@article{10_4064_sm194_1_6,
     author = {Chi-Kwong Li and Yiu-Tung Poon},
     title = {The joint essential numerical range of operators:
convexity and related results},
     journal = {Studia Mathematica},
     pages = {91--104},
     year = {2009},
     volume = {194},
     number = {1},
     doi = {10.4064/sm194-1-6},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/sm194-1-6/}
}