  We extend a recently proposed non-local version of Coleman's equivalence
between the Thirring and sine-Gordon models to the case in which the original
fermion fields interact with fixed impurities. We explain how our results can
be used in the context of one-dimensional strongly correlated systems (the so
called Tomonaga-Luttinger model) to study the dependence of the charge-density
oscillations on the range of the fermionic interactions.
