@article{CC_2007_36_3_a8,
     author = {Cegielski, Andrzej},
     title = {A generalization of the {Opial's} theorem},
     journal = {Control and Cybernetics},
     pages = {601--610},
     publisher = {mathdoc},
     volume = {36},
     number = {3},
     year = {2007},
     zbl = {1190.47067},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/CC_2007_36_3_a8/}
}