@article{MAN_1991__291_1_164883,
     author = {Qi Zhang},
     title = {Extremal rays on higher dimensional projective varieties.},
     journal = {Mathematische Annalen},
     pages = {497--504},
     publisher = {mathdoc},
     volume = {291},
     number = {1},
     year = {1991},
     zbl = {0762.14009},
     url = {http://geodesic.mathdoc.fr/item/MAN_1991__291_1_164883/}
}