2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/55363Performing functional integration of the free Lagrangian, we find the vacuum energy of a field. The functional integration is performed in a way which easily generalizes to systems at non-zero temperature. We use this technique to obtain the Casimir energy density and pressure at arbitrary temperatures.8 pages, Latex, uses epsf.sty for 1 embedded figure (some references added; no other significant change)High Energy Physics - TheoryCondensed MatterHigh Energy Physics - PhenomenologyQuantum Physicstext