Connected unipotent algebraic groups 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. 
 A: It is, for example, explained in paragraph $8$ (Appendix) of the book Unipotent Algebraic Groups, 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.
A: To add a little perspective to the question and Dietrich's answer, there has been a natural tendency in the literature to go beyond perfect ground fields and traditional algebraic geometry toward formulations for arbitrary fields in scheme language (SGA3, Demazure-Gabriel, Conrad-Gabber-Prasad).   Another tendency is to discuss connected solvable groups rather than just unipotent ones.    But there are some reasonable textbook treatments besides the Lecture Notes 414 volume.  
Much of the work on generalizations to solvable algebraic groups over arbitrary fields was done by M. Rosenlicht in the mid-1950s.   This is mostly presented with more "modern" proofs in the expanded second edition of Borel's Linear Algebraic Groups (GTM 126, Springer, 1991), $\S15$.  For the structure of a connected unipotent group $G$ over a perfect field $k$ of any characteristic, the key point here is that $G$ "splits" over $k$ (15.5).   But then Borel just quotes Rosenlicht without proof in 15.13(a) on the explicit structure of any $k$-split connected solvable group $G$ with $k$ arbitrary: $G$ is $k$-isomorphic as a variety to a product of copies of the additive and multiplicative groups (all being additive groups in the unipotent case).  
Springer's Linear Algebraic Groups (2nd ed., Birkhauser, 1998) has a somewhat more explicit treatment along these lines (Chapter 14): see in particular 14.2.6-14.2.7 for unipotent groups over an arbitrary field $F$.  Again the group is required to be $F$-split, which is satisfied when $F$ is perfect.   (While the ideas in the unipotent case go back to Lazard, the tendency in both of these books is to refer only to Rosenlicht's work on solvable groups.)  
