Jon Lee Vahab S. Mirrokni Viswanath Nagarajan Maxim Sviridenko Non-monotone submodular maximization under matroid and knapsack constraints. 323-332 2009 STOC http://doi.acm.org/10.1145/1536414.1536459 conf/stoc/2009 db/conf/stoc/stoc2009.html#LeeMNS09