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(1)

Various stable matching concepts

Tam´as Fleiner Zsuzsanna Jank´o

(2)

Stable marriages

Gale and Shapley (1962)

There aren men andm women, each of them having a preference order on the members of the other gender. We call a marriage schemestableif there is noblocking pair: a man and women that mutually prefer each other to their own partners (or he/she is single).

Theorem (Gale-Shapley)

There always exists a stable matching, and it can be found with the deferred acceptance algorithm.

(3)

Optimality-pessimality

We call a stable matchingS male-optimalit if is preferred by all men to any other stable matching: S ≥M S0 for every stable matchingS0. A stable matching S ismale-pessimal ifS ≤M S0 for every stable matchingS0.

Female-optimality and pessimality are defined similarly.

Theorem (Gale-Shapley)

The stable marriage scheme given by the Gale-Shapley algorithm is male-optimal and female-pessimal.

(4)

College admissions in Hungary

Givenn applicants: A1,A2, . . . ,An andm colleges: C1,C2, . . .Cm. Every applicant has a strict preference order over the colleges she applies to.

Every college assigns some score (an integer between 1 andM) to each of its applicants.

Moreover, each collegeC has a quotaq(C) on admissible applicants.

Each college has to declare a score limit. The score limit of college Ci is ti.

The vector of declared score limits (t1,t2, . . . ,tm) is called a score vector

Each applicant will become a student on her most preferred college where she has high enough score.

(5)

Properties of score vectors

Score vector (t1,t2, . . .tm) is validif no college exceeds its quota with these score limits.

Score vector (t1,t2, . . .tm) is criticalif for every college either tj = 0 or score vector (t1,t2, . . . ,tj−1,tj −1,tj+1, . . . ,tm) is not valid forCj. A score vector t is stableift is valid and critical. An student-college many-to-one matching isscore-stable if it can be realized by a stable score vector.

Note that if applicants have different scores and the qouta is one for every college, then we are back at the stable marriage problem.

(6)

Properties of score vectors

Score vector (t1,t2, . . .tm) is validif no college exceeds its quota with these score limits.

Score vector (t1,t2, . . .tm) is criticalif for every college either tj = 0 or score vector (t1,t2, . . . ,tj−1,tj −1,tj+1, . . . ,tm) is not valid forCj.

A score vector t is stableift is valid and critical. An student-college many-to-one matching isscore-stable if it can be realized by a stable score vector.

Note that if applicants have different scores and the qouta is one for every college, then we are back at the stable marriage problem.

(7)

Properties of score vectors

Score vector (t1,t2, . . .tm) is validif no college exceeds its quota with these score limits.

Score vector (t1,t2, . . .tm) is criticalif for every college either tj = 0 or score vector (t1,t2, . . . ,tj−1,tj −1,tj+1, . . . ,tm) is not valid forCj. A score vector t is stableift is valid and critical.

An student-college many-to-one matching isscore-stable if it can be realized by a stable score vector.

Note that if applicants have different scores and the qouta is one for every college, then we are back at the stable marriage problem.

(8)

Properties of score vectors

Score vector (t1,t2, . . .tm) is validif no college exceeds its quota with these score limits.

Score vector (t1,t2, . . .tm) is criticalif for every college either tj = 0 or score vector (t1,t2, . . . ,tj−1,tj −1,tj+1, . . . ,tm) is not valid forCj. A score vector t is stableift is valid and critical.

An student-college many-to-one matching isscore-stable if it can be realized by a stable score vector.

Note that if applicants have different scores and the qouta is one for every college, then we are back at the stable marriage problem.

(9)

Properties of score vectors

Score vector (t1,t2, . . .tm) is validif no college exceeds its quota with these score limits.

Score vector (t1,t2, . . .tm) is criticalif for every college either tj = 0 or score vector (t1,t2, . . . ,tj−1,tj −1,tj+1, . . . ,tm) is not valid forCj. A score vector t is stableift is valid and critical.

An student-college many-to-one matching isscore-stable if it can be realized by a stable score vector.

