ELA articles which reference this paper are

Louis Deaett, Ramiro H. Lafuente-Rodriguez, Juan Marin Jr., Erin Haller Martin, Linda J. Patton, Kathryn Rasmussen and Rebekah B. Johnson Yates
Trace conditions for symmetry of the numerical range.
Electron. J. Linear Algebra 26 (2013) pp. 591-603.