2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/18681The singular part of the finite-size free energy density $f_s$ of the O($n$) symmetric $ϕ^4$ field theory is calculated for confined geometries of linear size L with periodic boundary conditions in the large-N limit and with Dirichlet boundary conditions in one-loop order. We find that both a sharp cutoff and a subleading long-range interaction cause a non-universal L dependence of $f_s$ near $T_c$. For film geometry this implies a non-universal critical Casimir force with an algebraic L dependence that dominates the exponential finite-size scaling behavior above $T_c$ for both periodic and Dirichlet boundary conditions.4 pages, no figureStatistical Mechanicstext