# Defining a diffusion process for a swinging pendulum

In this tutorial we will define a diffusion and simulate trajectories from it.

## The model

We start with the differential equation for the angular position of a swinging pendulum, which is given by the second order ordinary differential equation

$\frac{\dd^2 x(t)}{\dd t^2} + θ^2 \sin(x(t)) = 0.$

Here $x(t)$ gives the angular position at time $t$ and $θ$ is the angular velocity of the linearised pendulum. Under the assumption that the acceleration is in fact a white-noise process, we obtain the Stochastic Differential Equation (SDE)

\begin{align} \dd X_t &=\begin{bmatrix} 0 & 1 \\ 0 & 0 \end{bmatrix} X_t \dd t + \begin{bmatrix} 0 \\ -θ^2 \sin(X_{1t})\end{bmatrix} \dd t + \begin{bmatrix} 0 \\ \gamma \end{bmatrix} \dd W_t, \end{align}

where $X_t=\begin{bmatrix} X_{t1} & X_{t2}\end{bmatrix}^\prime$.

## Defining diffusion

To define the diffusion for the pendulum, we use the DiffusionDefinition.jl-package.

using Plots, DiffusionDefinition, StaticArrays

To avoid typing DiffusionDefinition fully, we define

const DD = DiffusionDefinition;

To define the diffusion, we specify the dimensions of the state space and Wiener noise and the parameters appearing in the SDE using the diffusion_process-macro. In the call to this macro we specify the dimensions of the state space of the diffusion and driving Wiener process, as also parameters appearing in their definition.

@diffusion_process Pendulum begin
:dimensions
process --> 2
wiener --> 1

:parameters
(θ, γ) --> (2, Float64)
end

Note the printed message and that by issuing the command ?Pendulum information on the constructed struct is displayed. Suppose $θ=2$ and $γ=0.5$, then we can define an instance of the struct called Pendulum by setting

P = Pendulum(2.0, 0.5)

There are various convenience functions, among which

DD.parameters(P)
DD.parameter_names(P)

The general form in which the diffusion is specified uses the notation

$\dd X_t = b(t,X_t)\dd t + \sigma(t,X_t) \dd W_t$

So the drift is denoted by $b$ and the diffusivity by $σ$ (this is the notation in the well known books by Rogers and Williams on stochastic calculus). As the functions $b$ and $σ$ are not exported in the DiffusionDefinition-package (to avoid conflicts with other packages), these need to be defined using DiffusionDefinition.b and DiffusionDefinition.σ.

DD.b(t, x, P::Pendulum) = @SVector [x[2],-P.θ^2 * sin(x[1])]
DD.σ(t, x, P::Pendulum) = @SMatrix [0.0 ; P.γ]

While not strictly necessary, it is preferable to use static vectors and matrices using commands from the StaticArrays-package.

## Sampling trajectories

Now we can sample a trajectory of the diffusion. For that, we need a starting point and grid (on which the solution to the SDE is approximated using Euler discretisation).

tt = collect(0.0:0.005:8.0)  # grid
x0 = @SVector [1.0, 0.0]  # starting point
X = DD.rand(P, tt, x0);

Now X is a trajectory, with X.t denoting the timegrid, and X.x the simulated trajectory.

# Plotting

Next, we plot the simulated trajectories

#using PyPlot
pyplot()
plot(X, Val(:vs_time))