Note that if applicants have different scores and the qouta is one

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Example

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t t t

t

valid

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Example

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C1 C2

(q=1) (q=1)

1 2 2 1 1

C2 6

1 2 3

t t

t

valid stable

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Score-decreasing algorithm

Theorem

For any finite set of applicants, colleges and set of applications, for arbitrary positive scores of the applications there always exists a stable score vector.

The are two natural algorithms to find a stable score vector: 1. The score-decreasing algorithm: colleges start on a valid score vectortC := (M+ 1, . . . ,M+ 1) and they keep on decreasing their score limits by one at a time, if this results in another valid score vector. As soon as no college can decrease its score limit, the score vector is stable. LetsC note the stable score vector we get. Theorem

The score vector sC maximal among all stable score vectors, and this assignment is applicant-pessimal.

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Score-decreasing algorithm

Theorem

For any finite set of applicants, colleges and set of applications, for arbitrary positive scores of the applications there always exists a stable score vector.

The are two natural algorithms to find a stable score vector:

1. The score-decreasing algorithm: colleges start on a valid score vectortC := (M+ 1, . . . ,M+ 1) and they keep on decreasing their score limits by one at a time, if this results in another valid score vector. As soon as no college can decrease its score limit, the score vector is stable. LetsC note the stable score vector we get.

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Score-decreasing algorithm

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i

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Score-decreasing algorithm

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t

i

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Score-decreasing algorithm

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t i

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Score-decreasing algorithm

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Score-decreasing algorithm

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t t t

t i

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Score-decreasing algorithm

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(q=1)

3 2

1 2 2 1 1

C2 6

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t t

t

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stable

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Score-increasing algorithm

2. The score-increasing algorithm: Colleges start with critical score vectortA = (0, . . . ,0)) and keep on raising there score limits by one, if they receive more students than their quota. As soon as the score vector becomes valid, the score vector is also stable. Let sA the stable score vector the score-increasing algorithm outputs.

Theorem

Score vector sA is the minimum of all stable score vectors.

Additionally it is applicant-optimal.

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score-increasing algorithm

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C1

(q=1)

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t

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score-increasing algorithm

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t i

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score-increasing algorithm

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C1

(q=1)

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(q=1)

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t t

t stable

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Choice functions and properties

Choice function: F : 2E →2E s.t. F(A)⊆A ∀A⊆E.

Monotone: A⊆B ⊆E ⇒ F(A)⊆ F(B). Antitone: A⊆B ⊆E ⇒ F(B)⊆ F(A).

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Choice functions and properties

Choice function: F : 2E →2E s.t. F(A)⊆A ∀A⊆E. Monotone: A⊆B ⊆E ⇒ F(A)⊆ F(B).

Antitone: A⊆B ⊆E ⇒ F(B)⊆ F(A).

(26)

Choice functions and properties

Choice function: F : 2E →2E s.t. F(A)⊆A ∀A⊆E. Monotone: A⊆B ⊆E ⇒ F(A)⊆ F(B).

Antitone: A⊆B ⊆E ⇒ F(B)⊆ F(A).

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Substitutability

A choice functionF : 2E →2E issubstitutable if A\ F(A)⊆B\ F(B) for any A⊆B.

When the set of opportunities expands, the refused contracts expand. For example, if an applicant is refused out of 5 applicants, he will be still refused when 5 more people apply.

We usually assume this proprety.

(28)

Substitutability

A choice functionF : 2E →2E issubstitutable if A\ F(A)⊆B\ F(B) for any A⊆B.

When the set of opportunities expands, the refused contracts expand. For example, if an applicant is refused out of 5 applicants, he will be still refused when 5 more people apply.

We usually assume this proprety.

(29)

Substitutability

A choice functionF : 2E →2E issubstitutable if A\ F(A)⊆B\ F(B) for any A⊆B.

When the set of opportunities expands, the refused contracts expand. For example, if an applicant is refused out of 5 applicants, he will be still refused when 5 more people apply.

