Simplify doesn't recognize a zero

[andreasnoack]

The following system has no constant right-hand-side term but when I try to detect that programmatically I'm getting

julia> @parameters t CL Vc Qfast Vpfast Qslow Vpslow
(t, CL, Vc, Qfast, Vpfast, Qslow, Vpslow)

julia> @variables Central(t) Periph1(t) Periph2(t)
(Central(t), Periph1(t), Periph2(t))

julia> D = Differential(t);

julia> eqs = [
           D(Central) ~ -(CL/Vc)*Central + (Qfast/Vpfast)*Periph1 - (Qfast/Vc)*Central + (Qslow/Vpslow)*Periph2 - (Qslow/Vc)*Central,
           D(Periph1) ~ (Qfast/Vc)*Central - (Qfast/Vpfast)*Periph1,
           D(Periph2) ~ (Qslow/Vc)*Central - (Qslow/Vpslow)*Periph2
       ];

julia> @named sys = ODESystem(eqs);

julia> jac = calculate_jacobian(sys);

julia> linpart = jac*states(sys);

julia> rhs = [eq.rhs for eq ∈ equations(sys)];

julia> cnst = rhs - linpart
3-element Vector{Num}:
 -(-CL*(Vc^-1) - (Qfast*(Vc^-1)) - (Qslow*(Vc^-1)))*Central(t) - (CL*Central(t)*(Vc^-1)) - (Qfast*Central(t)*(Vc^-1)) - (Qslow*Central(t)*(Vc^-1))
                  0
                  0

julia> simplify(cnst[1])
-(-CL*(Vc^-1) - (Qfast*(Vc^-1)) - (Qslow*(Vc^-1)))*Central(t) - (CL*Central(t)*(Vc^-1)) - (Qfast*Central(t)*(Vc^-1)) - (Qslow*Central(t)*(Vc^-1))

This happens both with version 5.20.0 and master. With version 5.20.0, I can simply the expressions with expand

julia> expand(cnst[1])
0

but on master, I'm getting

julia> simplify(expand(cnst[1]))
(CL*Central(t)) / Vc + (Qfast*Central(t)) / Vc + (Qslow*Central(t)) / Vc + (-CL*Central(t)) / Vc + (-Qfast*Central(t)) / Vc + (-Qslow*Central(t)) / Vc

Update: The versions of Symbolics are v0.1.22 and v4.1.0 respectively.

[andreasnoack]

I can reduce this to just a Symbolics example

Symbolics 0.1.22

julia> @variables x y z
(x, y, z)

julia> expand(2*((y + z)/x) - 2*y/x - z/x*2)
0

julia> simplify(2*((y + z)/x) - 2*y/x - z/x*2)
2(x^-1)*(y + z) - (2y*(x^-1)) - (2z*(x^-1))

Symbolics 4.1.0

julia> expand(2*((y + z)/x) - 2*y/x - z/x*2)
(2y + 2z) / x + (-2y) / x + (-2z) / x

julia> simplify(2*((y + z)/x) - 2*y/x - z/x*2)
(2y + 2z) / x + (-2y) / x + (-2z) / x

so maybe this issue should be moved to Symbolics

[ChrisRackauckas]

What if you use simplify_fractions?

[andreasnoack]

It doesn't handle the original example

julia> simplify_fractions(cnst[1])
(-CL*Central(t) - Qfast*Central(t) - Qslow*Central(t) - (-CL - Qfast - Qslow)*Central(t)) / Vc

[ChrisRackauckas]

@shashi

[YingboMa]

You can do it by

julia> simplify(simplify_fractions(cnst[1], polyform=true), expand=true)
0//1

[shashi]

should be fixed by JuliaSymbolics/SymbolicUtils.jl#422