  We consider input-output systems in descriptor form and ask when such systems
can be rendered SPR (strictly positive real) via output feedback. Time and
frequency domain conditions are given to determine when and how this is
possible. In addition, a synthesis procedure for controller design is also
derived. Together, the results provide a complete answer to when a linear
descriptor system can be made SPR via output feedback, and give a recipe for
design of the feedback controller when it exists. Simple examples are given to
illustrate our results and to demonstrate their efficacy.
