“…Regular linear systems were introduced in [40], and most well-posed systems encountered in applications are regular. There is now a rich literature using this concept, ranging from partial differential equations to engineering applications, see for instance Avalos et al [2], Hinrichsen and Pritchard [8], Jacob and Zwart [10], Logemann et al [14], Logemann and Ryan [15], Logemann, Ryan and Townley [16,17], Logemann and Townley [18,19], Morris [20], Rebarber [25,26], Rebarber and Townley [27], Staffans [30,32,33,34], Weiss and Curtain [44], Weiss and Häfele [46] and Weiss 2 [45]. The concept of a well-posed linear system is closely related to that of a scattering semigroup, as studied in Lax and Phillips [12,13] -we shall say more about this connection later.…”