(3) Concentrator: P1 >conc= P2 is the concentrator that selects some problem elements of P1, which are put into P2. Therefore the cardinality of P2 is usually smaller than that of P1.

The difference between eliminator and concentrator is that the eliminator only eliminates non-solution elements, i.e., P1-P2 does not contain elements in S, while the concentrator may eliminate solution elements, i.e., P1-P2 may contain elements in S. In other words the concentrator may deliberately throw away less important solution elements in order to concentrate on more important solution elements.