In the class NPO minus APX we have seen examples which are not approximable at all, like minimum TSP, maximum independent set, maximum clique and minimum graph coloring.
In the class APX minus PTAS we have seen examples like minimum bin packing, maximum satisfability, minimum vertex cover, maximum cut, which are approximable within a given performance ratio but cannot be approximate for any performance ratio.
We have then seen an example which is in PTAS but not in PO, this problem is minimum partition.
Finally we have seen that the class PO contains hundreds of problems, but I shown you only one simple example.
