Say, an infinite isotropic medium with a spherical cavity cut out, and a sphere just a little too big jammed in the void.
I'm trying to set this problem up from scratch, but am mainly scratching my head so far. Obviously we want to come up with a single equation in r or rho or such. Visions of stretched thin spherical shells dance in my head. It should be "simple" because of symmetry.
Any helpful advice or standard reference would be appreciated.
Disclaimer: I am not an engineer, and this is not HW. I think I once set this up correctly, but have since got dumber.