@article {ekim2004approximation, title = {Approximation preserving reductions for set covering, vertex covering and independent set hierarchies under differential approximationa}, journal = {International Journal of Computer Mathematics}, volume = {81}, number = {5}, year = {2004}, pages = {569{\textendash}582}, publisher = {Taylor \& Francis Group}, author = {Ekim, Tinaz and Paschos, Vangelis Th} }