.frame(x=fpt2d$fptx,y=fpt2d$fpty)
library(ggplot2)
m <- ggplot(out, aes(x = x, y = y))
m + stat_density_2d(aes(fill = ..level..), geom = "polygon")
A \(3\)D plot of the density obtained with sm, from a realization of \((\tau_{(S(t),X_{t})},\tau_{(S(t),Y_{t})})\).
library(sm)
sm.density(out,display="persp")## Warning: weights overwritten by binning
FPT for 3-Dim SDE’s
The following \(3\)-dimensional SDE’s with a vector of drift and a diagonal 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}\(W_{1,t}\), \(W_{2,t}\) and \(W_{3,t}\) is a 3 independent standard Wiener process. First passage time (3D) \((\tau_{(S(t),X_{t})},\tau_{(S(t),Y_{t})},\tau_{(S(t),Z_{t})})\) is defined as:
\[ \left\{ \begin{array}{ll} \tau_{S(t),X_{t}}=\inf \left\{t: X_{t} \geq S(t)|X_{t_{0}}=x_{0} \right\} & \hbox{if} \quad x_{0} \leq S(t_{0}) \\ \tau_{S(t),Y_{t}}=\inf \left\{t: Y_{t} \geq S(t)|Y_{t_{0}}=y_{0} \right\} & \hbox{if} \quad y_{0} \leq S(t_{0}) \\ \tau_{S(t),Z_{t}}=\inf \left\{t: Z_{t} \geq S(t)|Z_{t_{0}}=z_{0} \right\} & \hbox{if} \quad z_{0} \leq S(t_{0}) \end{array} \right. \] and \[ \left\{ \begin{array}{ll} \tau_{S(t),X_{t}}= \inf \left\{t: X_{t} \leq S(t)|X_{t_{0}}=x_{0} \right\} & \hbox{if} \quad x_{0} \geq S(t_{0}) \\ \tau_{S(t),Y_{t}}= \inf \left\{t: Y_{t} \leq S(t)|Y_{t_{0}}=y_{0} \right\} & \hbox{if} \quad y_{0} \geq S(t_{0}) \\ \tau_{S(t),Z_{t}}= \inf \left\{t: Z_{t} \leq S(t)|Z_{t_{0}}=z_{0} \right\} & \hbox{if} \quad z_{0} \geq S(t_{0}) \\ \end{array} \right. \]
Assume that we want to describe the following SDE’s (3D): \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}and \[ S(t)=-3+5t \]
Set the system \((X_t , Y_t , Z_t)\):
fx <- expression(4*(-1-x)*y) ; gx <- expression(0.2)
fy <- expression(4*(1-y)*x) ; gy <- expression(0.2)
fz <- expression(4*(1-z)*y) ; gz <- expression(0.2)
mod3d <- snssde3d(driftx=fx,diffx=gx,drifty=fy,diffy=gy,driftz=fz,diffz=gz,x0=2,y0=-2,z0=0,M=1000)Generate the triplet \((\tau_{(S(t),X_{t})},\tau_{(S(t),Y_{t})},\tau_{(S(t),Z_{t})})\), with fptsde3d() function::
St <- expression(-3+5*t)
fpt3d <- fptsde3d(mod3d, boundary = St)
names(fpt3d) ## names of output## [1] "SDE" "boundary" "fptx" "fpty" "fptz"summary(fpt3d)##
## Monte-Carlo Statistics for the F.P.T of (X(t),Y(t),Z(t))
## | T(S,X) = inf{t >= 0 : X(t) <= -3 + 5 * t }
## | T(S,Y) = inf{t >= 0 : Y(t) >= -3 + 5 * t }
## | T(S,Z) = inf{t >= 0 : Z(t) <= -3 + 5 * t }
## T(S,X) T(S,Y) T(S,Z)
## NA's 0 0 0
## Mean 0.58147 0.78676 0.7695
## Variance 0.00013 0.00122 0.00029
## Median 0.58138 0.78253 0.76943
## First quartile 0.57342 0.76354 0.7578
## Third quartile 0.58941 0.80572 0.78135
## Skewness 0.11006 1.18629 -0.08683
## Kurtosis 2.86346 6.33489 3.12204
## Moment of order 3 0 5e-05 0
## Moment of order 4 0 1e-05 0
## Moment of order 5 0 0 0
## Int.conf Inf (95%) 0.56067 0.73227 0.73681
## Int.conf Sup (95%) 0.60318 0.8662 0.80184
A \(3\)D plot of the density obtained with sm, from a realization of \((\tau_{(S(t),X_{t})},\tau_{(S(t),Y_{t})},\tau_{(S(t),Z_{t})})\).
out <- data.frame(x=fpt3d$fptx,y=fpt3d$fpty,z=fpt3d$fptz)
library(sm)
sm.density(out,display="rgl")