“…The key ingredients of the HTE method are the Quantum Transfer Matrix (QTM) [134,135,136,137,138,139,140,141,142,143] and a functional relation called the T -system [144,145,146], from which one derives nonlinear integral equations which can be solved in an exact perturbative fashion. To date this approach has been applied to the Heisenberg model [129], the osp(1|2s) model [130], the su(N ) Uimin-Lai-Sutherland model [131], the higher spin Heisenberg model [147], the su(N ) Perk-Schultz model [148] and the su(m|n) Perk-Schultz model [149]. In particular, in this way, i.e., via the HTE expansion, it has been demonstrated that the integrable su(4) ladder model can be used to study the thermodynamics and magnetic properties of the strongly coupled ladder compounds [8].…”