# Constructs and Analysis of Bridges Stochastic Differential Equations

#### A.C. Guidoum1 and K. Boukhetala2

#### 2025-12-13

# Bridges SDE’s

Consider now a \(d\)-dimensional stochastic process \(X_{t}\) defined on a probability space \((\Omega, \mathfrak{F},\mathbb{P})\). We say that the bridge associated to \(X_{t}\) conditioned to the event \(\{X_{T}= a\}\) is the process: \[ \{\tilde{X}_{t}, t_{0} \leq t \leq T \}=\{X_{t}, t_{0} \leq t \leq T: X_{T}= a \} \] where \(T\) is a deterministic fixed time and \(a \in \mathbb{R}^d\) is fixed too.

# The `bridgesdekd()` functions

The (S3) generic function `bridgesdekd()` (where `k=1,2,3`) for simulation of 1,2 and 3-dim bridge stochastic differential equations,Ito or Stratonovich type, with different methods. The main arguments consist:

- The `drift` and `diffusion` coefficients as R expressions that depend on the state variable `x` (`y` and `z`) and time variable `t`.
- `corr` the correlation structure of standard Wiener process ; must be a real symmetric positive-definite square matrix (2D and 3D, default: `corr=NULL`).
- The number of simulation steps `N`.
- The number of the solution trajectories to be simulated by `M` (default: `M=1`).
- Initial value `x0` at initial time `t0`.
- Terminal value `y` final time `T`
- 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 (by default `type="ito"`).
- The numerical method to be used by `method` (default `method="euler"`).

By Monte-Carlo simulations, the following statistical measures (`S3 method`) for class `bridgesdekd()` (where `k=1,2,3`) can be approximated for the process at any time \(t \in [t_{0},T]\) (default: `at=(T-t0)/2`):

- 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 result summaries of the results of Monte-Carlo simulation at time \(t\), using the command `summary`.

We can just make use of the `rsdekd()` function (where `k=1,2,3`) to build our random number for class `bridgesdekd()` (where `k=1,2,3`) at any time \(t \in [t_{0},T]\). the main arguments consist:

- `object` an object inheriting from class `bridgesdekd()` (where `k=1,2,3`).
- `at` time between \(s=t0\) and \(t=T\).

The function `dsde()` (where `k=1,2,3`) approximate transition density for class `bridgesdekd()` (where `k=1,2,3`), the main arguments consist:

- `object` an object inheriting from class `bridgesdekd()` (where `k=1,2,3`).
- `at` time between \(s=t0\) and \(t=T\).
- `pdf` probability density function `Joint` or `Marginal`.

The following we explain how to use this functions.

# `bridgesde1d()`

Assume that we want to describe the following bridge sde in Ito form: \[\begin{equation}\label{eq0166} dX_t = \frac{1-X_t}{1-t} dt + X_t dW_{t},\quad X_{t_{0}}=3 \quad\text{and}\quad X_{T}=1 \end{equation}\] We simulate a flow of \(1000\) trajectories, with integration step size \(\Delta t = 0.001\), and \(x_0 = 3\) at time \(t_0 = 0\), \(y = 1\) at terminal time \(T=1\).

    R> set.seed(1234, kind = "L'Ecuyer-CMRG")
    R> f <- expression((1-x)/(1-t))
    R> g <- expression(x)
    R> mod <- bridgesde1d(drift=f,diffusion=g,x0=3,y=1,M=1000)
    R> mod

    Itô Bridge Sde 1D:
        | dX(t) = (1 - X(t))/(1 - t) * dt + X(t) * dW(t)
    Method:
        | Euler scheme with order 0.5
    Summary:
        | Size of process   | N = 1001.
        | Crossing realized | C = 978 among 1000.
        | Initial value     | x0 = 3.
        | Ending value      | y = 1.
        | Time of process   | t in [0,1].
        | Discretization    | Dt = 0.001.

In Figure 1, we present the flow of trajectories, the mean path (red lines) of solution of \(X_{t}|X_{0}=3,X_{T}=1\):

    R> plot(mod,ylab=expression(X[t]))
    R> lines(time(mod),apply(mod$X,1,mean),col=2,lwd=2)
    R> legend("topleft","mean path",inset = .01,col=2,lwd=2,cex=0.8,bty="n")

Hence we can just make use of the `rsde1d()` function to build our random number generator for \(X_{t}|X_{0}=3,X_{T}=1\) at time \(t=0.55\):

    R> x <- rsde1d(object = mod, at = 0.55)
    R> head(x, n = 3)

    [1] 0.72282 0.96118 0.94990

The function `dsde1d()` can be used to show the kernel density estimation for \(X_{t}|X_{0}=3,X_{T}=1\) at time \(t=0.55\) (`hist=TRUE` based on `truehist()` function in MASS package):

