Abstract
In object reallocation problems, if preferences are strict but otherwise unrestricted, TTC is the leading rule: It is the only rule satisfying efficiency, the endowment lower bound, and strategy-proofness. However, on the subdomain of single-peaked preferences, Bade (J Econ Theory 180:81–99, 2019) defines a new rule, the “crawler”, which also satisfies these properties, and in fact enjoys a stronger strategic property. We identify additional interesting properties that the crawler satisfies, and provide two characterizations of this rule. The first characterization is based on the endowment lower bound and two invariance properties, “adjacent-endowment-swapping invariance” and “separability”. The second characterization is based on the endowment lower bound, strategy-proofness, adjacent-endowment-swapping invariance, and another invariance property, “non-bossiness”.
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Notes
If agents are visited from right to left and each of them in turn is asked the symmetric question, the resulting allocation remains the same (Tamura and Hosseini 2021). Moreover, the crawler always selects the same allocation regardless of the choice of the order on the object set as long as single-peakedness holds.
Throughout, “\(A \Longrightarrow B\)” means that “A implies B”, and “\(A \iff B\)” means that “A if and only if B”.
The agent whose endowment is the leftmost has no one to his left, and the agent whose endowment is the rightmost has no one to his right.
TTC also satisfies these properties.
Refer to Li (2017) for the definition of obvious strategy-proofness.
If there are at most three agents, TTC is also obviously strategy-proof.
For the agent who currently owns the leftmost object, we ask ‘among the available objects, do you most prefer the object you currently own?’
Another common name is “individual rationality”.
Two objects \(o,o' \in {\mathbb {O}}\) are adjacent if there is no \(o^{''} \in {\mathbb {O}}\) such that \(o \prec o^{''} \prec o'\).
A special case of group strategy-proofness is when a group contains only two agents, “pairwise strategy-proofness”. Alva (2017) also shows that if a domain satisfies the properties, pairwise strategy-proofness is equivalent to group strategy-proofness.
For efficient rules, there would of course be at least one other agent who is made worse off.
In reality, more complicated endowment swapping may occur. However, it is difficult for rules to be robust to such manipulation.
Another property concerning endowment swap by two agents is that no pair of agents ever be made better off by such a move, “endowment-swapping-proofness”. It is known that TTC satisfies this property (Fujinaka and Wakayama 2018). On the other hand, the crawler does not.
We refer to Thomson (2022) for an extensive analysis of consistency.
Thomson (1988) studies a group version of the property in the context of exchange economies. In his paper, the property requires that a rule should not be affected by the existence of a group of agents if their assignments add up to the sum of their endowments.
None of constant rules, the no-trade rule, nor the rule in Example 3 satisfies efficiency.
Here “\(\omega _2\) is between \(\omega _1\) and \(\omega _3\)” means either \(\omega _1 \prec \omega _2 \prec \omega _3\) or \(\omega _3 \prec \omega _2 \prec \omega _1\).
Here “\(\omega _2\) and \(\omega _3\) are between \(\omega _1\) and \(\omega _4\)” means that one of the followings holds: \(\omega _1 \prec \omega _2 \prec \omega _3 \prec \omega _4\), \(\omega _1 \prec \omega _3 \prec \omega _2 \prec \omega _4\), \(\omega _4 \prec \omega _2 \prec \omega _3 \prec \omega _1\), or \(\omega _4 \prec \omega _3 \prec \omega _2 \prec \omega _1\).
We denote by \(SP^f\) the sequential priority rule with respect to f.
References
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Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
I am grateful to William Thomson for his invaluable advice, support, and encouragement, and for his patient reviews and detailed suggestions. I thank Hadi Hosseini for very helpful comments and discussions. I also thank Paulo Barelli, Christopher Chambers, Srihari Govindan, Sean Horan, Jingyi Xue, and seminar participants at the 2019 Ottawa Microeconomic Theory Workshop, the University of Rochester, Osaka University, Keio University, and Waseda University. I thank an associate editor and anonymous referees for their valuable comments and suggestions. I am grateful for financial support from Tamkeen under the NYUAD Research Institute award for Project CG005. I thank the Institute of Social and Economic Research at Osaka University for their summer research support. This paper is based on a chapter of my Ph.D. dissertation at the University of Rochester. All remaining errors are my own.
