Arnab Bhattacharyya Elena Grigorescu Kyomin Jung Sofya Raskhodnikova David P. Woodruff Transitive-closure spanners. 932-941 2009 SODA http://doi.acm.org/10.1145/1496770.1496871 conf/soda/2009 db/conf/soda/soda2009.html#BhattacharyyaGJRW09