The tensor product of the module of a linear system with the quotient
field of the ring of linear differential operators is a vector space w
here, even in the time-varying case, a (formal) Laplace transform and
the transfer matrix are most naturally defined. Several classic proble
ms are examined in this algebraic setting: the relationship between le
ft (right) coprime matrix decomposition and controllability (observabi
lity), the state-variable canonical realization, the transfer algebra
with respect to parallel and series connections, the input-output inve
rsion, and model matching.