Optimization of Stochastic Differential Equations

Here we demonstrate sensealg = ForwardDiffSensitivity() (provided by DiffEqSensitivity.jl) for forward-mode automatic differentiation of a small stochastic differential equation. For large parameter equations, like neural stochastic differential equations, you should use reverse-mode automatic differentiation. However, forward-mode can be more efficient for low numbers of parameters (<100). (Note: the default is reverse-mode AD which is more suitable for things like neural SDEs!)

Example 1: Fitting Data with SDEs

Let's do the most common scenario: fitting data. Let's say our ecological system is a stochastic process. Each time we solve this equation we get a different solution, so we need a sensible data source.

using DiffEqFlux, DifferentialEquations, Plots, Flux, Optim, DiffEqSensitivity
function lotka_volterra!(du,u,p,t)
  x,y = u
  α,β,γ,δ = p
  du[1] = dx = α*x - β*x*y
  du[2] = dy = δ*x*y - γ*y
end
u0 = [1.0,1.0]
tspan = (0.0,10.0)

function multiplicative_noise!(du,u,p,t)
  x,y = u
  du[1] = p[5]*x
  du[2] = p[6]*y
end
p = [1.5,1.0,3.0,1.0,0.3,0.3]

prob = SDEProblem(lotka_volterra!,multiplicative_noise!,u0,tspan,p)
sol = solve(prob)
plot(sol)

Let's assume that we are observing the seasonal behavior of this system and have 10,000 years of data, corresponding to 10,000 observations of this timeseries. We can utilize this to get the seasonal means and variances. To simulate that scenario, we will generate 10,000 trajectories from the SDE to build our dataset:

using Statistics
ensembleprob = EnsembleProblem(prob)
@time sol = solve(ensembleprob,SOSRI(),saveat=0.1,trajectories=10_000)
truemean = mean(sol,dims=3)[:,:]
truevar  = var(sol,dims=3)[:,:]

From here, we wish to utilize the method of moments to fit the SDE's parameters. Thus our loss function will be to solve the SDE a bunch of times and compute moment equations and use these as our loss against the original series. We then plot the evolution of the means and variances to verify the fit. For example:

function loss(p)
  tmp_prob = remake(prob,p=p)
  ensembleprob = EnsembleProblem(tmp_prob)
  tmp_sol = solve(ensembleprob,SOSRI(),saveat=0.1,trajectories=1000,sensealg=ForwardDiffSensitivity())
  arrsol = Array(tmp_sol)
  sum(abs2,truemean - mean(arrsol,dims=3)) + 0.1sum(abs2,truevar - var(arrsol,dims=3)),arrsol
end

function cb2(p,l,arrsol)
  @show p,l
  means = mean(arrsol,dims=3)[:,:]
  vars = var(arrsol,dims=3)[:,:]
  p1 = plot(sol[1].t,means',lw=5)
  scatter!(p1,sol[1].t,truemean')
  p2 = plot(sol[1].t,vars',lw=5)
  scatter!(p2,sol[1].t,truevar')
  p = plot(p1,p2,layout = (2,1))
  display(p)
  false
end

We can then use sciml_train to fit the SDE:

pinit = [1.2,0.8,2.5,0.8,0.1,0.1]
@time res = DiffEqFlux.sciml_train(loss,pinit,ADAM(0.05),cb=cb2,maxiters = 100)

The final print out was:

(p, l) = ([1.5242134195974462, 1.019859938499017, 2.9120928257869227, 0.9840408090733335, 0.29427123791721765, 0.3334393815923646], 1.7046719990657184)

Notice that both the parameters of the deterministic drift equations and the stochastic portion (the diffusion equation) are fit through this process! Also notice that the final fit of the moment equations is close:

The time for the full fitting process was:

250.654845 seconds (4.69 G allocations: 104.868 GiB, 11.87% gc time)

approximately 4 minutes.

Example 2: Controlling SDEs to an objective

In this example, we will find the parameters of the SDE that force the solution to be close to the constant 1.

using DifferentialEquations, Flux, Optim, DiffEqFlux, DiffEqSensitivity, Plots

function lotka_volterra!(du, u, p, t)
  x, y = u
  α, β, δ, γ = p
  du[1] = dx = α*x - β*x*y
  du[2] = dy = -δ*y + γ*x*y
end

function lotka_volterra_noise!(du, u, p, t)
  du[1] = 0.1u[1]
  du[2] = 0.1u[2]
end

u0 = [1.0,1.0]
tspan = (0.0, 10.0)
p = [2.2, 1.0, 2.0, 0.4]
prob_sde = SDEProblem(lotka_volterra!, lotka_volterra_noise!, u0, tspan)


function predict_sde(p)
  return Array(solve(prob_sde, SOSRI(), p=p,
               sensealg = ForwardDiffSensitivity(), saveat = 0.1))
end

loss_sde(p) = sum(abs2, x-1 for x in predict_sde(p))

For this training process, because the loss function is stochastic, we will use the ADAM optimizer from Flux.jl. The sciml_train function is the same as before. However, to speed up the training process, we will use a global counter so that way we only plot the current results every 10 iterations. This looks like:

callback = function (p, l)
  display(l)
  remade_solution = solve(remake(prob_sde, p = p), SOSRI(), saveat = 0.1)
  plt = plot(remade_solution, ylim = (0, 6))
  display(plt)
  return false
end

Let's optimize

result_sde = DiffEqFlux.sciml_train(loss_sde, p, ADAM(0.1),
                                    cb = callback, maxiters = 100)