On Fattorini's paper

In 1968 Fattorini published in SIAM Journal on Control his well-known paper with the title Boundary Control Systems. In this paper he sets up the abstract theory for systems described by partial differential equations with a control at the boundary. The theory as presented is complete, meaning that it could be copied into textbooks and articles without major alternations. In his paper and in these textbooks one finds examples illustrating the use of this abstract concept. However, as we will show, these examples are much older, and are more or less standard within the theory of partial differential equations with an inhomogeneous boundary condition.