Classification of units in H/sub /spl infin// and an alternative proof Kharitonov's theorem

The authors present an alternative proof of Kharitonov's theorem, using the property of a ratio of odd and even parts of a Hurwitz polynomial and the Nyquist stability criterion. The ratio of Kharitonov's polynomials in the classification of units in H/sub /spl infin//, along with its relation to the problem of simultaneous stabilization of one parameter family of plants is discussed. A new theorem on the existence of a Hurwitz polynomial such that its ratio with a Hurwitz interval polynomial family with either the same even or odd part, is a strictly positive real (SPR) function is proved. It is also proved that if the ratio of a polynomial /spl beta/(s) with four Kharitonov's polynomials is an SPR function, then the ratio of /spl beta/(s) with the interval family is an SPR function.