# Parameter argument in ODE definition

In the demo listed at `https://docs.sciml.ai/Overview/dev/showcase/missing_physics/`,
there is the following call that defines a Universal differential equation, where `U` is a neural network model:

``````# Define the hybrid model
function ude_dynamics!(du,u, p, t, p_true)
û = U(u, p, st)[1] # Network prediction
du[1] = p_true[1]*u[1] + û[1]
du[2] = -p_true[4]*u[2] + û[2]
end
``````

Why aren’t the parameters `p` and `p_true` concatenated into a single parameter argument or put into a tuple `(p, p_true)`, for example by doing:

``````params = (p, p_true)
function ude_dynamics!(du, u, params, t)
p, p_true = params
û = U(u, p, st)[1]
du[1] = p_true[1]*u[1] + û[1]
du[2] = -p_true[4]*u[2] + û[2]
end

Thanks,``````

In that demo it’s not trying to optimize `p_true`, so it excludes it from the parameters list being optimized. Indeed, if you want to optimize those parameters, you’d add them to the list.