Dimension of elementary amenable groups

The paper has three parts. It is conjectured that for every elementary amenable group G and every non-zero commutative ring k, the homological dimension of G over k is equal to the Hirsch length of G whenever G has no k-torsion. In Part I this is proved for several classes, including the abelian-by-polycyclic groups. In Part II it is shown that elementary amenable groups of homological dimension one are filtered colimits of systems of groups of cohomological dimension one. Part III is devoted to the deeper study of cohomological dimension with particular emphasis on the nilpotent-by-polycyclic case.