# Parameter argument in ODE definition

**URL:** <https://discourse.julialang.org/t/parameter-argument-in-ode-definition/92390>\
**Category:** General Usage\
**Created:** [January 1, 2023, 4:28pm UTC](https://discourse.julialang.org/t/parameter-argument-in-ode-definition/92390 "2023-01-01T16:28:31Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![erlebach](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/erlebach/32/12973_2.png) [@erlebach](https://discourse.julialang.org/u/erlebach)\
**Post date:** [January 1, 2023, 4:28pm UTC](https://discourse.julialang.org/t/parameter-argument-in-ode-definition/92390/1 "2023-01-01T16:28:31Z")

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@ChrisRackauckas,

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:

```julia
# 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:

```julia
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,
```

---

<div class="post-metadata">

**Author:** ![ChrisRackauckas](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/chrisrackauckas/32/77_2.png) [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)\
**Post date:** [January 2, 2023, 5:25am UTC](https://discourse.julialang.org/t/parameter-argument-in-ode-definition/92390/2 "2023-01-02T05:25:19Z")

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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.