@article{MZ_1990__205_3_174176,
     author = {Harry A.W.M. Kneppers},
     title = {Two-dimensional formal groups: from isomorphism to isogeny.},
     journal = {Mathematische Zeitschrift},
     pages = {309--314},
     publisher = {mathdoc},
     volume = {205},
     number = {3},
     year = {1990},
     zbl = {0685.14025},
     url = {http://geodesic.mathdoc.fr/item/MZ_1990__205_3_174176/}
}