Abstract

Let A,B∈M n . The numerical range of a linear pencil Aλ−B is defined by W(Aλ−B)={t∈ C:tw *Aw−w *Bw=0 for some non-zero w∈ C n}. The numerical range of a linear pencil of n-by- n matrices coincides with the union of the numerical ranges of linear pencils of 2-by-2 matrices. We emphasize on computation of the numerical range of linear pencils of 2-by-2 matrices. Let A,B∈M 2 with constant diagonals. We show that the boundary of W(Aλ−B) lies on a rational curve of degree at most 4. By using Lagrange's multiplier and implicit function, we describe the boundary of W(Aλ−B) for general 2-by-2 matices.

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