# The true jump method

A while ago, I learned a numerical recipe for simulating state-dependent jump processes from this paper by Romain Veltz. After some digging, this book by Graham and Talay seems to introduce the method. It’s a cute trick that even my advisor was excited by, so I figured it seemed like a nice short thing to share.

# Setup

Suppose we have some switching process $X_t$, which between jump events evolves via $$$\frac{\mathrm{d}}{\mathrm{d}t} X_t = F(X_t).\tag{1}$$$ For simplicity, we’ll assume for now that $F(\cdot)$ is deterministic, so that our process is a piecewise-deterministic Markov process (PDMP), but I’m fairly confident that this need not be the case (discussed later).

Along side this, superimpose a random switching process with intensity (rate) $\lambda(X_t)$. The important feature here is that $\lambda$ depends on the process, which is evolving by (1). Denote the jump times $$$\tau_1, \tau_2, \ldots, \tau_n.$$$

A few things could happen at the jump times: $F$ could switch (as in the PDMP case) or a jump could occur in $X_t$. However, we won’t focus on this aspect since the overall theme of this post is: how do we generate the jump times $\tau_j$?

## Most naïve technique

I’m embarrassed to admit that as an early grad student, this is how I simulated this type of process, but this technique is worth mentioning just for how bad it is.

Effectively, the idea is to take fixed time steps $\Delta t$, and each step, check whether to jump or evolve $X_t$.

In pseudocode, this looks like

at step i:
eff_rate <- lambda(x(i)) # current value of the effective rate
next_jump <- randexp(rate) # generate an exponential rand num with this rate

if next_jump < dt: # check whether jump occurred
x(i+1) = # do jump procedure
else
x(i+1) =  # evolve with F(x(i))

Note that this can be thought of as making the approximation that $$$\int_{\tau_{n-1}}^{\tau_{n-1}+\Delta t} \lambda(X_t) \, \mathrm{d} t \approx \lambda(X_{\tau_{n-1}})\Delta t,$$$ so we’re treating the rate as unchanging in this time step. Another thing to note is that this introduces irregular times, as the next event either occurs at time $\Delta t$ or whatever the time is to the next event. Furthermore, this technique is slow, as we’re doing the check at every time step. Overall, never do this.

## Fictitious jump method

Although the aforementioned technique is bad, it lays the groundwork for a much better technique called the Fictitious jump method. In spirit, the idea is the same: generate possible jump times and “reject” some based on probability of it actually occurring. This procedure is ubiquitous in stochastic simulation, and goes under the apt name rejection sampling.

The crux of this method is finding some $\lambda_{\infty}$ such that $$$\sup_{x \in \mathbb{R}} \lambda(x) < \lambda_{\infty}.$$$ That is, we want an upper bound for our jump rate and then use this to generate possible jump times. The algorithm is then

### Fictitious jump method (FJM)

At step $n$,

1. generate $S_n \sim \mathcal{E}(\lambda_\infty)$, where $\mathcal{E}$ is the exponential distribution.
2. set $\tau_n = \tau_{n-1} + S_n$
3. evolve $X_t = F(X_t)$ for $t \in [\tau_{n-1},\tau_{n}^-]$.
4. either, with probability $1-\lambda(X_{\tau_n^-})/\lambda_\infty$ set $X_{\tau_n} = X_{\tau_n^-}$ (no jump occurs) or a jump occurs, and set $X_{\tau_n}$ accordingly.

There are certainly a few perks to this technique from the previous. Note that the use of constant $\lambda_{\infty}$ allows all the times to be generated upfront (as opposed to the naïve technique where the times were generated on the fly). Because these times are generated up front, evolving $X_t$ for a known time window (step 3) is easy: basically throw it into your favorite ODE solver. This wasn’t the case in the previous method, where the jump check was performed at each time step.

