A note on spectral conditions for positive realness of single-input-single-output systems with strictly proper transfer functions

Necessary and sufficient conditions for strict positive realness and positive realness of strictly proper functions are derived. The conditions are expressed in terms of eigenvalues of the state matrices representation of the system. Previous results rendered conditions which were significantly more complex than those for proper (but not strictly proper) functions. The present conditions for strictly proper functions are simpler than the ones for proper functions, which is consistent with intuition in this case. Illustrative numerical examples are provided.