Abstract
We prove that twisted ℓ2 -Betti numbers of locally indicable groups are equal to the usual ℓ2 -Betti numbers rescaled by the dimension of the twisting representation; this answers a question of Lück for this class of groups. It also leads to two formulae: given a fibration E with base space B having locally indicable fundamental group, and with a simply-connected fiber F, the first formula bounds ℓ2 -Betti numbers bi(2)(E) of E in terms of ℓ2 -Betti numbers of B and usual Betti numbers of F; the second formula computes bi(2)(E) exactly in terms of the same data, provided that F is a high-dimensional sphere. We also present an inequality between twisted Alexander and Thurston norms for free-by-cyclic groups and 3-manifolds. The technical tools we use come from the theory of generalised agrarian invariants, whose study we initiate in this paper.