@article{MV_1975_27_1_a24,
     author = {A. Torga\v{s}ev},
     title = {Certain notes on the numerical range of an unbounded operator},
     journal = {Matemati\v{c}ki vesnik},
     pages = {189 },
     publisher = {mathdoc},
     volume = {27},
     number = {1},
     year = {1975},
     url = {http://geodesic.mathdoc.fr/item/MV_1975_27_1_a24/}
}