@article{CMUC_1968__9_4_a2,
     author = {Istr\u{a}\c{t}escu, V.},
     title = {Some remarks on the spectra and numerical range},
     journal = {Commentationes Mathematicae Universitatis Carolinae},
     pages = {527--531},
     publisher = {mathdoc},
     volume = {09},
     number = {4},
     year = {1968},
     mrnumber = {0246147},
     zbl = {0174.44304},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/CMUC_1968__9_4_a2/}
}