The Helmholtz differential equation is not separable in toroidal coordinates
Helmholtz Differential Equation--Toroidal Coordinates
See also
Helmholtz Differential Equation, Laplace's Equation--Toroidal Coordinates, Toroidal CoordinatesExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Helmholtz Differential Equation--Toroidal Coordinates." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/HelmholtzDifferentialEquationToroidalCoordinates.html