However, there are considerable drawbacks to FJM as well. What if $\lambda(x)$ has no obvious upper bound? Furthermore, if $X_t$ spends lot of time in a region where $\lambda(X_t) \ll \lambda_\infty$, this technique spends most of its iterations rejecting possible jump times while taking relatively small step sizes, producing a considerably inefficient result.

There are improvements that can be made to this technique, called the “sub-domain” method, which basically tries to use a local approximation of $\lambda_\infty$ based on a partitioning of the state space, but finding these partitions is non-trivial or even rarely possible.

# The true jump method

While the aforementioned techniques are in reality, fine for simulating these processes, they have some annoying drawbacks. I’ll lastly present the true jump method (TJM), which I believe is the sweet spot of all of these, and is also the technique I use for my simulations in this context.

The whole idea behind this technique is to do a clever time change. Akin to the previous methods, for a fixed next jump time $u$, the effective $n$th jump rate is then $S_n$ $$$\int_{\tau_{n-1}}^{\tau_{n-1}+u} \lambda(X_t) \, \mathrm{d}t = S_n.\tag{2}$$$ However, we can think of (2) in the inverse manner: if we generate $S_n$ up front, can we solve for $u$? Rewriting it slightly $$$\int_{\tau_{n-1}}^{\sigma(s)} \lambda(X_t) \, \mathrm{d}t = s.$$$ and differentiating with respect to $s$, we get back the ODE $$$\dot{\sigma}(s) \lambda(X_{\sigma(s)}) = 1.$$$ This, while seemingly unimpressive gives us the whole technique: we’ll perform a random time change $t \rightarrow s$ so that between jumps, the effective jump rate is always intensity $1$.

That is, define the time-changed solution to be $$$y(s) = X_{\sigma(s)},$$$ so that we are left with the system of equations $$$\begin{cases} \tag{3} \dot{y}(s) = F(y)/\lambda(y),\\ \dot{\sigma}(s) = 1/\lambda(y). \end{cases}$$$

In summary, the algorithm is then

### True jump method (TJM)

At each step $n$,

1. generate $S_n \sim \mathcal{E}(1)$, where $\mathcal{E}$ is the exponential distribution.
2. Set $y(0) = X_{\tau_{n-1}}$ and evolve the system (3) for $y(s), \sigma(s)$ for $s \in [0,1]$.
3. Set $\tau_{n} = \tau_{n-1}+\sigma(1)$ and the solution before the jump is $X_t = X_{\sigma(s)}$.
4. Perform the jump procedure on $X_{\tau_n}$.

Notice that this procedure has basically all the benefits of the previous with none of the drawbacks. For one, all the random times $S_n$ can be generated up front easily. Between them, evolving the system (3) can be done with any pre-canned solver (e.g. ode45) without further thought. This technique does not require that $\lambda(x)$ is bounded, so long as there isn’t any explosion.

Overall, I think the TJM is elegant, simple, and a worthwhile time investment for anyone that regularly does these types of simulations.

## Final thoughts

One particularly nice thing about TJM is that it can be extended easily to do a Gillespie-type simulation with multiple possible jumps. That is, suppose at any given step we had $j$ possible transitions, indexed by their rates $\lambda_1, \lambda_2, \ldots, \lambda_j$. Then, in the TJM algorithm, we just replace $$$\lambda_{tot}(x) = \sum_j \lambda_j(x)$$$ and the algorithm is exactly the same. At the jump times, we perform a Gillespie selection of which “reaction” to choose, by assigning relative probabilities $$$p_j := \frac{\lambda_j(X_{\tau_n})}{\lambda_{tot}(X_{\tau_n})}$$$ and sampling from this distribution to choose the appropriate reaction.

Note that at no point in the analysis did we require that $F$ be deterministic. Consequently, either technique is totally valid for jump-SDEs. For instance, Lawley and Bressloff use the FJM (which they refer to as thinning) for Brownian motion with switching diffusion coefficient $D$.