@article{10_4064_sm197_3_5,
     author = {Vladim{\'\i}r M\"uller},
     title = {The joint essential numerical range, compact perturbations,
 and the {Olsen} problem},
     journal = {Studia Mathematica},
     pages = {275--290},
     publisher = {mathdoc},
     volume = {197},
     number = {3},
     year = {2010},
     doi = {10.4064/sm197-3-5},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/sm197-3-5/}
}