Brown's representability theorem
theorem in homotopy theory that a set-valued contravariant functor on the homotopy category of connected pointed CW-complexes is representable iff it maps wedge sums to product and homotopy pushouts to weak pullbacks
theorem in homotopy theory that a set-valued contravariant functor on the homotopy category of connected pointed CW-complexes is representable iff it maps wedge sums to product and homotopy pushouts to weak pullbacks