    R> dens <- dsde1d(mod, at = 0.55)
    R> plot(dens,hist=TRUE) ## histgramme
    R> plot(dens,add=TRUE)  ## kernel density

Bridge sde 1D. Histgramme and kernel density estimation for \(X_{t}|X_{0}=3,X_{T}=1\)

Return to bridgesde1d()

# `bridgesde2d()`

Assume that we want to describe the following \(2\)-dimensional bridge SDE’s in Stratonovich form:

\[\begin{equation}\label{eq:09} \begin{cases} dX_t = -(1+Y_{t}) X_{t} dt + 0.2 (1-Y_{t})\circ dB_{1,t},\quad X_{t_{0}}=1 \quad\text{and}\quad X_{T}=1\\ dY_t = -(1+X_{t}) Y_{t} dt + 0.1 (1-X_{t}) \circ dB_{2,t},\quad Y_{t_{0}}=-0.5 \quad\text{and}\quad Y_{T}=0.5 \end{cases} \end{equation}\] with \((B_{1,t},B_{2,t})\) are two correlated standard Wiener process: \[ \Sigma= \begin{pmatrix} 1 & 0.3\\ 0.3 & 1 \end{pmatrix} \]

We simulate a flow of \(1000\) trajectories, with integration step size \(\Delta t = 0.01\):

    R> set.seed(1234, kind = "L'Ecuyer-CMRG")
    R> fx <- expression(-(1+y)*x , -(1+x)*y)
    R> gx <- expression(0.2*(1-y),0.1*(1-x))
    R> Sigma <-matrix(c(1,0.3,0.3,1),nrow=2,ncol=2)
    R> mod2 <- bridgesde2d(drift=fx,diffusion=gx,x0=c(1,-0.5),y=c(1,0.5),Dt=0.01,M=1000,type="str",corr=Sigma)
    R> mod2

    Stratonovich Bridge Sde 2D:
        | dX(t) = -(1 + Y(t)) * X(t) * dt + 0.2 * (1 - Y(t)) o dB1(t)
        | dY(t) = -(1 + X(t)) * Y(t) * dt + 0.1 * (1 - X(t)) o dB2(t)
        | Correlation structure:
                 1.0 0.3
                 0.3 1.0
    Method:
        | Euler scheme with order 0.5
    Summary:
        | Size of process   | N = 1001.
        | Crossing realized | C = 1000 among 1000.
        | Initial values    | x0 = (1,-0.5).
        | Ending values     | y = (1,0.5).
        | Time of process   | t in [0,10].
        | Discretization    | Dt = 0.01.

In Figure 2, we present the flow of trajectories of \(X_{t}|X_{0}=1,X_{T}=1\) and \(Y_{t}|Y_{0}=-0.5,Y_{T}=0.5\):

    R> plot(mod2,col=c('#FF00004B','#0000FF82'))

Bridge sde 2D

Hence we can just make use of the `rsde2d()` function to build our random number generator for the couple \(X_{t},Y_{t}|X_{0}=1,Y_{0}=-0.5,X_{T}=1,Y_{T}=0.5\) at time \(t=5\):

    R> x2 <- rsde2d(object = mod2, at = 5)
    R> head(x2, n = 3)

             x          y
    1 0.051742  0.0095811
    2 0.135792  0.0436799
    3 0.021494 -0.0348084

The marginal density of \(X_{t}|X_{0}=1,X_{T}=1\) and \(Y_{t}|Y_{0}=-0.5,Y_{T}=0.5\) at time \(t=5\) are reported using `dsde2d()` function. A `contour` plot of joint density obtained from a realization of the couple \(X_{t},Y_{t}|X_{0}=1,Y_{0}=-0.5,X_{T}=1,Y_{T}=0.5\) at time \(t=5\), see e.g. Figure 3:

    R> ## Marginal
    R> denM <- dsde2d(mod2,pdf="M",at = 5)
    R> plot(denM, main="Marginal Density")
    R> ## Joint
    R> denJ <- dsde2d(mod2, pdf="J", n=100,at = 5)
    R> plot(denJ,display="contour",main="Bivariate Transition Density at time t=5")

The marginal and joint density of \(X_{t}|X_{0}=1,X_{T}=1\) and \(Y_{t}|Y_{0}=-0.5,Y_{T}=0.5\) at time \(t=5\)

A \(3\)D plot of the transition density at \(t=5\) obtained with:

    R> plot(denJ,main="Bivariate Transition Density at time t=5")

\(3\)D plot of the transition density of \(X_{t}|X_{0}=1,X_{T}=1\) and \(Y_{t}|Y_{0}=-0.5,Y_{T}=0.5\) at time \(t=5\)

We approximate the bivariate transition density over the set transition horizons \(t\in [1,9]\) with \(\Delta t = 0.005\) using the code:

