Mechanisms for complement-free procurement.

We study procurement auctions when the buyer has complement-free (subadditive) objectives in the budget feasibility model (Singer 2010). For general subadditive functions we give a randomized universally truthful mechanism which is an O(log2 n) approximation, and an O(log3 n) deterministic truthful approximation mechanism; both mechanisms are in the demand oracle model. For cut functions, an interesting case of nonincreasing objectives, we give both randomized and deterministic truthful and budget feasible approximation mechanisms that achieve a constant approximation factor.