Congruence numerical ranges and their radii
Tian-Gang Lei · Linear and Multilinear Algebra · 1998
The earliest congruence numerical range was introduced by Thompson in 1980. Since then, congruence numerical ranges and their radii have been studied by some authors. The purpose of this article is to discuss several aspects of this subject. We survey some known results and propose some problems for furthur study. A few new results are also presented.