On the minimal-order of dynamic output regulators

The minimal-order problem of the dynamic output regulators of linear time-invariant systems is considered, and a lower bound for the minimal-order is given. Unlike in previous works, the lower bound presented is a simple function of the system dimension and ranks of the input and output matrices, computation involves neither a rank minimization problem nor a semi-definite programming problem. An example is given to illustrate the non-conservativeness of the lower bound