U. Feige, A threshold of ln n for approximating set cover, Journal of the ACM, vol.45, issue.4, pp.634-652, 1998.
DOI : 10.1145/285055.285059

S. Yuan, S. Varma, and J. P. Jue, Minimum-color path problems for reliability in mesh networks, In: INFOCOM. pp, pp.2658-2669, 2005.