Appendices
Appendices
Let \(N \in {\mathcal {N}}\) and \(\bigcup _{i \in N} \omega _i \equiv O \in {\mathcal {O}}\). Unless explicitly specified, without loss of generality, suppose that \(N = \{1,\ldots ,n\}\) and \(O = \{o_1,\ldots ,o_n\}\).
Proofs for Sect. 3
1.1 Proof of Proposition 1
Let \(\varphi \) satisfy efficiency, the endowment lower bound, and strategy-proofness.
Let \({\mathcal {D}}^N \subset {\mathcal {P}}^N\) and \(o_m,o_{m+1} \in O\) be such that for each \(P \in {\mathcal {D}}^N\) and each \(i~\in ~N\), \(X_i(O) \in \{o_m,o_{m+1}\}\). Without loss of generality, let \(\omega \in {\mathcal {X}}^N\) be such that \(\omega ~=~(o_1,\ldots ,o_n)\). We first have the following fact:
Fact 1
For each \(P \in {\mathcal {D}}^N\),
The proof is by induction on n.
Base step Suppose that \(n = 2\). There are four possible preference profiles.
Because \(\varphi \) satisfies efficiency and the endowment lower bound,
Induction hypothesis Let \(k \ge 3\). Suppose that for each \(n \le k - 1\), \(\varphi = TTC\).
Induction step Suppose that \(n = k\). Let \(P \in {\mathcal {D}}^N\). Suppose that either (i) \(X_m(O) = o_m\) or (ii) \(X_{m+1}(O) = o_{m+1}\). Without loss of generality, suppose that (i) holds. Because \(\varphi \) satisfies the endowment lower bound, \(\varphi _m(P,\omega ) = o_m\). Then by the induction hypothesis together with Fact 1,
Suppose that \(X_m(O) = o_{m+1}\) and \(X_{m+1}(O) = o_m\).
Let \(P'_m,P'_{m+1} \in {\mathcal {D}}\) be such that
By the endowment lower bound,
By efficiency,
By strategy-proofness,
Suppose that agents m and \(m+1\) leave with their assignments. Then there are \(n~-~2\) agents and each remaining agent has single-peaked preferences over the remaining objects. Moreover, there are two adjacent objects in \(O \setminus \{o_m,o_{m+1}\}\) such that the most preferred object for each remaining agent in the remaining objects is one of them. Then by the induction hypothesis,
Finally by Fact 1,
1.2 Proof of Lemma 2
Let \((P,\omega ) \in {\mathcal {P}}^N \times {\mathcal {X}}^N\), \(i \in N\), and \(P'_i \in {\mathcal {P}}\) be such that \(CR_i(P,\omega ) = CR_i(P'_i,P_{-i},\omega )\). Then for each pair \(j,k \in N \backslash \{i\}\), agent j receives his assignment and leaves with it earlier than agent k when the crawler is applied to \((P,\omega )\) if and only if they leave with same order when the crawler is applied to \((P'_i,P_{-i},\omega )\). Hence, \(CR(P,\omega ) = CR(P'_i,P_{-i},\omega )\).
1.3 Proof of Proposition 2
By definition, group strategy-proofness implies both strategy-proofness and non-bossiness. We show the converse. Let \((P,\omega ) \in {\mathcal {P}}^N \in {\mathcal {X}}^N\), \(N' \equiv \{i_1,\ldots ,i_{n'}\} \subseteq N\), and \(P'_{N'} \in {\mathcal {P}}^{N'}\) be such that for each \(i \in N'\),
For each \(i \in N'\), let \({\widehat{P}}_i \in {\mathcal {P}}\) be such that
For each \(i \in N\), because \(\varphi _i(P'_{N'},P_{-N'},\omega )~R_i~\varphi _i(P_{N'},P_{-N'},\omega )\), we have \(\varphi _i(P'_{N'},P_{-N},\omega ) \in U(P_i,\varphi _i(P,\omega ))\). Hence, preferences that satisfy (1) exist (Fig. 2).
