Dear all,

I’m running a simple NN that can also be understood as a linear regression problem (there are specific reasons I’m posing it as a NN). Instead of a classic least square approach in the loss function, mine has a ODE being solved inside the loss function.

I’m trying to do this using Flux and my code is the following. Here, the input data is contained in a Matrix called InputData.

```
using DiffEqFlux, OrdinaryDiffEq, Flux,
tspan=(0.0,4.0)
init_cond_range=(0.05:0.05:0.95)
#NeuralNetwork
NN = Chain(Dense((M+1)*(N+1),1))
W = params(NN)
opt = ADAM()
#Control Approximation
control_app(W, Input) = dot(W, Input)
#Solving the Trajectory
function SolveODE(x₀, W, input)
g(x,p,t) = 1/2*x*(1 - x)*(control_app(W, input) - 1/2)
prob = ODEProblem(g, x₀, tspan)
sol = solve(prob, Tsit5(), reltol=1e-8, abstol=1e-8)
end
timesize=size(time)[1]
#Optimization Loop
j=0
for x0 in init_cond_range
x₀=x0
for i in ((j*80) + 1):((j*80) + timesize)
x_t = SolveODE(x₀, W[1], InputData[i,:]).t
x_sol = SolveODE(x₀, W[1], InputData[i,:]).u
x_spl = Spline1D(x_t, x_sol; k=3, bc="nearest", s=0.0)
grads = Flux.gradient(W) do
loss_sum += -0.05*(2*control_app(W[1], InputData[i,:])*x_spl(i) - control_app(W[1], InputData[i,:])^2)
end
Flux.Optimise.update!(opt, W, grads)
end
end
```

I want to update the loss for each row of my matrix, therefore I included the loss in the very inner loop that accounts for each row of InputData.

I have constructed this problem in many different ways, and always seem to get the same error which is unknown to me. It reads:

```
MethodError: no method matching +(::IRTools.Inner.Undefined, ::Float64)
Closest candidates are:
+(::Any, ::Any, !Matched::Any, !Matched::Any...) at operators.jl:538
+(!Matched::ChainRulesCore.One, ::Any) at /Users/Gabriel/.julia/packages/ChainRulesCore/qbmEe/src/differential_arithmetic.jl:146
+(!Matched::Missing, ::Number) at missing.jl:115
...
Stacktrace:
[1] macro expansion at /Users/Gabriel/.julia/packages/Zygote/zowrf/src/compiler/interface2.jl:0 [inlined]
[2] _pullback(::Zygote.Context, ::typeof(+), ::IRTools.Inner.Undefined, ::Float64) at /Users/Gabriel/.julia/packages/Zygote/zowrf/src/compiler/interface2.jl:9
[3] #9 at ./In[33]:34 [inlined]
[4] _pullback(::Zygote.Context, ::var"#9#10"{Int64,Spline1D}) at /Users/Gabriel/.julia/packages/Zygote/zowrf/src/compiler/interface2.jl:0
[5] pullback(::Function, ::Params) at /Users/Gabriel/.julia/packages/Zygote/zowrf/src/compiler/interface.jl:250
[6] gradient(::Function, ::Params) at /Users/Gabriel/.julia/packages/Zygote/zowrf/src/compiler/interface.jl:58
[7] top-level scope at In[33]:33
[8] include_string(::Function, ::Module, ::String, ::String) at ./loading.jl:1091
```

I came across this matching +(::IRTools.Inner.Undefined,… many times while trying to get around this issue.

Please help me understand what I am doing wrong or provide me hints where I shuold look closely. Thank you a lot,

Gabriel