We usually assume this proprety.

(30)

IRC, LOB

Choice functionF : 2E →2E satisfies the IRC (Irrelevance of rejected contracts)ifF(A)⊆B ⊆A⇒ F(A) =F(B).

My favorite subset of setAis F(A), and F(A) is a subset ofB, then this is also my favorite subset ofB.

A choice functionF islinear order based(LOB) if it can be defined by a strict preference order over all subsets ofE, such that F(A) is best subset ofA according to this order.

F(A) = max{X :X ⊆A}.

Many articles define every choice function as linear order based. Then it implies IRC too.

However, the choice funtion of colleges is usually not IRC. For example, given two applicants: a,b, both of them with score 100, and the quota of the college is 1.

G({a}) ={a} G({b}) ={b} G({a,b}) =∅.

(31)

IRC, LOB

Choice functionF : 2E →2E satisfies the IRC (Irrelevance of rejected contracts)ifF(A)⊆B ⊆A⇒ F(A) =F(B).

My favorite subset of setAis F(A), and F(A) is a subset ofB, then this is also my favorite subset ofB.

A choice functionF islinear order based(LOB) if it can be defined by a strict preference order over all subsets ofE, such that F(A) is best subset ofA according to this order.

F(A) = max{X :X ⊆A}.

Many articles define every choice function as linear order based. Then it implies IRC too.

However, the choice funtion of colleges is usually not IRC. For example, given two applicants: a,b, both of them with score 100, and the quota of the college is 1.

G({a}) ={a} G({b}) ={b} G({a,b}) =∅.

(32)

IRC, LOB

Choice functionF : 2E →2E satisfies the IRC (Irrelevance of rejected contracts)ifF(A)⊆B ⊆A⇒ F(A) =F(B).

My favorite subset of setAis F(A), and F(A) is a subset ofB, then this is also my favorite subset ofB.

A choice functionF islinear order based(LOB) if it can be defined by a strict preference order over all subsets ofE, such that F(A) is best subset ofA according to this order.

F(A) = max{X :X ⊆A}.

Many articles define every choice function as linear order based. Then it implies IRC too.

However, the choice funtion of colleges is usually not IRC. For example, given two applicants: a,b, both of them with score 100, and the quota of the college is 1.

G({a}) ={a} G({b}) ={b} G({a,b}) =∅.

(33)

IRC, LOB

Choice functionF : 2E →2E satisfies the IRC (Irrelevance of rejected contracts)ifF(A)⊆B ⊆A⇒ F(A) =F(B).

My favorite subset of setAis F(A), and F(A) is a subset ofB, then this is also my favorite subset ofB.

A choice functionF islinear order based(LOB) if it can be defined by a strict preference order over all subsets ofE, such that F(A) is best subset ofA according to this order.

F(A) = max{X :X ⊆A}.

Many articles define every choice function as linear order based.

However, the choice funtion of colleges is usually not IRC. For example, given two applicants: a,b, both of them with score 100, and the quota of the college is 1.

G({a}) ={a} G({b}) ={b} G({a,b}) =∅.

(34)

IRC, LOB

Choice functionF : 2E →2E satisfies the IRC (Irrelevance of rejected contracts)ifF(A)⊆B ⊆A⇒ F(A) =F(B).

My favorite subset of setAis F(A), and F(A) is a subset ofB, then this is also my favorite subset ofB.

A choice functionF islinear order based(LOB) if it can be defined by a strict preference order over all subsets ofE, such that F(A) is best subset ofA according to this order.

F(A) = max{X :X ⊆A}.

Many articles define every choice function as linear order based.

Then it implies IRC too.

However, the choice funtion of colleges is usually not IRC. For example, given two applicants: a,b, both of them with score 100, and the quota of the college is 1.

G({a}) ={a} G({b}) ={b} G({a,b}) =∅.

(35)

IRC, LOB

Choice functionF : 2E →2E satisfies the IRC (Irrelevance of rejected contracts)ifF(A)⊆B ⊆A⇒ F(A) =F(B).