First, we show that \(\varphi ({\widehat{P}}_{N'},P_{-N'},\omega ) = \varphi (P_{N'},P_{-N'},\omega )\).
Begin with P and suppose that agent \(i_1\)’s preferences change to \({\widehat{P}}_{i_1}\). By strategy-proofness,
Then (2) implies that
On the other hand, (3) implies that
Because
we have
By non-bossiness,
Consider \(({\widehat{P}}_{i_1},P_{-i_1})\) and suppose that agent \(i_2\)’s preferences change to \({\widehat{P}}_{i_2}\). By strategy-proofness,
Then (4) implies that
On the other hand, (5) implies that
Because
we have
By non-bossiness,
Repeating this argument for each \(i \in \{i_3,\ldots ,i_{n'}\}\), we have
Second, we show that \(\varphi (P'_{N'},P_{-N'},\omega ) = \varphi ({\widehat{P}}_{N'},P_{-N'},\omega )\).
Begin with \((P'_{N'},P_{-N'})\) and suppose that agent \(i_1\)’s preferences change to \({\widehat{P}}_{i_1}\). Because for each \(o \in O\),
by strategy-proofness,
By non-bossiness,
Consider \(({\widehat{P}}_{i_1},P'_{N' \backslash \{i_1\}},P_{-N'})\) and suppose that agent \(i_2\)’s preferences change to \({\widehat{P}}_{i_2}\). Because for each \(o \in O\),
by strategy-proofness,
By non-bossiness,
Repeating this argument for each \(i \in \{i_3,\ldots ,i_{n'}\}\), we have
Overall,
Thus, \(\varphi \) is group strategy-proof.
1.4 Proof of Lemma 3
Let \((P,\omega ) \in {\mathcal {P}}^N \times {\mathcal {X}}^N\) and \(i,i+1 \in N\) be such that \(\omega _i \prec p(P_i)\) and \(p(P_{i+1}) \prec \omega _{i+1}\). Suppose that agent \(i+1\) receives his assignment \(CR_{i+1}(P,\omega )\) at Step t when the crawler is applied to \((P,\omega )\). Then by definition of the crawler, the ownership of agent i is shifted to \(\omega _{i+1}\) at Step t.
Suppose that agents i and \(i+1\) swap their endowments. First, until Step t, the underlying procedure remains the same as above when the crawler is applied to \((P,\omega ^{i,i+1})\). Hence, \(CR_{i+1}(P,\omega ^{i,i+1}) = CR_{i+1}(P,\omega )\). Moreover, after agent \(i+1\) receives his assignment, the problem becomes the same as the problem of Step \(t+1\) when the crawler is applied to \((P,\omega )\).
1.5 Proof of Theorem 1
We have seen that the crawler satisfies the endowment lower bound and adjacent endowment-swapping invariance. Moreover, it is clear that it satisfies separability.
Let \(\varphi \) be a rule satisfying the properties listed in the theorem. Let \(\omega \in {\mathcal {X}}^N\). Without loss of generality, suppose that \(\omega = (o_1,o_2,\ldots ,o_n)\). We prove by induction on n that \(\varphi \) is the crawler.
Base step Suppose that \(n = 2\). There are four possible preference profiles.
Note that
By the endowment lower bound,
Consider \((P',\omega )\). Suppose that agents 1 and 2 swap their endowments. Then by the endowment lower bound,
Hence, by adjacent endowment-swapping invariance,
Therefore, \(\varphi = CR\).
Induction hypothesis Let \(k \ge 3\). Suppose that for each \(n \le k-1\), \(\varphi = CR\).
