2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/48137Well known from the sixties, the pressure of e.g. massless phi-four theory may be written as a series of 2PI-diagrams (skeletons) with the lines fully dressed. Varying the self-energy Pi in this expression, it turns into a functional U[Y] having a maximum in function space at Y=Pi. There is also the Feynman-Jensen thermal variational principle V[S], a potentially non-perturbative tool. Here actions S are varied. It is shown, through a few formal but exact steps, that the functional U is covered by V. The corresponding special subset of trial actions is made explicit.4 pages RevTeX; talk at the 5th Thermal Field Theory Workshop 1998 at Regensburg, Germany, August 10-14; text around (9), (10) improvedHigh Energy Physics - Phenomenologytext