Errors and wrong solutions when solving NonlinearSystem with initialization_eqs
#3458
Comments
sol[X2] # ERROR: ArgumentError: Symbol X2(t) is not present in the system. (should be 2.0).There is no equation for X2, so I don't see how or why this should be 2.0. You're missing an equation?
Since X2 is not defined, Γ[1] can be anything. |
|
Ok, that might make a bit of sense. My understanding was that initialization_eqs = [
X2 ~ Γ[1] - X1
]is the equation that allow us to compute Previously I instead did observed = [
X2 ~ Γ[1] - X1
]which still would be my preferred way, but I am also trying to explore alternatives. |
It was pointed out in that conversation that it's a bit ambiguous since using ModelingToolkit, OrdinaryDiffEqDefault, Test
using ModelingToolkit: t_nounits as t, D_nounits as D
# Defines the model.
@parameters k1 k2
@variables X1(t) X2(t)
@parameters Γ[1:1] = missing [guess = [1.0]]
eqs = [
0 ~ k1 * (Γ[1] - X1) - k2 * X1
X2 ~ Γ[1] - X1
]
initialization_eqs = [
Initial(X2) ~ Γ[1] - Initial(X1)
]
@mtkbuild nlsys = NonlinearSystem(eqs, [X1, X2], [k1, k2, Γ]; initialization_eqs)
# Creates the problem.
u0 = [X1 => 1.0, X2 => 2.0]
ps = [k1 => 0.1, k2 => 0.2]
nlprob = NonlinearProblem(nlsys, u0, ps)
# Solves the problem and check the solution.
sol = solve(nlprob)
sol[X1]
sol[X2]
sol.ps[k1]
sol.ps[k2]
sol.ps[Γ[1]]This won't work quite yet as we need to setup |
From the latest understanding this is what one should use with
structural_simplify? Although, admittedly, I am not really sure anymore.Either case, there is problem with the solution, generating a mix of errors and wrong outputs when you check the solution.
Either this is a bug, or I still do not know how you are supposed to do this.
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