2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/57797We prove ferromagnetic ordering of energy levels for XXX Heisenberg chains of any spin and XXZ spin chains with all spins equal to 1/2. Ferromagnetic ordering means that the minimum energies in the invariant subspaces of fixed total spin are monotone decreasing as a function of the total spin. This result provides a ferromagnetic analogue of the well-known theorem by Lieb and Mattis about ordering of energy levels in antiferromagnetic and ferrimagnetic systems on bipartite graphs.4 pages, final version, some references addedMathematical PhysicsStatistical Mechanics82B10; 82D40text