Let $G$ be a connected unipotent algebraic (affine) group defined over a perfect field $k$. Then $G$ is $k$-isomorphic as an algebraic $k$-variety to the affine space $\mathbb A^n\_k$.


Is there any good reference for this result?


It is probably in Demazure, Michel; Gabriel, Pierre Groupes algébriques, but I do not have an access to this book. 



Connected unipotent algebraic groups
It is, for example, explained in paragraph $8$ (Appendix) of the book [Unipotent Algebraic Groups](http://link.springer.com/book/10.1007/BFb0070517), Lecture Notes in Mathematics Volume 414, 1974, by Tatsuji Kambayashi, Masayoshi Miyanishi, and Mitsuhiro Takeuchi, which is accessible online. It is also discussed in the first chapter *On the theory of unipotent algebraic groups over an arbitrary ground field*, with the reference to § $8$, where it is referred to Lazard's original proof.


