2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/27001The stability of the insulating regime of the Hubbard model on a $d$-dimensional lattice, which is characterized by an exponential decay of the Green's functions, is investigated in terms of a cluster expansion. This expansion for the Green's function is organized in terms of connected clustered transfer matrices. An upper bound for the expansion terms is derived for the hopping rate ${\bar t}$ depending on the coupling constant $U$ as ${\bar t}<U/4d$. This implies an upper bound for the decay length of the Green's function.18 pages, Plain-TeX, TKM 08/93Condensed Mattertext