Induction step Suppose that \(n = k\). Let \(P \in {\mathcal {P}}^N\). Let \(i \in N\) and \(P^0_i \in {\mathcal {P}}(\omega _i)\). Suppose that \(P_i = P^0_{i}\), i.e., his most preferred object is his endowment. By the endowment lower bound,
By separability,
There are \(k-1\) agents in \(r_{N \backslash \{i\}}(P,\omega )\). Hence, by the induction hypothesis,
Moreover, because the crawler meets the endowment lower bound,
Let \(i^* \in N\) be the first agent who is assigned an object when the crawler is applied to \((P,\omega )\). Let \(N' \equiv \{i_1,i_2,\ldots ,i_m\} \subseteq N\), where \(i_m \equiv i^*\), be such that \(CR_{i^*}(P,\omega ) = \omega _{i_1} = p(P_{i^*})\), \(o_{i_1}~\prec ~o_{i_2}~\prec ~\cdots ~\prec ~o_{i_m}\), and for each \(k \in \{1,\ldots ,m-1\}\), agents \(i_k\) and \(i_{k+1}\) are adjacent. Let \((P,\omega ^{N'})~\in ~{\mathcal {P}}^N \times {\mathcal {X}}^N\) be the problem that differs from \((P,\omega )\) only in that the agents in \(N'\) have exchanged their endowments as follows: For each \(k \in \{1,\ldots ,m\}\), agent k owns object \(o_{i_{k+1}}\) (agent \(i_m\) owns object \(o_1\)). Because \(p(P_{i^*}) = o_1\) and agent \(i^*\) owns it at \((P,\omega ^{N'})\), by (7),
Let \(N^{''} = N' \setminus \{i_1\}\). Let \((P,\omega ^{N^{''}})~\in ~{\mathcal {P}}^N \times {\mathcal {X}}^N\) be the problem that differs from \((P,\omega )\) only in that the agents in \(N^{''}\) have exchanged their endowments as follows: For each \(k~\in ~\{2,\ldots ,m\}\), agent k owns object \(o_{k+1}\) (agent \(i_m\) owns \(o_2\)). Hence, agent \(i_1\) owns \(o_1\) and agent \(i_m\) owns \(o_2\). Then
-
\(o_1 \prec p(P_{i_1})\);
-
\(p(P_{i^*}) \prec o_2\);
-
\(o_1 \prec o_2\); and
-
there is no \(o \in O\) such that \(o_1 \prec o \prec o_2\).
By adjacent endowment-swapping invariance,
Applying this argument repeatedly,
Because
together with (8) and (9), we have
1.6 Proof of Theorem 2
We have seen that the crawler satisfies the endowment lower bound, strategy-proofness, non-bossiness, and adjacent endowment-swapping invariance.
We show the converse. The proof is almost the same as that of Theorem 1. The only difference is that we derive (6) from strategy-proofness and non-bossiness. Let \(\varphi \) be a rule satisfying the properties. Let \(\omega \in {\mathcal {X}}^N\). Without loss of generality, suppose that \(\omega = (o_1,o_2,\ldots ,o_n)\). We prove by induction on n that \(\varphi \) is the crawler.
Base step Suppose that \(n = 2\). There are four possible preference profiles.
Note that
By the endowment lower bound,
Consider \((P',\omega )\). Suppose that agents 1 and 2 swap their endowments. Then by the endowment lower bound,
Hence, by adjacent endowment-swapping invariance,
Therefore, \(\varphi = CR\).
Induction hypothesis Let \(k \ge 3\). Suppose that for each \(n \le k-1\), \(\varphi = CR\).
Induction step Suppose that Let \(n = k\). Let \(P \in {\mathcal {P}}^N\). Let \(i \in N\) and \(P^0_i~\in ~{\mathcal {P}}(\omega _i)\). Suppose that \(P_i = P^0_i\), i.e., his most preferred object is his endowment. By the endowment lower bound,
Let \(j \in N \setminus \{i\}\) and \(P'_j \in {\mathcal {P}}\) be such that for each pair \(k,l \in N \backslash \{i\}\),
By strategy-proofness,
By non-bossiness,
This implies that
There are \(k-1\) agents in \(r_{N \backslash \{i\}}(P^0_i,P_{-i})\). Hence, by the induction hypothesis,
Because the crawler meets the endowment lower bound,
The rest of the proof is the same as that of Theorem 1.