My favorite subset of setAis F(A), and F(A) is a subset ofB, then this is also my favorite subset ofB.

A choice functionF islinear order based(LOB) if it can be defined by a strict preference order over all subsets ofE, such that F(A) is best subset ofA according to this order.

F(A) = max{X :X ⊆A}.

Many articles define every choice function as linear order based.

Then it implies IRC too.

However, the choice funtion of colleges is usually not IRC. For example, given two applicants: a,b, both of them with score 100, and the quota of the college is 1.

(36)

IRC, LOB

Choice functionF : 2E →2E satisfies the IRC (Irrelevance of rejected contracts)ifF(A)⊆B ⊆A⇒ F(A) =F(B).

My favorite subset of setAis F(A), and F(A) is a subset ofB, then this is also my favorite subset ofB.

A choice functionF islinear order based(LOB) if it can be defined by a strict preference order over all subsets ofE, such that F(A) is best subset ofA according to this order.

F(A) = max{X :X ⊆A}.

Many articles define every choice function as linear order based.

Then it implies IRC too.

However, the choice funtion of colleges is usually not IRC. For example, given two applicants: a,b, both of them with score 100, and the quota of the college is 1.

G({a}) ={a}

G({b}) ={b}

G({a,b}) =∅.

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Path-independence

A choice functionF : 2E →2E ispath-independent if

F(A∪B) =F(F(A)∪B) holds for all subsets AandB of E.

Lemma (Fleiner)

A choice functionF is path-independent if and only ifF is IRC and substitutable.

Theorem

IfF path-independent then it is linear order based.

(38)

Path-independence

A choice functionF : 2E →2E ispath-independent if

F(A∪B) =F(F(A)∪B) holds for all subsets AandB of E. Lemma (Fleiner)

A choice functionF is path-independent if and only ifF is IRC and substitutable.

Theorem

IfF path-independent then it is linear order based.

(39)

Path-independence

A choice functionF : 2E →2E ispath-independent if

F(A∪B) =F(F(A)∪B) holds for all subsets AandB of E. Lemma (Fleiner)

A choice functionF is path-independent if and only ifF is IRC and substitutable.

Theorem

IfF path-independent then it is linear order based.

(40)

Path-independence

A choice functionF : 2E →2E ispath-independent if

F(A∪B) =F(F(A)∪B) holds for all subsets AandB of E. Lemma (Fleiner)

A choice functionF is path-independent if and only ifF is IRC and substitutable.

Theorem

IfF path-independent then it is linear order based.

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Two-sided market

Atwo-sided market can be represented as a bipartite graph. On one side, the applicants have a choice functionF over the set of contracts and the on the other side the colleges have a choice functionG over the contracts.

LetE be the set of all possible contracts.

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Choice functions for college admissions

For subsetX ⊆E of applications F(X) denotes the set of most preferred applications of each applicant.

Similarly,G(X) denotes the set of applications that colleges would choose. From a given set of applicants, they choose the most possible applicants by giving a score limit, not exceeding their quota.

Choice functionF of the applicants is IRC, butG for the colleges is not. F and G are both substitutable.

Example:

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Choice functions for college admissions

For subsetX ⊆E of applications F(X) denotes the set of most preferred applications of each applicant.

Similarly,G(X) denotes the set of applications that colleges would choose. From a given set of applicants, they choose the most possible applicants by giving a score limit, not exceeding their quota.

Choice functionF of the applicants is IRC, butG for the colleges is not. F and G are both substitutable.

Example:

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2nd F

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Choice functions for college admissions

For subsetX ⊆E of applications F(X) denotes the set of most preferred applications of each applicant.

Similarly,G(X) denotes the set of applications that colleges would choose. From a given set of applicants, they choose the most possible applicants by giving a score limit, not exceeding their quota.

Choice functionF of the applicants is IRC, butG for the colleges is not. F and G are both substitutable.

