  There are four variants of passive, linear time-invariant systems, described
by rational functions: Continuous or Discrete time, Positive or Bounded real.
By introducing a quadratic matrix inequality formulation, we present a unifying
framework for state-space characterization (a.k.a. Kalman-Yakubovich-Popov
Lemma) of the above four classes of passive systems.
  These four families are matrix-convex as rational functions, and a slightly
weaker version holds for the corresponding balanced, state-space realization
arrays.
