  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.
