@article{BSMF_1990__118_2_193_0,
     author = {Letzter, Gail and Makar-Limanov, Leonid},
     title = {Rings of differential operators over rational affine curves},
     journal = {Bulletin de la Soci\'et\'e Math\'ematique de France},
     pages = {193--209},
     publisher = {Soci\'et\'e math\'ematique de France},
     volume = {118},
     number = {2},
     year = {1990},
     doi = {10.24033/bsmf.2143},
     mrnumber = {91m:16023},
     zbl = {0722.16013},
     language = {en},
     url = {http://archive.numdam.org/articles/10.24033/bsmf.2143/}
}