  We prove Freudenburg's Freeness Conjecture:
  Let B be the polynomial ring in three variables over a field of
characteristic zero, let D : B --> B be a nonzero locally nilpotent derivation,
and let A = ker(D). Then B is a free A-module, and there exists a basis
$(e_i)_{i \in \mathbb{N}}$ of B such that deg$_D(e_i) = i$ for all $i \in
\mathbb{N}$.
