Composition of points and Mordell-Weil problem for cubic surfaces
  Let $V$ be a plane smooth cubic curve over a finitely generated field $k.$ The Mordell-Weil theorem for $V$ states that there is a finite subset $P\subset V(k)$ such that the whole $V(k)$ can be obtained from $P$ by drawing secants and tangents through pairs of previously constructed points and consecutively adding their new intersection points with $V.$ Equivalently, the group of birational transformations of $V$ generated by reflections with respect to $k$-points is finitely generated. In this paper, elaborating an idea from [M3], we establish a Mordell-Weil type finite generation result for some birationally trivial cubic surfaces $W$. To the contrary, we prove that the birational automorphism group generated by reflections cannot be finitely generated if $W(k)$ is infinite. 
D. Kanevsky, Yu. Manin