@article{PIM_1964_N_S_04_18_a20,
     author = {Ivan Vidav},
     title = {On {Idempotent} {Operators} {In} {A} {Hilbert} {Space}},
     journal = {Publications de l'Institut Math\'ematique},
     pages = {157 },
     year = {1964},
     volume = {_N_S_04},
     number = {18},
     url = {http://geodesic.mathdoc.fr/item/PIM_1964_N_S_04_18_a20/}
}