@article{10_4064_sm174_2_4,
     author = {Chi-Kwong Li and Nung-Sing Sze},
     title = {Product of operators and  numerical range preserving maps},
     journal = {Studia Mathematica},
     pages = {169--182},
     publisher = {mathdoc},
     volume = {174},
     number = {2},
     year = {2006},
     doi = {10.4064/sm174-2-4},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/sm174-2-4/}
}