Class of exactly soluble models of one-dimensional spinless fermions and   its application to the Tomonaga-Luttinger Hamiltonian with nonlinear   dispersion
  It is shown that for some special values of Hamiltonian parameters the Tomonaga-Luttinger model with nonlinear dispersion is unitary equivalent to the system of noninteracting fermions. For such parameter values the density-density propagator of the Tomonaga-Luttinger Hamiltonian with nonlinear dispersion can be found exactly. In a generic situation the exact solution can be used as a reference point around which a perturbative expansion in orders of certain irrelevant operators may be constructed. 
A.V. Rozhkov