The coarse Baum–Connes conjecture for certain relative expanders

Let (1→N_m→G_m→Q_m→1)_m∈N be a sequence of extensions of finite groups such that their coarse disjoint unions have bounded geometry. In this paper, we show that if the coarse disjoint unions of (N_m)_m∈N and (Q_m)_m∈N are coarsely embeddable into Hilbert space, then the coarse Baum–Connes conjecture holds for the coarse disjoint union of (G_m)_m∈N. As an application, the coarse Baum–Connes conjecture holds for the relative expanders constructed by G. Arzhantseva and R. Tessera, and the special box spaces of free groups discovered by T. Delabie and A. Khukhro, which do not coarsely embed into Hilbert space, yet do not contain a weakly embedded expander. This enlarges the class of metric spaces known to satisfy the coarse Baum–Connes conjecture. In particular, it solves an open problem raised by G. Arzhantseva and R. Tessera on the coarse Baum–Connes conjecture for relative expanders.