@article{MM2_1992__75_4_155721,
     author = {A. Ragusa and G. Paxia and G. Raciti},
     title = {On the {L\"uroth} {Semigroup} of curves lying on a smooth quadric.},
     journal = {Manuscripta mathematica},
     pages = {225--246},
     publisher = {mathdoc},
     volume = {75},
     number = {4},
     year = {1992},
     zbl = {0769.14012},
     url = {http://geodesic.mathdoc.fr/item/MM2_1992__75_4_155721/}
}