@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}
}
