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Sönmez(1999)による一般的な離散的配分モデルにおけるアロー流の社会選択的公理の含意を検討し以下を示す：もしアローの社会選択の公理(独立性，満場一致性)をみたす社会厚生関数が存在するならば，いかなる選好組に対しても，この社会厚生関数が最上に順位付ける選択肢はコア配分であり，しかもコアは単価である．

貨幣が流通するためには，それが無限に交換され続けることが論理的には前提となるが，それは経済主体の合理性といかに整合するか．ゲーム理論を使って分析してみる．

Adachi(2000)は契約なしの1対1マッチングにおいてタルスキの不動点定理を用いて安定マッチングの存在を証明した．本稿ではこれを契約付きの多対多マッチングに一般化する．

経済分析においては，経済主体の目的として，多くの場合で物質的な選好の充足を想定するが，一方で非物質的な選好が本質的な役割を担う場合がある。非物質的選好の重要性を理解するには，経済状況と選好との相互作用を具体的に究明せねばならない。本稿ではこの究明の一つとして，公共財経済での提携形成において「承認」という非物質的要素への選好がもたらす効果について検討する。ここで「承認」とは主体がその行動の善悪と突出性とにもとづいて評価されることをいう。この経済においては，各主体は提携に参加し他の主体と連体して公共財生産を行うか，あるいは参加せず孤立して生産を行うかを選択する。大きな提携の形成には利があるにもかかわらず，財の消費から得られる（すなわち物質的な）効用のみを想定する場合，安定的に形成が可能な提携のサイズは非常に小さいことが示される。しかしここに承認からの効用を導入すると，これが物質的な効用に対していかに僅少であっても，経済に十分な数の主体がいれば，全体提携が安定的に形成可能であることが示される。これらの結果は，気候変動対策での国際的行動において現実に観測される状況を説明する。

ISER discussion paper, March 2006 Revised July 2006 (Originally entitled "Computational Complexity in the Design of Voting Systems."), This paper discusses an aspect of computational complexity in social choice theory. We consider the problem of designing voting rules, which is formulated in terms of simple games. We prove that it is an NP-complete problem to decide whether a given simple game is stable, or not.

In a simple game, coalitions belonging to a given class are "absolutely powerful" while others have no power. We attempt to make this distinction operational. Toward this end, we propose two axioms on social choice correspondences, Strong Non-Discrimination and Exclusion. Strong Non-Discrimination describes circumstances under which certain coalitions, the losing coalitions, have no influence over social choice. Exclusion requires that there are situations in which certain coalitions, the winning coalitions, can exercise their power. We show that the weak core correspondence is the minimal correspondence satisfying Maskin Monotonicity and Strong Non-Discrimination. We also show that the weak core is the unique correspondence satisfying Nash implementability, Strong Non-Discrimination, and Exclusion.

This paper reexamines the paradoxical aspect of the electronic mail game (Rubinstein, 1989). The electronic mail game is a coordination game with payoff uncertainty. At a Bayesian Nash equilibrium of the game, players cannot achieve the desired coordination of actions even when a high order of mutual knowledge of payoff functions obtains. We want to make explicit the role of knowledge about rationality of players, not only that of payoff functions. For this purpose, we use an extended version of the belief system model developed by Aumann and Brandenburger (1995). We propose a certain way of embedding the electronic mail game in an belief system. And we show that for rational players to coordinate their actions, for any embedding belief systems, it is necessary that the upper bound order of mutual knowledge of payoff functions exceeds the upper bound order of mutual knowledge of rationality. This result implies that under common knowledge of rationality, the coordination never occurs, which is similar to Rubinstein's result. We point out, however, that there exists a class embedding belief systems for which the above condition is also sufficient for the desired coordination.

This paper studies allocation correspondences in the house allocation problems with collective initial endowments. We examine the implications of two axioms, namely 'consistency' and 'unanimity.' Consistency requires the allocation correspondence be invariant under reductions of population. Unanimity requires the allocation correspondence respect unanimity, that is, it assigns to every agent the object that ranks best for him whenever possible. We prove that if an allocation correspondence satisfies these two axioms, then it is a subcorrespondence of the Pareto correspondence. Further, we give a characterization of the Pareto correspondence using a version of 'converse consistency.'

This paper discusses an aspect of computational complexity in social choice theory. We consider the problem of designing voting rules, which is formulated in terms of simple games. We prove that it is an NP-complete problem to decide whether a given simple game is stable, or not.

In the marriage problem (two-sided one-to-one matching problem), it is well-known that the weak core, the strong core and the set of stable matchings are all equivalent. This paper generalizes the above observation considering the G-weak core and the G-strong core. These are core concepts in which blocking power is restricted to the coalitions belonging to the prescribed class of coalitions G. I give a necessary and sufficient condition that G should satisfy for the equivalence of the G-weak core and the G-strong core.

This paper studies the incentive compatibility of solutions to generalized indivisible good allocation problems introduced by Sönmez (1999), which contain the well-known marriage problems (Gale and Shapley, 1962) and the housing markets (Shapley and Scarf, 1974) as special cases. In particular, I consider the vulnerability to manipulation of solutions that are individually rational and Pareto optimal. By the results of Sönmez (1999) and Takamiya (2003), any individually rational and Pareto optimal solution is strategy-proof if and only if the strong core correspondence is essentially single-valued, and the solution is a strong core selection. Given this fact, this paper examines the equilibrium outcomes of the preference revelation games when the strong core correspondence is not necessarily essentially single-valued. I show that for the preference revelation games induced by any solution which is individually rational and Pareto optimal, the set of strict strong Nash equilibrium outcomes coincides with the strong core. This generalizes one of the results by Shin and Suh (1996) obtained in the context of the marriage probelms. Further, I examine the other preceding results proved for the marriage problems (Alcalde, 1996; Shin and Suh, 1996; Sönmez, 1997) to find that none of those results are generalized to the general model.