@article{1178207479, author = {Sekiguchi, Tsutomu and Suwa, Noriyuki}, title = {A note on extensions of algebraic and formal groups, IV. Kummer-Artin-Schreier-Witt theory of degree $p^2$}, journal = {Tohoku Math. J. (2)}, volume = {53}, number = {4}, year = {2001}, pages = { 203-240}, language = {en}, url = {http://dml.mathdoc.fr/item/1178207479} }
Sekiguchi, Tsutomu; Suwa, Noriyuki. A note on extensions of algebraic and formal groups, IV. Kummer-Artin-Schreier-Witt theory of degree $p^2$. Tohoku Math. J. (2), Tome 53 (2001) no. 4, pp. 203-240. http://gdmltest.u-ga.fr/item/1178207479/