We construct a topological invariant of the renormalization group
trajectories of a large class of 2D quantum integrable models, described by the
thermodynamic Bethe ansatz approach. A geometrical description of this
invariant in terms of triangulations of three-dimensional manifolds is proposed
and associated dilogarithm identities are proven.Comment: 12 pages, 6 figures. Presented at the Euroconference on New
Symmetries in Statistical Mech. and Cond. Mat. Physics, Torino, July 20-
August 1 1998. typos correcte