“…The functor CW A turns out to be augmented and idempotent and is characterized by the facts that map * (A, CW A X) is weakly homotopy equivalent to map * (A, X) and every map A ā X factors through CW A X in a unique way, up to homotopy. These ideas have made a great impact both in Homotopy Theory ( [CDI06], [CCS07], [RS08]) and outside of it, because they can be generalized in a natural way to any framework where there is a notion of limit (see examples in [DGI01], [BKI08] or [Kie08]). A line of research of great relevance in the last years, and very related to our work, is cellular approximation in the category of groups ([RS01], [Flo07], [DGS07], [DGS08]).…”