@article{AM2_1989__38_1_137330,
     author = {Wing-Sum Cheung},
     title = {On {Opial-type} inequalities in two varaiables.},
     journal = {Aequationes mathematicae},
     pages = {236--244},
     publisher = {mathdoc},
     volume = {38},
     number = {1},
     year = {1989},
     zbl = {0684.26006},
     url = {http://geodesic.mathdoc.fr/item/AM2_1989__38_1_137330/}
}