On the numerical range of some block matrices with scalar diagonal blocks
Titas Geryba, Ilya M. Spitkovsky · Linear and Multilinear Algebra · 2020
Several new verifiable conditions are established for matrices of the form αIn−kCDβIk to have the numerical range equal the convex hull of at most k ellipses. For k = 2, these conditions are also necessary, provided that the ellipses are co-centred.