@article{ASENS_1991_4_24_3_339_0,
     author = {Koll\'ar, J\'anos},
     title = {Extremal rays on smooth threefolds},
     journal = {Annales scientifiques de l'\'Ecole Normale Sup\'erieure},
     pages = {339--361},
     publisher = {Elsevier},
     volume = {Ser. 4, 24},
     number = {3},
     year = {1991},
     doi = {10.24033/asens.1630},
     mrnumber = {92f:14034},
     zbl = {0753.14036},
     language = {en},
     url = {http://www.numdam.org/articles/10.24033/asens.1630/}
}