# Delay Differential Equations

Other differential equation problem types from DifferentialEquations.jl are supported. For example, we can build a layer with a delay differential equation like:

```
using DifferentialEquations, Flux, Optim, DiffEqFlux, DiffEqSensitivity
# Define the same LV equation, but including a delay parameter
function delay_lotka_volterra!(du, u, h, p, t)
x, y = u
α, β, δ, γ = p
du[1] = dx = (α - β*y) * h(p, t-0.1)[1]
du[2] = dy = (δ*x - γ) * y
end
# Initial parameters
p = [2.2, 1.0, 2.0, 0.4]
# Define a vector containing delays for each variable (although only the first
# one is used)
h(p, t) = ones(eltype(p), 2)
# Initial conditions
u0 = [1.0, 1.0]
# Define the problem as a delay differential equation
prob_dde = DDEProblem(delay_lotka_volterra!, u0, h, (0.0, 10.0),
constant_lags = [0.1])
function predict_dde(p)
return Array(solve(prob_dde, MethodOfSteps(Tsit5()),
u0=u0, p=p, saveat = 0.1,
sensealg = ReverseDiffAdjoint()))
end
loss_dde(p) = sum(abs2, x-1 for x in predict_dde(p))
#using Plots
cb = function (p,l...)
display(loss_dde(p))
#display(plot(solve(remake(prob_dde,p=p),MethodOfSteps(Tsit5()),saveat=0.1),ylim=(0,6)))
return false
end
cb(p,loss_dde(p))
result_dde = DiffEqFlux.sciml_train(loss_dde, p, ADAM(0.1),
cb = cb, maxiters = 100)
```

Notice that we chose `sensealg = ReverseDiffAdjoint()`

to utilize the ReverseDiff.jl reverse-mode to handle the delay differential equation.

We define a callback to display the solution at the current parameters for each step of the training:

```
#using Plots
cb = function (p,l...)
display(loss_dde(p))
#display(plot(solve(remake(prob_dde,p=p),MethodOfSteps(Tsit5()),saveat=0.1),ylim=(0,6)))
return false
end
cb(p,loss_dde(p))
```

We use `sciml_train`

to optimize the parameters for our loss function:

```
result_dde = DiffEqFlux.sciml_train(loss_dde, p, ADAM(0.1),
cb = cb, maxiters = 100)
```