| Summary: | 碩士 === 國立中央大學 === 數學研究所 === 94 === In this thesis, we study some properties of reducible companion matrices. We first prove that if a nonunitary reducible companion matrix A is unitarily equivalent to the direct sum A_1oplus A_2 on mathbb{C}^koplusmathbb{C}^{n-k} with sigma(A_1)={aom_n^{j_1},cdots,aom_n^{j_k}} and sigma(A_2)={(1/ ar{a})om_n^{j_{k+1}},cdots,(1/ ar{a})om_n^{j_n}},where |a|>1 and om_n denotes the nth primitive root of 1,then rank(I_k-A^{*}_1A_1)=rank(I_{n-k}-A^{*}_2A_2)=1. We denote mathcal{S}^{al}_nequiv{Ain M_n:rank(I_n-A^{*}A)=1 and |la|=al,forall: lainsigma(A)}, thus A_1 is in mathcal{S}^{al}_k and A_2 is in mathcal{S}^{1/al}_{n-k}. Next, we prove that every mathcal{S}^{al}_n-matrix is irreducible, cyclic, and the boundary of its numerical range is a differentiable curve.
Furthermore, we show that the following statements are equivalent:
(a) W(A)=W(A_1); (b)W(J_{n-1})subseteq W(A_1); (c)W(A_2)subseteq W(J_{n-1}).
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