  The purpose of this article is to develop a new approach to the robust
regulation problem for plants which do not necessarily admit coprime
factorizations. The approach is purely algebraic and allows us dealing with a
very general class of systems in a unique simple framework. We formulate the
famous internal model principle in a form suitable for plants defined by
fractional representations which are not necessarily coprime factorizations. By
using the internal model principle, we are able to give necessary and
sufficient solvability conditions for the robust regulation problem and to
parameterize all robustly regulating controllers.
