The automorphism theorem and additive group actions on the affine plane
  Due to Rentschler, Miyanishi and Kojima, the invariant ring for a ${\bf G}_a$-action on the affine plane over an arbitrary field is generated by one coordinate. In this note, we give a new short proof for this result using the automorphism theorem of Jung and van der Kulk. 
Shigeru Kuroda
math.AC math.AG