Piotr Berman Bhaskar DasGupta Marek Karpinski Approximating Transitive Reductions for Directed Networks. 74-85 2009 WADS http://dx.doi.org/10.1007/978-3-642-03367-4_7 conf/wads/2009 db/conf/wads/wads2009.html#BermanDK09