  A large family of linear, usually overdetermined, systems of partial
differential equations that admit a multiplication of solutions, i.e, a
bi-linear and commutative mapping on the solution space, is studied. This
family of PDE's contains the Cauchy-Riemann equations and the cofactor pair
systems, included as special cases. The multiplication provides a method for
generating, in a pure algebraic way, large classes of non-trivial solutions
that can be constructed by forming convergent power series of trivial
solutions.
