Reuven Bar-Yehuda One for the Price of Two: A Unified Approach for Approximating Covering Problems. 49-62 http://link.springer.de/link/service/series/0558/bibs/1444/14440049.htm 1998 conf/approx/1998 APPROX db/conf/approx/approx1998.html#Bar-Yehuda98