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