The higher rank numerical range is convex
Hugo J. Woerdeman · Linear and Multilinear Algebra · 2007
In this short note we provide the final step in showing that the higher rank numerical range is convex. The previous steps appear in the paper “Geometry of Higher-Rank Numerical Ranges” by Choi, M.-D., Giesinger, M., Holbrook, J.A. and Kribs, D.W.