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@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/} }
A. Torgašev. Certain notes on the numerical range of an unbounded operator. Matematički vesnik, Tome 27 (1975) no. 1, p. 189 . http://geodesic.mathdoc.fr/item/MV_1975_27_1_a24/