Relation between the properties in Sect. 3
Figure 3 shows how the properties in Theorem 1 are related.
Example 5
Let \(N' \in {\mathcal {N}}\) be such that \(|N'| \ge 3\).
Let \(\varphi \) be defined by setting for each \((P,\omega )~\in ~{\mathcal {P}}^{N'}~\times ~{\mathcal {X}}^{N'}\), \(\varphi (P,\omega ) = \omega \), and for each \(N \in {\mathcal {N}}\) such that \(N \ne N'\) and each \((P,\omega ) \in {\mathcal {P}}^N \times {\mathcal {X}}^N\), \(\varphi (P,\omega ) = CR(P,\omega )\). Because the crawler meets the endowment lower bound, so does \(\varphi \).
Let \(N' = \{1,2,3\}\) and \(\omega \in {\mathcal {X}}^{N'}\). Let \(P' \in {\mathcal {P}}^{N'}\) be such that
Then \(\varphi (P',\omega ) = (\omega _1,\omega _2,\omega _3)\). Because
\(\varphi \) is not adjacent-endowment-swapping invariant. Because
\(\varphi \) is not separable.
Example 6
Let \(N' = \{1,2,3\}\). Let \(\varphi \) be defined by setting for each \((P,\omega ) \in {\mathcal {P}}^{N'} \times {\mathcal {X}}^{N'}\),
and for each \(N \in {\mathcal {N}}\) such that \(N \ne N'\) and each \((P,\omega ) \in {\mathcal {P}}^N \times {\mathcal {X}}^N\), \(\varphi (P,\omega ) = CR(P,\omega )\).
By the same logic as in Example 3, \(\varphi \) is adjacent-endowment-swapping invariant.
Consider the set \(N'\) of agents. Let \(\omega \in {\mathcal {X}}^{N'}\). Let \(P' \in {\mathcal {P}}^{N'}\) be such that
Then \(\varphi (P',\omega ) = (\omega _3,\omega _2,\omega _1)\). Because
\(\varphi \) does not meet the endowment lower bound. Moreover, because
\(\varphi \) is not separable.
Example 7
For each \(N \in {\mathcal {N}}\), each \((P,\omega ) \in {\mathcal {P}}^N \times {\mathcal {X}}^N\), and each \(i \in N\), let \(\varphi _i(P) = \omega _{i+1}\), where the agent whose endowment is the rightmost in the order is assigned the leftmost object.
Because for each \(N \in {\mathcal {N}}\), each \((P,\omega ) \in {\mathcal {P}}^N \times {\mathcal {X}}^N\), and each \(i \in N\), \(\varphi _i(P,\omega ) \ne \omega _i\), \(\varphi \) is separable.
Let \(N \in {\mathcal {N}}\). Let \((P,\omega ) \in {\mathcal {P}}^N \times {\mathcal {X}}^N\) and \(i \in N\) be such that agent i’s endowment is not the rightmost object and \(p(P_i) \prec \omega _i\). Because \(p(P_i) \prec \omega _i \prec \omega _{i+1} = \varphi _i(P,\omega )\), we have \(\omega _i ~P_i~\varphi _i(P,\omega )\). Hence, \(\varphi \) does not meet the endowment lower bound.
Let \((P,\omega ) \in {\mathcal {P}}^N \times {\mathcal {X}}^N\) and \(i,i+1 \in N\) be such that agent \(i+1\)’s endowment is not the rightmost object, \(\omega _i \prec p(P_i)\) and \(p(P_{i+1}) \prec \omega _{i+1}\). Because
\(\varphi \) is not adjacent-endowment-swapping invariant.
Figure 4 shows how the properties in Theorem 2 are related. In some of the examples below, we refer to problems consisting of a specific agent set. For those examples, for any other agent set, we apply the crawler.
