2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/151575We prove rigorous lower and upper bounds for the mass gap of the ferromagnetic spin 1/2 XXZ chain. The two bounds coincide asymptotically in the Ising limit $Δ\to\infty$. Near the isotropic point, $Δ=1$, the estimates are good enough to determine the critical behaviour of the mass gap unambiguously. The derivation does not rely on exact solutions.18 pages, LaTeX, 1 figure as uuencoded compressed Postscript fileCondensed MatterFunctional AnalysisHigh Energy Physics - Theorytext