Actions of nilpotent groups on complex algebraic varieties

We study nilpotent groups acting faithfully on complex algebraic varieties. We use a method of base change. For finite p-groups, we go from $k$, a number field, to a finite field in order to use counting lemmas. We show that a finite $p$-group of polynomial automorphisms of $k^d$ is isomorphic to a subgroup of GL${}_d(k)$. For infinite groups, we go from $\mathbb{C}$ to ${\mathbb{Z}}_p$ and use p-adic analytic tools and the theory of p-adic Lie groups. We show that a finitely generated nilpotent group $H$ acting faithfully on a complex quasiprojective variety $X$ of dimension $d$ can be embedded into a $p$-adic Lie group acting faithfully and analytically on ${\mathbb{Z}}_p^d$; we deduce that $d$ is larger than the virtual derived length of $H$.