On pointed Hopf algebras over nilpotent groups

We classify finite-dimensional Nichols algebras over finite nilpotent groups of odd order in group-theoretical terms. The main step is to show that the conjugacy classes of such finite groups are either abelian or of type C; this property also holds for finite conjugacy classes of finitely generated nilpotent groups whose torsion has odd order. To extend our approach to the setting of finite GK-dimension, we propose a new Conjecture on racks of type C. We also prove that the bosonization of a Nichols algebra of a Yetter-Drinfeld module over a group whose support is an infinite conjugacy class has infinite GK-dimension. We apply this to the study of the finite GK-dimensional pointed Hopf algebras over finitely generated torsion-free nilpotent groups.