# Handling Exogenous Input Signals

The key to using exogeneous input signals is the same as in the rest of the SciML universe: just use the function in the definition of the differential equation. For example, if it's a standard differential equation, you can use the form

```
I(t) = t^2
function f(du,u,p,t)
du[1] = I(t)
du[2] = u[1]
end
```

so that `I(t)`

is an exogenous input signal into `f`

. Another form that could be useful is a closure. For example:

```
function f(du,u,p,t,I)
du[1] = I(t)
du[2] = u[1]
end
_f = (du,u,p,t) = f(du,u,p,t,x -> x^2)
```

which encloses an extra argument into `f`

so that `_f`

is now the interface-compliant differential equation definition.

Note that you can also learn what the exogenous equation is from data. For an example on how to do this, you can use the Optimal Control Example which shows how to parameterize a `u(t)`

by a universal function and learn that from data.

## Example of a Neural ODE with Exogenous Input

In the following example, a discrete exogenous input signal `ex`

is defined and used as an input into the neural network of a neural ODE system.

```
using DifferentialEquations, Flux, Optim, DiffEqFlux, DiffEqSensitivity, Plots
tspan = (0.1f0, Float32(10.0))
tsteps = range(tspan[1], tspan[2], length = 100)
t_vec = collect(tsteps)
ex = vec(ones(Float32,length(tsteps), 1))
f(x) = (atan(8.0 * x - 4.0) + atan(4.0)) / (2.0 * atan(4.0))
function hammerstein_system(u)
y= zeros(size(u))
for k in 2:length(u)
y[k] = 0.2 * f(u[k-1]) + 0.8 * y[k-1]
end
return y
end
y = Float32.(hammerstein_system(ex))
plot(collect(tsteps), y, ticks=:native)
nn_model = FastChain(FastDense(2,8, tanh), FastDense(8, 1))
p_model = initial_params(nn_model)
u0 = Float32.([0.0])
function dudt(u, p, t)
#input_val = u_vals[Int(round(t*10)+1)]
nn_model(vcat(u[1], ex[Int(round(10*0.1))]), p)
end
prob = ODEProblem(dudt,u0,tspan,nothing)
function predict_neuralode(p)
_prob = remake(prob,p=p)
Array(solve(_prob, Tsit5(), saveat=tsteps, abstol = 1e-8, reltol = 1e-6))
end
function loss(p)
sol = predict_neuralode(p)
N = length(sol)
return sum(abs2.(y[1:N] .- sol'))/N
end
# start optimization (I played around with several different optimizers with no success)
res0 = DiffEqFlux.sciml_train(loss,p_model ,ADAM(0.01), maxiters=100)
res1 = DiffEqFlux.sciml_train(loss,res0.minimizer,BFGS(initial_stepnorm=0.01), maxiters=300, allow_f_increases = true)
Flux.gradient(loss,res1.minimizer)
sol = predict_neuralode(res1.minimizer)
plot(tsteps,sol')
N = length(sol)
scatter!(tsteps,y[1:N])
savefig("trained.png")
```