The Shapley-Folkman Theorem and the Range of a Bounded Measure: An Elementary and Unified Treatment

We present proofs, based on the Shapley-Folkman theorem, of the convexity of the range of a strongly continuous, finitely additive measure, as well as that of an atomless, countably additive measure. We also present proofs, based on diagonalization and separation arguments respectively, of the closure of the range of a purely atomic or purely nonatomic countably additive measure. A combination of these results yields Lyapunov's celebrated theorem on the range of a countably additive measure. We also sketch, through a comprehensive bibliography, the pervasive diversity of the applications of the Shapley-Folkman theorem in mathematical economics.

Sellers with misspecified models

Principals often operate on misspecified models of their agents’ preferences. When preferences are such that non-local incentive constraints may bind in the optimum, even slight misspecification of the preferences can lead to large and non-vanishing losses. Instead, we propose a two-step scheme whereby the principal: (1) identifies the model-optimal menu; and (2) modifies prices by offering to share with the agent a fixed proportion of the profit she would receive if an item were sold at the model-optimal price. We show that her loss is bounded and vanishes smoothly as the model converges to the truth. Finally, two-step mechanisms without a sharing rule like (2) will not yield a valid approximation