Example 8
Let \(N \in {\mathcal {N}}\). Let \(\varphi \) be defined by setting for each \((P,\omega ) \in {\mathcal {P}}^N \times {\mathcal {X}}^N\),
Because the crawler satisfies the endowment lower bound, so does \(\varphi \). No agent would benefit by pretending that he most prefers his endowment. Moreover, if an agent’s most preferred object is his endowment and he is truthful, he receives his endowment. Hence, \(\varphi \) is strategy-proof.
Let \(N \in {\mathcal {N}}\) and \(\omega \in {\mathcal {X}}^N\). Let \({\mathcal {D}}^N \subset {\mathcal {P}}^N\) be such that for each \(P' \in {\mathcal {D}}^N\), there is \(i \in N\) such that \(p(P'_i) \ne \omega _i\) and \(\varphi _i(P',\omega ) = \omega _i\). Also, for each \(P^0_i \in {\mathcal {P}}(\omega _i)\), we have \(\varphi _i(P^0_i,P'_{-i},\omega ) = \omega _i\). However, there are \(P' \in {\mathcal {D}}^N\), \(i \in N\), and \(P^0_i \in {\mathcal {P}}(\omega _i)\) such that \(\varphi (P^0_i,P'_{-i},\omega ) \ne \varphi (P'_i,P'_{-i},\omega )\). Hence, \(\varphi \) violates non-bossiness.
Let \(\omega \in {\mathcal {X}}^N\). Let \(P' \in {\mathcal {P}}^N\) be such that there is \(i \in N\) such that
Moreover, suppose that there are \(j,j+1 \in N\) such that \(\omega _j \prec p(P'_j)\) and \(p(P'_{j+1}) \prec \omega _{j+1}\). Then \(\varphi _{\{j,j+1\}}(P',\omega ) = (\omega _j,\omega _{j+1})\). Because
\(\varphi \) is not adjacent-endowment-swapping invariant.
Example 9
Let \(n \in \{2,\ldots ,{\overline{n}}\}\) and \(N = \{1,2,\ldots ,n\} \in {\mathcal {N}}\). Let \(\varphi \) be defined by setting for each \((P,\omega )~\in ~{\mathcal {P}}^N~\times ~{\mathcal {X}}^N\),
It is clear that \(\varphi \) satisfies the endowment lower bound. If for each \(i \in N\), \(p(P_i) \ne \omega _i\), we have \(\varphi (P,\omega ) \ne \omega \). This implies that \(\varphi \) is non-bossy.
Let \(\omega \in {\mathcal {X}}^N\). Let \(P' \in {\mathcal {P}}^N\) be such that there are \(i,j \in N\) such that \(p(P'_i) = p(P'_j)\). Moreover, suppose that there are \(k,k+1 \in N\) such that \(\omega _k \prec p(P'_k)\) and \(p(P'_{k+1}) \prec \omega _{k+1}\). Then \(\varphi _{\{k,k+1\}}(P',\omega )~=~(\omega _k,\omega _{k+1})\). Because
\(\varphi \) is not adjacent endowment-swapping invariant.
Example 10
Let \(N \in {\mathcal {N}}\). Let \(\varphi \) be defined by setting for each \((P,\omega ) \in {\mathcal {P}}^N \times {\mathcal {X}}^N\), \(\varphi (P,\omega ) = TTC^*(P,\omega )\), where at each step, no agent points to his endowment unless he is the only remaining agent. The only difference between \(\varphi \) and TTC is at the step when an agent points to himself when applying TTC. Also, \(\varphi \) assigns an agent his endowment only if he is the only agent present. This implies that being assigned his endowment is independent of his preferences. Then because TTC is strategy-proof, so is \(\varphi \). Because TTC is non-bossy, so is \(\varphi \).
It is trivial that \(\varphi \) does not satisfy the endowment lower bound. Because TTC is not adjacent-endowment-swapping invariant, neither is \(\varphi \).
