@article{MAN_1964__153_161131,
     author = {F. OORT},
     title = {Yoneda {Extensions} in {Abelian} {Categories.}},
     journal = {Mathematische Annalen},
     pages = {227--235},
     year = {1964},
     volume = {153},
     zbl = {0126.03401},
     url = {http://geodesic.mathdoc.fr/item/MAN_1964__153_161131/}
}