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In this paper, we consider robust stability of interval polynomials of which stability region is the special left sector. The argument of the boundary of the special left sector is expressible as an irrational number multiplied by the circle ratio. We show that a family of interval polynomials is robustly stable if and only if a small set of vertex polynomials are robustly stable. This new result comes from the construction algorithm of the value set and the zero exclusion principle.