2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/104619We present a simple formalism for the evaluation of the Casimir energy for two spheres and a sphere and a plane, in case of a scalar fluctuating field, valid at any separations. We compare the exact results with various approximation schemes and establish when such schemes become useful. The formalism can be easily extended to any number of spheres and/or planes in three or arbitrary dimensions, with a variety of boundary conditions or non-overlapping potentials/non-ideal reflectors.published version; 13 pages, 2 figures; add. material (in Sec.VI) and corrections (esp. in App.B)High Energy Physics - TheoryStatistical MechanicsHigh Energy Physics - PhenomenologyChaotic DynamicsQuantum Physicstext