“…However, knowing an input-output representation is sometimes essential for finding a solution to a specific control problem. Typical representatives are various system equivalence and controllability problems [18,19,22,26,30], model matching problems [8,15,16], observer design [9,14,21], observability and identifiability of parameters [1,29], and others. Eventually, any transfer function approach to a control problem deals, in general, with input-output properties of a system.…”