On families of nilpotent subgroups and associated coset posets

We study some properties of the coset poset associated with the family of subgroups of class \(\le 2\) of a nilpotent group of class \(\le 3\). We prove that under certain assumptions on the group the coset poset is simply-connected if and only if the group is 2-Engel, and 2-connected if and only if the group is nilpotent of class 2 or less. We determine the homotopy type of the coset poset for the group of \(4\times 4\) upper unitriangular matrices over \(\mathbb {F}_p\), and for the Burnside groups of exponent 3.