Keywords: divisible rigid group, group of automorphisms, group of normal automorphisms.
an Abelian group and is torsion-free as a (right) Z[G/G i ]-module.A rigid group is one that is m-rigid for some m. The specified series is determined by a given rigid group uniquely; so it consists of characteristic subgroups and is called a rigid series; the solvability length of a group is exactly m.