The concept of a transfer function is extended to general dynamic syst
ems described by non-regular linear algebraic and integro-differential
equations. The input consistency and uniqueness of the system output
for the given consistent input are discussed. These two properties are
closely related to the existence of a transfer function which itself
does not need to be unique. Input-output equivalence in the sense that
two systems have the same output for the given consistent input is pr
oved to be necessary and sufficient for the existence of an input-outp
ut isomorphic map between the solution spaces of the systems under zer
o initial conditions.