Example 11
Let \(N' = \{1,2,3,4\}\). Let \(\varphi \) be defined by setting for each \((P,\omega ) \in {\mathcal {P}}^{N'} \times {\mathcal {X}}^{N'}\),
Because both TTC and the crawler satisfy the endowment lower bound, so does \(\varphi \). Consider the set \(N'\) of agents. If \(p(P_2)~=~\omega _2\) and \(p(P_3) = \omega _3\), neither one of agents 2 and 3 would benefit from swapping his endowment with either of his neighbor. Also, the crawler is adjacent-endowment-swapping invariant. Hence, so is \(\varphi \).
Let \(\omega \in {\mathcal {X}}^{N'}\). Let \(P' \in {\mathcal {P}}^{N'}\) be such that
Then \(\varphi (P',\omega ) = (\omega _1,\omega _2,\omega _3,\omega _4)\).
Let \({\widehat{P}}_4 \in {\mathcal {P}}\) be such that
Then \(\varphi ({\widehat{P}}_4,P'_{\{1,2,3\}},\omega ) = (\omega _4,\omega _2,\omega _3,\omega _1)\). Because
\(\varphi \) is not strategy-proof.
Let \({\widehat{P}}_2 \in {\mathcal {P}}\) be such that
Then \(\varphi ({\widehat{P}}_2,P'_{\{1,3,4\}},\omega ) = (\omega _4,\omega _2,\omega _3,\omega _1)\). Because
\(\varphi \) violates non-bossiness.
Example 12
Let \(N' = \{1,2,3\}\) and denote \(\bigcup _{i \in N'} \omega _i = \{o_1,o_2,o_3\}\). Let \(f = (1,2,3,\ldots )\) and \(f' = (1,3,2,\ldots )\). Let \(\varphi \) be defined by setting for each \((P,\omega ) \in {\mathcal {P}}^{N'} \times {\mathcal {X}}^{N'}\),Footnote 20
Consider the set \(N'\) of agents. Agent 1 is always assigned an object first. Hence, \(\varphi \) is strategy-proof for him. Moreover, all sequential priority rules are strategy-proof. Because the ordering of agents depends only on agent 1’s preferences, \(\varphi \) is strategy-proof. All sequential priority rules are adjacent-endowment-swapping invariant. Because the ordering of agents does not depend on the endowment profile, \(\varphi \) is adjacent-endowment-swapping invariant. Because none of the sequential priority rules meets the endowment lower bound, neither does \(\varphi \).
Let \(\omega \in {\mathcal {X}}^{N'}\) and let \(P' \in {\mathcal {P}}^{N'}\) be such that
Let \({\widehat{P}}_1 \in {\mathcal {P}}\) be such that
Then
Hence, \(\varphi \) violates non-bossiness.
Example 13
Let \(N' = \{1,2,3\}\) and denote \(\bigcup _{i \in N'} \omega _i = \{o_1,o_2,o_3\}\). Let \(\varphi \) be defined by setting for each \((P,\omega ) \in {\mathcal {P}}^{N'} \times {\mathcal {X}}^{N'}\),
Consider the set \(N'\) of agents. It is trivial that \(\varphi \) is non-bossy. Because \(\varphi \) is independent of the endowment profile, it is adjacent-endowment-swapping invariant.
Let \(\omega = (o_1,o_2,o_3)\). Let \(P' \in {\mathcal {P}}^{N'}\) be such that
Then \(\varphi (P',\omega ) = (o_3,o_2,o_1)\). Because \(o_1~P'_1~o_3 = \varphi _1(P',\omega )\), \(\varphi \) does not satisfy the endowment lower bound.
Let \({\widehat{P}}_3 \in {\mathcal {P}}\) be such that
Then \(\varphi ({\widehat{P}}_3,P'_{\{1,2\}},\omega ) = (o_1,o_3,o_2)\). Because
\(\varphi \) is not strategy-proof.
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Tamura, Y. Object reallocation problems under single-peaked preferences: two characterizations of the crawler. Int J Game Theory 51, 537–565 (2022). https://doi.org/10.1007/s00182-022-00803-6
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DOI: https://doi.org/10.1007/s00182-022-00803-6