Higher Rank Numerical Ranges of Normal Matrices
Hwa-Long Gau, Chi-Kwong Li, Yiu-Tung Poon, Nung-Sing Sze · SIAM Journal on Matrix Analysis and Applications · 2011
The higher rank numerical range is closely connected to the construction of quantum error correction code for a noisy quantum channel. It is known that if a normal matrix $A\in M_n$ has eigenvalues $a_1,\dots,a_n$, then its higher rank numerical range $\Lambda_k(A)$ is the intersection of convex polygons with vertices $a_{j_1},\dots,a_{j_{n-k+1}}$, where $1\leq j_1<\dots