Function exp is not supported

[kikiggg]

Hi

I want to check StructuralIdentifiability some differential equation which include "exp" but it didnt work with message "Function exp is not supported"

Is there any solutiong for that? or internally not supported?

Thank you
Park

[iliailmer]

Hi @kikiggg , StructuralIdentifiability.jl supports ODE models with rational (polynomial) functions. You can try to modify your ODE so that exp is "implicit", for example

# original:
x' = A * exp(B * t) * x

# modified
x' = A * g * x
g' = B * g

[ChrisRackauckas]

We could make that a ModelingToolkit pass of sorts, worth an issue.

[pogudingleb]

Small addition to the comment of @iliailmer
It is better to set g = A * exp(B * t), this way the resulting system will be

x' = g * x
g' = B * g

So the number degrees of freedom (num of states plus num of parameters will be the same), so the identifiability results can be transferred between the original system and the new one.

@ChrisRackauckas Here is a subtlety: one should ideally try to make the transformation carefully, keeping the dimension of the problem. Not impossible, definitely worth an issue, I will create one.

[pogudingleb]

I created a separate issue for handling non-rational models containing my thoughts on the matter. It subsumes the present issue, so I close it.