2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/48153I compute the derivative expansion of an effective action at finite temperature using the imaginary time approach. I show that it is a well behaved expansion giving a unique seriers contrary to previous results. This disparity is shown to originate in the choice of thermal Green functions used in the calculations.RevTeX, 4 pages, no figuresHigh Energy Physics - Phenomenologytext