As shown by Morse and Feshbach (1953) and Arfken (1970), the Helmholtz differential equation is separable in oblate spheroidal coordinates.
Helmholtz Differential Equation--Oblate Spheroidal Coordinates
See also
Helmholtz Differential Equation, Oblate Spheroidal CoordinatesExplore with Wolfram|Alpha
References
Arfken, G. "Oblate Spheroidal CoordinatesCite this as:
Weisstein, Eric W. "Helmholtz Differential Equation--Oblate Spheroidal Coordinates." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/HelmholtzDifferentialEquationOblateSpheroidalCoordinates.html