Dimension of Elementary Amenable Groups

Martin R. Bridson and Peter H. Kropholler

To appear in Crelle's Journal. Submitted 2 Sept 2012, accepted 29 Nov 2012.

This paper has three parts. It is conjectured that for every
elementary amenable group G and every non-zero commutative ring k,
the homological dimension hd_k(G) is equal to the Hirsch length h(G)
whenever G has no k-torsion. In Part I this conjecture is proved for
several classes, including the abelian-by-polycyclic groups. In Part II it
is shown that the elementary amenable groups of homological dimension
one are colimits of systems of groups of cohomological dimension one.
In Part III the deep problem of calculating the cohomological dimension
of elementary amenable groups is tackled with particular emphasis on
the nilpotent-by-polycyclic case, where a complete answer is obtained
over Q for countable groups.

31 pages, 2 figures.