The envelope of the generalized numerical range∗
Mao-Ting Chien · Linear and Multilinear Algebra · 1998
We generalize a well known result on the classical numerical ranges of tridiagonal Toeplitz matrices to the c-numerical ranges of matrices associated to some tree graphs. We also characterize idempotent matrices whose c-numerical range is an elliptical disk by considering the envelope of a family of supporting lines of the c-numerical range.