This paper considers the problem of identifying regions in the complex-plane, such that polynomials having roots in those regions, have their phase bounded by that of a few extreme polynomials. We present sufficient and testable conditions for different regions satisfying this property. Applications of the results to the robust SPR analysis and synthesis problems are illustrated.
Pérez, F. and C.T. Abdallah. "Phase-Convex Arcs in Root Space and Their Application to Robust SPR Problem." (1994). http://digitalrepository.unm.edu/ece_rpts/11