2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/18509We discuss some of the difficulties that have been mentioned in the literature in connection with the Bethe ansatz for the six-vertex model and XXZ chain, and for the eight-vertex model. In particular we discuss the ``beyond the equator'', infinite momenta and exact complete string problems. We show how they can be overcome and conclude that the coordinate Bethe ansatz does indeed give a complete set of states, as expected.40 pages, no figuresStatistical MechanicsHigh Energy Physics - TheoryMathematical Physicstext