@article{JRAM_1969__0239_0240_150960,
     author = {M. Maruyama and M. Nagata},
     title = {Note on the structure of a ruled surface.},
     journal = {Journal f\"ur die reine und angewandte Mathematik},
     pages = {68--73},
     publisher = {mathdoc},
     volume = {0239_0240},
     year = {1969},
     zbl = {0184.46802},
     url = {http://geodesic.mathdoc.fr/item/JRAM_1969__0239_0240_150960/}
}