Completion preserves homotopy fibre squares of connected nilpotent spaces

Abstract

We prove that completion at a set of primes preserves homotopy fibre squares of connected nilpotent spaces. As a consequence, we deduce the Hasse fracture square associated to a connected nilpotent space. Along the way, we give a quick proof of the well-known result that if the base of a fibre sequence has the homotopy type of a CW complex, then the total space has the homotopy type of a CW complex iff each fibre does.