@article{CM_1969__21_3_253_0,
     author = {Franklin, S. P.},
     title = {Natural covers},
     journal = {Compositio Mathematica},
     pages = {253--261},
     publisher = {Wolters-Noordhoff Publishing},
     volume = {21},
     number = {3},
     year = {1969},
     mrnumber = {250252},
     zbl = {0188.27901},
     language = {en},
     url = {http://www.numdam.org/item/CM_1969__21_3_253_0/}
}