This paper studies the problem of assigning a set of indivisible objects to a set of agents when monetary transfers are not allowed and agents reveal only ordinal preferences, but random assignments are possible. We offer two characterizations of the probabilistic serial mechanism, which assigns lotteries over objects. We show that it is the only mechanism that satisfies non-wastefulness and ordinal fairness, and the only mechanism that satisfies sd-efficiency, sd-envy-freeness, and weak invariance or weak truncation robustness (where "sd" stands for first-order stochastic dominance). 6 We thank an anonymous referee for suggesting that we weaken HH's definition of truncation robustness to the current definition (Definition 3). Upon showing that this new definition is strong enough to characterize PS, we observed that the proof also extends to the general case where the null object may not exist. This motivated us to obtain our second characterization result using the current definition of weak invariance (Definition 2), which is the counterpart of Definition 3 in environments without the null object. 7 Also, our proof immediately implies that we can weaken sd-efficiency in Theorem 2 and Corollary 2 as in HH and BH. 8 A more recent paper by Heo and Yılmaz (2012) extends the results of BH to the case with weak preferences for Katta and Sethuraman's (2006) extended probabilistic serial correspondence. 9 This axiom was previously introduced by Heo (2013) as one of her auxiliary axioms. She referred to it as "limited invariance."
This paper investigates mixed strategy equilibria in a capacity-constrained price competition among three firms. It is shown that the equilibria in an asymmetric oligopoly are substantially different from those in a duopoly and symmetric oligopoly. In an asymmetric triopoly, it is possible that (i) a continuum of equilibria exists and that (ii) the lowest price of the smallest firm is higher than that of the others and the smallest firm earns more than the max-min profit in undominated strategies. In particular, the second finding sheds light on a new pricing incentive in Bertrand competitions. As an application, the equilibrium characterizations give rise to a new class of merger paradoxes.
This paper studies stable and (one-sided) strategy-proof matching rules in manyto-one matching markets with contracts. First, the number of such rules is shown to be at most one. Second, the doctor-optimal stable rule, whenever it exists, is shown to be the unique candidate for a stable and strategy-proof rule. Third, a stable and strategyproof rule, when exists, is shown to be second-best optimal for doctor welfare, in the sense that no individually-rational and strategy-proof rule can dominate it. This last result is further generalized to non-wasteful and strategy-proof rules. Notably, all those results are established without any substitutes conditions on hospitals' choice functions, and hence, the proofs do not rely on the "rural hospital" theorem. We also show by example that the outcomes of a stable and strategy-proof rule do not always coincide with those of the cumulative offer process; hence, the above results hold not because the cumulative offer process is the only candidate for stable and strategyproof rules.
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