Moshe Babaioff Michal Feldman Noam Nisan Free-Riding and Free-Labor in Combinatorial Agency. 109-121 2009 SAGT http://dx.doi.org/10.1007/978-3-642-04645-2_11 conf/sagt/2009 db/conf/sagt/sagt2009.html#BabaioffFN09