2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/22709The calculation of the third order susceptibility still is a long standing fundamental problem of particular importance in nonlinear nanooptics: Indeed, cancellation of size-dependent terms coming from uncorrelated excitations is expected, but up to now shown for very simple Hamiltonians only. Using a many-body theory recently developed to handle interacting close-to-bosons, we prove it here for \emph{arbitrary} H. This new formalism actually provides the first clean way to calculate nonlinear susceptibilities, with results different from previous ones.Mesoscale and Nanoscale PhysicsStrongly Correlated Electronstext