Exceptional solutions to the eight-vertex model and integrability of anisotropic extensions of massive fermionic models

We consider several anisotropic extensions of the Belavin model, and show that integrability holds also for the massive case for some specific relations between the coupling constants. This is done by relating the S-matrix factorization property to the exceptional solutions of the eight-vertex model. The relation of exceptional solutions to the XXZ and six-vertex models is also shown. (C) 2018 The Author(s). Published by Elsevier B.V.