Example:

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d d d

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2nd 3rd F

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, , , , ,

, , , , , ,

C1(q = 3)

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Pairwise stability

A contract-setS ofE is calledpairwise stable(or dominating stable), if

1. F(S) =G(S) =S and

2. There is no contract x ∈/ S such that x∈ F(S ∪ {x}) and x ∈ G(S ∪ {x})

This is a natural generalization of the original stable marriages.

(46)

Pairwise stability

A contract-setS ofE is calledpairwise stable(or dominating stable), if

1. F(S) =G(S) =S and

2. There is no contract x ∈/ S such that x∈ F(S ∪ {x}) and x ∈ G(S ∪ {x})

This is a natural generalization of the original stable marriages.

(47)

Group-stability

Hatfield-Milgrom (2005) used the following concept (for many-to-one matchings), we will name itgroup-stable:

A set of contractsS ⊆E is group-stableif 1. F(S) =G(S) =S and

2. there exists no college h and set of contractsX0 6=Gh(S) such that X0 =Gh(S∪X0)⊆ F(S∪X0).

Lemma

IfF and G are substitutable, and the market contains at most one

(48)

3-stability

SubsetS of E is 3-stable, if 1. F(S) =G(S) =S and

2. there exists subsetsA andB ofE, such that F(A) =S =G(B) andA∪B =E,A∩B =S.

Pair (A,B) with this property is called an3-stable pair, andS is an 3-stable core.

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3-stability

SubsetS of E is 3-stable, if 1. F(S) =G(S) =S and

2. there exists subsetsA andB ofE, such that F(A) =S =G(B) andA∪B =E,A∩B =S.

Pair (A,B) with this property is called an3-stable pair, andS is an 3-stable core.

@@@@@@@ A B

B

B B B

E S

(50)

Score-stability versus 3-stability

For a set of applicationsS, score-stablitity is not equivalent with 3-stability.

Example:

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A1

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(q = 2)

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C2A1,C2A2 C1A1,C1A2

A

B

LetA=S ={C2A1,C2A2}and B=E therefore

F(A) =G(B) =S,A∪B=E,A∩B=S. So S is 3-stable, and it can be realized with score vector (1,0). Although, (1,0) is not score-stable, ifC1 lowers its limit to (0,0), the admission changes toC1A1,C2A2 which is still valid.

(51)

Score-stability versus 3-stability

For a set of applicationsS, score-stablitity is not equivalent with 3-stability.

Example:

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d C1 1

(q = 1)

A1

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B

LetA=S ={C2A1,C2A2}and B=E therefore

F(A) =G(B) =S,A∪B=E,A∩B=S. So S is 3-stable, and it

(52)

Score-stability versus 3-stability

For a set of applicationsS, score-stablitity is not equivalent with 3-stability.

Example:

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d

d C1 0

(q = 1)

A1

C2 0 (q = 2)

A2 F

G

0

0 0 0

i 1

C2A1,C2A2 C1A1,C1A2

A

B

LetA=S ={C2A1,C2A2}and B=E therefore

F(A) =G(B) =S,A∪B=E,A∩B=S. So S is 3-stable, and it can be realized with score vector (1,0). Although, (1,0) is not score-stable, ifC1 lowers its limit to (0,0), the admission changes toC1A1,C2A2 which is still valid.

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Determinants

We sayD: 2E →2E is a determinant of choice function F if F(A) =A∩ D(A) for everyA⊆E.

Lemma

Choice functionF : 2E →2E is substitutable if and only if there exists an antitone determinantDof F.

For every substitutableF, there is a canonical determinant, which is minimal among all possible antitone determinant ofF.

(54)

4-stability

SubsetS of E is 4-stable, if 1. F(S) =G(S) =S and

2. there exists subsetsA andB ofE, such that

F(A) =S =G(B) andA∩B =S,DF(A) =B,DG(B) =A

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A B

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Connection between stability concepts

Theorem

If F and G are substitutable and IRC, 3-part, 4-part and dominating stability are equivalent.

bothF,G are IRC

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F andG may not be IRC

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Connection between stability concepts 2

If there are no parallel contracts, i.e. there is only one possible contract between a college and an applicant.

bothF,G are IRC

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one side is IRC

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Partial order over stable marriages

There is a partial order over marriage schemes:

S ≥M S0, if every men got at least as good wife in matchingS as inS0. Similarly S ≥W S0, if every woman got at least as good husband inS, compared toS0.

