The subject of this book is the hierarchies of integrable equations
connected with the one-component and multi component loop groups. There
are many publications on this subject, and it is rather well defined.
Thus, the author would like t.o explain why he has taken the risk of
revisiting the subject. The Sato Grassmannian approach, and other
approaches standard in this context, reveal deep mathematical structures
in the base of the integrable hi- erarchies. These approaches
concentrate mostly on the algebraic picture, and they use a language
suitable for applications to quantum field theory. Another well-known
approach, the a-dressing method, developed by S. V. Manakov and V.E.
Zakharov, is oriented mostly to particular systems and ex- act classes
of their solutions. There is more emphasis on analytic properties, and
the technique is connected with standard complex analysis. The language
of the a-dressing method is suitable for applications to integrable
nonlinear PDEs, integrable nonlinear discrete equations, and, as
recently discovered, for t.he applications of integrable systems to
continuous and discret.e geometry. The primary motivation of the author
was to formalize the approach to int.e- grable hierarchies that was
developed in the context of the a-dressing method, preserving the
analytic struetures characteristic for this method, but omitting the
peculiarit.ies of the construetive scheme. And it was desirable to find
a start.-