Alias elimination internal error

[tshort]

This model is two resistors in series.

@variables t
@variables v1(t) v2(t) i1(t) i2(t)
eq = [
    v1 ~ sin(2pi*t)
    v1 - v2 ~ i1
    v2 ~ i2
    i1 ~ i2
]
@named sys = ODESystem(eq)
sys = structural_simplify(sys)

The error is:

julia> sys = structural_simplify(sys)
ERROR: AssertionError: r == 0
Stacktrace:
 [1] exactdiv(a::Int64, b::Int64)
   @ ModelingToolkit C:\Users\xxxxxx\.julia\packages\ModelingToolkit\frHXN\src\systems\alias_elimination.jl:303
 [2] locally_structure_simplify!(adj_row::ModelingToolkit.CLILVector{Int64, Int64}, pivot_col::Int64, ag::ModelingToolkit.AliasGraph, may_eliminate::Bool)
   @ ModelingToolkit C:\Users\xxxxxx\.julia\packages\ModelingToolkit\frHXN\src\systems\alias_elimination.jl:347
 [3] (::ModelingToolkit.var"#lss!#576"{ModelingToolkit.AliasGraph, ModelingToolkit.SparseMatrixCLIL{Int64, Int64}, Vector{Int64}})(ei::Int64)
   @ ModelingToolkit C:\Users\xxxxxx\.julia\packages\ModelingToolkit\frHXN\src\systems\alias_elimination.jl:261
 [4] foreach(f::ModelingToolkit.var"#lss!#576"{ModelingToolkit.AliasGraph, ModelingToolkit.SparseMatrixCLIL{Int64, Int64}, Vector{Int64}}, itr::StepRange{Int64, Int64})
   @ Base .\abstractarray.jl:2141
 [5] alias_eliminate_graph!(graph::BipartiteGraph{Int64, Nothing}, varassoc::Vector{Int64}, mm_orig::ModelingToolkit.SparseMatrixCLIL{Int64, Int64})
   @ ModelingToolkit C:\Users\xxxxxx\.julia\packages\ModelingToolkit\frHXN\src\systems\alias_elimination.jl:265
 [6] alias_elimination(sys::ODESystem)
   @ ModelingToolkit C:\Users\xxxxxx\.julia\packages\ModelingToolkit\frHXN\src\systems\alias_elimination.jl:12
 [7] structural_simplify(sys::ODESystem; simplify::Bool)
   @ ModelingToolkit C:\Users\xxxxxx\.julia\packages\ModelingToolkit\frHXN\src\systems\abstractsystem.jl:857
 [8] structural_simplify(sys::ODESystem)
   @ ModelingToolkit C:\Users\xxxxxx\.julia\packages\ModelingToolkit\frHXN\src\systems\abstractsystem.jl:857
 [9] top-level scope
   @ REPL[53]:1

[ChrisRackauckas]

This isn't an ODE. There are no differential variables. Did you mean NonlinearSystem?

[tshort]

I realize it's not an ODE, but I'm hoping MTK can handle DAE's without differential variables.

The MTK Standard Library has several examples of this type of system (like this), and these work with ODESystem/structural_simplify.

[ChrisRackauckas]

Recognizing and handling this is something that hasn't been implemented.

[tshort]

Thanks, Chris. Currently, it's hard to know which non-D models will work with ODESystem/structural_simplify and which need a NonlinearSystem.

[ChrisRackauckas]

All

[tshort]

I also managed to get the same AssertionError: r == 0 error after including a differential term:

using ModelingToolkit
const D = Differential(t)
@variables t
@variables v1(t) v2(t) i1(t) i2(t) i3(t)
eq = [
    v1 ~ sin(2pi*t)
    v1 - v2 ~ i1
    v2 ~ i2
    i1 ~ i2
    D(i3) ~ v1
]
@named sys = ODESystem(eq)
sys = structural_simplify(sys)

It's a DAE, but the source drives one differential term and another set of equations without differential terms.

[ChrisRackauckas]

Interesting, that's a separate issue then that currently hits the same error. If the error gets fixed the first case will still not work well though as it will fail in the solver.

[tshort]

The Rosenbrock23 solver works for at least some systems without differential terms. It's used in the tests for the ModelingToolkitStandardLibrary (this example has no differential terms).

[YingboMa]

Thanks for the bug report. This is an internal error in MTK that should never happen in principle.