    R> for (i in seq(1,9,by=0.005)){
    + plot(dsde2d(mod2, at = i,n=100),display="contour",main=paste0('Transition Density \n t = ',i))
    + }

Return to bridgesde2d()

# `bridgesde3d()`

Assume that we want to describe the following bridges SDE’s (3D) in Ito form:

\[\begin{equation} \begin{cases} dX_t = -4 (1+X_{t}) Y_{t} dt + 0.2 dW_{1,t},\quad X_{t_{0}}=0 \quad\text{and}\quad X_{T}=0\\ dY_t = 4 (1-Y_{t}) X_{t} dt + 0.2 dW_{2,t},\quad Y_{t_{0}}=-1 \quad\text{and}\quad Y_{T}=-2\\ dZ_t = 4 (1-Z_{t}) Y_{t} dt + 0.2 dW_{3,t},\quad Z_{t_{0}}=0.5 \quad\text{and}\quad Z_{T}=0.5 \end{cases} \end{equation}\]

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

    R> set.seed(1234, kind = "L'Ecuyer-CMRG")
    R> fx <- expression(-4*(1+x)*y, 4*(1-y)*x, 4*(1-z)*y)
    R> gx <- rep(expression(0.2),3)
    R> mod3 <- bridgesde3d(x0=c(0,-1,0.5),y=c(0,-2,0.5),drift=fx,diffusion=gx,M=1000)
    R> mod3

    Itô Bridge 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.
        | Crossing realized | C = 998 among 1000.
        | Initial values    | x0 = (0,-1,0.5).
        | Ending values     | y  = (0,-2,0.5).
        | Time of process   | t in [0,1].
        | Discretization    | Dt = 0.001.

For plotting (back in time) using the command `plot`, and `plot3D` in space the results of the simulation are shown in Figure 5:

    R> plot(mod3) ## in time
    R> plot3D(mod3,display = "persp",main="3D Bridge SDE's") ## in space

Bridge sde 3D

Hence we can just make use of the `rsde3d()` function to build our random number generator for the triplet \(X_{t},Y_{t},Z_{t}|X_{0}=0,Y_{0}=-1,Z_{0}=0.5,X_{T}=0,Y_{T}=-2,Z_{T}=0.5\) at time \(t=0.75\):

    R> x3 <- rsde3d(object = mod3, at = 0.75)
    R> head(x3, n = 3)

           x        y        z
    1 2.0636 0.074044 -0.30984
    2 1.9445 0.111705 -0.26487
    3 1.9288 0.054322 -0.50509

the density of \(X_{t}|X_{0}=0,X_{T}=0\), \(Y_{t}|Y_{0}=-1,Y_{T}=-2\) and \(Z_{t}|Z_{0}=0.5,Z_{T}=0.5\) process at time \(t=0.75\) are reported using `dsde3d()` function. For an approximate joint density for triplet \(X_{t},Y_{t},Z_{t}|X_{0}=0,Y_{0}=-1,Z_{0}=0.5,X_{T}=0,Y_{T}=-2,Z_{T}=0.5\) at time \(t=0.75\) (for more details, see package sm or ks.)

    R> ## Marginal
    R> denM <- dsde3d(mod3,pdf="M",at =0.75)
    R> plot(denM, main="Marginal Density")
    R> ## Joint
    R> denJ <- dsde3d(mod3,pdf="J",at=0.75)
    R> plot(denJ,display="rgl")

Return to bridgesde3d()

# Further reading

1. `snssdekd()` & `dsdekd()` & `rsdekd()`- Monte-Carlo Simulation and Analysis of Stochastic Differential Equations.
2. `bridgesdekd()` & `dsdekd()` & `rsdekd()` - Constructs and Analysis of Bridges Stochastic Differential Equations.
3. `fptsdekd()` & `dfptsdekd()` - Monte-Carlo Simulation and Kernel Density Estimation of First passage time.
4. `MCM.sde()` & `MEM.sde()` - Parallel Monte-Carlo and Moment Equations for SDEs.
5. `TEX.sde()` - Converting Sim.DiffProc Objects to LaTeX.
6. `fitsde()` - Parametric Estimation of 1-D Stochastic Differential Equation.

# References

1. Bladt, M. and Sorensen, M. (2007). Simple simulation of diffusion bridges with application to likelihood inference for diffusions. Working Paper, University of Copenhagen.

2. Guidoum AC, Boukhetala K (2024). Sim.DiffProc: Simulation of Diffusion Processes. R package version 4.9, URL https://cran.r-project.org/package=Sim.DiffProc.

1. Department of Mathematics and Computer Science, Faculty of Sciences and Technology, University of Tamanghasset, Algeria, E-mail (acguidoum@univ-tam.dz)↩︎

2. 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)↩︎