Ken Brown's lemma

In mathematics, specifically in category theory, Ken Brown's lemma gives a sufficient condition for a functor on a category of fibrant objects to preserve weak equivalences; the sufficient condition is that acyclic fibrations go to weak equivalences. Passed to the dual, the co version of the lemma also holds. The lemma or, more precisely, a result of which the lemma is a corollary, was introduced by Kenneth Brown.