Topological Rigidity for FJ by the Infinite Cyclic Group

TL;DR

This paper proves topological rigidity results for groups satisfying the Farrell-Jones conjecture, showing that certain group extensions preserve these properties and extend the class of groups satisfying the Novikov conjecture.

Contribution

It demonstrates that the Farrell-Jones conjecture is preserved under group extensions by $bZ$, broadening the scope of groups for which the Novikov and Borel conjectures hold.

Findings

Rigidity results for groups satisfying the Farrell-Jones conjecture.

Closure of the Farrell-Jones conjecture under extensions by $bZ$.

Extension of the class of groups satisfying the Novikov conjecture.

Abstract

We call a group FJ if it satisfies the $K$- and $L$-theoretic Farrell-Jones conjecture with coefficients in $\mathbb Z$. We show that if $G$ is FJ, then the simple Borel conjecture (in dimensions $\ge 5$) holds for every group of the form $G\rtimes\mathbb Z$. If in addition $Wh(G\times \mathbb Z)=0$, which is true for all known torsion free FJ groups, then the bordism Borel conjecture (in dimensions $n\ge 5$) holds for $G\rtimes\mathbb Z$. One of the key ingredients in proving these rigidity results is another main result, which says that if a torsion free group $G$ satisfies the $L$-theoretic Farrell-Jones conjecture with coefficients in $\mathbb Z$, then any semi-direct product $G\rtimes\mathbb Z$ also satisfies the $L$-theoretic Farrell-Jones conjecture with coefficients in $\mathbb Z$. Our result is indeed more general and implies the $L$-theoretic Farrell-Jones conjecture with coefficients in additive categories is closed under extensions of torsion free groups. This enables us to extend the class of groups which satisfy the Novikov conjecture.