On Quadratic Hurwitz Stability of Interval Polynomials

Quadratic Hurwitz stability is discussed for interval polynomials. Taking cues from the weak version of Kharitonov's theorem, we pose two questions: 1)does Hurwitz stability of interval polynomials imply quadratic Hurwitz stability? and 2) if not, is there any case where this implication holds? By focusing on interval polynomials of degree two, we show two examples, one of which negates the question 1) and the other supports the question 2). Based on these facts, we further discuss several aspects of quadratic Hurwitz stability property in the polynomial coefficient space.