Lemma (Knuth)

If everyone has a strict preference order, if S ≥M S0 then S0W S .

Theorem (Conway)

Let S1 and S2 be two stable marriage schemes, and every man picks the better one out of this wives in S1 and S2. Then we obtain a stable matching.

Corollary:

Theorem (Conway)

The stable marriages form a distributive lattice for the partial order

M.

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Partial order over stable marriages

There is a partial order over marriage schemes:

S ≥M S0, if every men got at least as good wife in matchingS as inS0. Similarly S ≥W S0, if every woman got at least as good husband inS, compared toS0.

Lemma (Knuth)

If everyone has a strict preference order, if S ≥M S0 then S0W S .

Theorem (Conway)

Let S1 and S2 be two stable marriage schemes, and every man picks the better one out of this wives in S1 and S2. Then we obtain a stable matching.

Corollary:

Theorem (Conway)

The stable marriages form a distributive lattice for the partial order

M.

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Partial order over stable marriages

There is a partial order over marriage schemes:

S ≥M S0, if every men got at least as good wife in matchingS as inS0. Similarly S ≥W S0, if every woman got at least as good husband inS, compared toS0.

Lemma (Knuth)

If everyone has a strict preference order, if S ≥M S0 then S0W S . Theorem (Conway)

Corollary:

Theorem (Conway)

The stable marriages form a distributive lattice for the partial order

M.

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Partial order over stable marriages

There is a partial order over marriage schemes:

S ≥M S0, if every men got at least as good wife in matchingS as inS0. Similarly S ≥W S0, if every woman got at least as good husband inS, compared toS0.

Lemma (Knuth)

If everyone has a strict preference order, if S ≥M S0 then S0W S .

Theorem (Conway)

Let S1 and S2 be two stable marriage schemes, and every man picks the better one out of this wives in S1 and S2. Then we obtain a stable matching.

Corollary:

Theorem (Conway)

The stable marriages form a distributive lattice for the partial order

M.

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Lattice properties

Theorem (Tarski’s fixed point theorem)

LetL be complete lattice, and f :L → Lbe a monotone function onL. ThenLf is a nonempty, complete lattice on the restricted partial order whereLf ={x∈ L:f(x) =x} is the set of fixed points of f .

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Lattice of 3-stable cores

Given a choice functionF, define a partial order on contract sets:

S0F S if F(S ∪S0) =S. Theorem (Blair)

IfF,G: 2E →2E are substitutable, IRC choice functions, then the 3-stable cores form a lattice for partial order≤F.

Recall that if both sides have IRC choice functions, 3-stability, 4-stability and dominating stability are equivalent, so all of them form a lattice.

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Lattice properties of 4-stability

Theorem (Generalization of Blair’s theorem)

IfF and G are substitutable andF is IRC (G doesn’t need to be IRC), the 4-stable sets form a nonempty complete lattice for partial order≤F.

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Lattice property of score-stability

If the market has no parallel contracts, 4-stability and score-stability are equivalent.

Theorem

IfF and G are substitutable, andF is IRC, score-stable solutions form a lattice.

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Conclusion

definition always exists lattice

dominating no no

group no no

3-stable yes yes

4-stable yes yes

score-stable yes yes

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Usefulness of determinants

We have a choice funtion over a latticeF :L → L. For example every agent plays tennis with the others, and wants to allocate her free time. If she have 3 possible partners, and her free time all together is 1 hour, the choice function isF : [0,1]3 →[0,1]3. A choice functionF :L → Lis substitutable if and only if there exists an antitone determinantDof F.

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Thank you for your attention!

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Outline

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