`pde_separate_mul` returns `None`

[Ícaro]

I am using pde_separate_mul to solve some problems in electrodynamics. However, the function sometimes just returns nothing without giving an error or warning.

Here's a code example:

import sympy as sp
from sympy.vector import CoordSys3D, laplacian

S = CoordSys3D('S', transformation='cylindrical', variable_names=('r', 'θ', 'z'), vector_names=('e_r', 'e_θ', 'e_z'))
V0, a = sp.symbols('V_0 a', positive=True)

R = sp.Function('R')
T = sp.Function('T')
V = sp.Function('V')

r = sp.symbols('r', positive=True)
θ = sp.symbols('θ', real=True)
V_eq = laplacian(V(S.r, S.θ)).subs(S.r,r).subs(S.θ,θ)
V_sol = sp.pde_separate_mul(V_eq, V(r, θ), [R(r), T(θ)])

The output of V_sol in this example is None