the mathematics of 1950’s dating: who wins the battle of the sexes?

The Mathematics Of 1950’s The Mathematics Of 1950’s Dating: Who wins the battle of the Dating: Who wins the battle of the

sexes?sexes?Adapted from a presentation by Stephen RudichAdapted from a presentation by Stephen Rudich

Steven Rudich: www.discretemath.com www.rudich.net

WARNING: This lecture contains mathematical content that may be shocking to some students.


Dating ScenarioDating Scenario• There are n boys and n girls• Each girl has her own ranked

preference list of all the boys• Each boy has his own ranked

preference list of the girls• The lists have no ties

Question: How do we pair them off?


C,B,A,D

1B,A,D,C

2D,C,A,B

3A,B,C,D

4

A

3,4,1,2

B

1,2,3,4

C

4,3,2,1

D

1,3,4,2


Rogue CouplesRogue CouplesSuppose we pair off all the boys and Suppose we pair off all the boys and girls. girls. Now suppose that some boy Now suppose that some boy and some girl prefer each other to and some girl prefer each other to the people to whom they are paired.the people to whom they are paired. They will be called a They will be called a rogue couplerogue couple..


Why be with them when we can be with each

other?


Stable PairingsStable Pairings

A pairing of boys and girls is called A pairing of boys and girls is called stablestable if it contains no rogue if it contains no rogue couples.couples.

C,B,A,D

1B,A,D,C

D,C,A,B

3A,B,C,D

4

A3,4,1,2

B1,2,3,4

C4,3,2,1

D1,3,4,2

2


Given a set of preference lists, Given a set of preference lists, how do we find a stable pairing?how do we find a stable pairing?


Given a set of preference lists, Given a set of preference lists, how do we find a stable pairing?how do we find a stable pairing?

Wait! We don’t even know that such a

pairing always exists!


Idea: Allow the pairs to keep breaking up Idea: Allow the pairs to keep breaking up and reforming until they become stable. and reforming until they become stable.


Can you argue that the couples will not

continue breaking up and reforming forever?

Female


The Traditional Marriage The Traditional Marriage AlgorithmAlgorithm

String

Worshipping males


Traditional Marriage AlgorithmTraditional Marriage AlgorithmWhile there is an unmatched boy, do:While there is an unmatched boy, do:• Some unmatched boy proposes to next girl on his listSome unmatched boy proposes to next girl on his list• If girl is unmatched:If girl is unmatched:

– boy & girl get engaged• If girl is matched but prefers boy to her current fiancé:If girl is matched but prefers boy to her current fiancé:

– boy & girl get engaged– previous fiancé becomes unmatched

• If girl is matched and prefers fiancé to proposerIf girl is matched and prefers fiancé to proposer– proposer is rejected

Each girl marries the boy to whom she is last engagedEach girl marries the boy to whom she is last engaged


Does the Traditional Marriage Does the Traditional Marriage Algorithm always produce a Algorithm always produce a

stablestable pairing? pairing?


Does the Traditional Marriage Does the Traditional Marriage Algorithm always produce a Algorithm always produce a

stablestable pairing? pairing?

Wait! There is a more primary

question!


Does TMA always terminate?Does TMA always terminate?

• It might encounter a situation where algorithm does not specify what to do next (core dump error)

• It might keep on going for an infinite number of days


Traditional Marriage AlgorithmTraditional Marriage AlgorithmWhile there is an unmatched boy, do:While there is an unmatched boy, do:• Some unmatched boy proposes to next girl on his listSome unmatched boy proposes to next girl on his list• If girl is unmatched:If girl is unmatched:

– boy & girl get engaged• If girl is matched but prefers boy to her current fiancé:If girl is matched but prefers boy to her current fiancé:

– boy & girl get engaged– previous fiancé becomes unmatched

• If girl is matched and prefers fiancé to proposerIf girl is matched and prefers fiancé to proposer– proposer is rejected

Each girl marries the boy to whom she is last engagedEach girl marries the boy to whom she is last engaged


Improvement LemmaImprovement Lemma: If a girl is : If a girl is engaged to a boy, then she will engaged to a boy, then she will always be engaged (or married) to always be engaged (or married) to someone at least as good.someone at least as good.

•She would only let go of him in order to get engaged to someone better

•She would only let go of that guy for someone even better


•AND SO ON . . . . . . . . . . . . .


Improvement LemmaImprovement Lemma: If a girl is : If a girl is engaged to a boy, then she will engaged to a boy, then she will always be engaged (or married) to always be engaged (or married) to someone at least as good.someone at least as good.

•She would only let go of him in order to get engaged to someone better



•AND SO ON . . . . . . . . . . . . .

Proof by Induction


LemmaLemma: No boy can be rejected : No boy can be rejected by all the girlsby all the girls

Proof by contradiction.Proof by contradiction.Suppose boy b is rejected by all the Suppose boy b is rejected by all the girls.girls. At that point: At that point:• Each girl must have a suitor other than b

(By Improvement Lemma, once a girl has a suitor she will always have at least one)

• The n girls have n suitors, b not among them. Thus, there are at least n+1 boys

Contradiction


TheoremTheorem: The TMA always : The TMA always terminates in at most nterminates in at most n22 days days

• A “master list” of all n of the boys lists starts with a total of n X n = n2 girls on it.

• Each day that at least one boy gets a “No”, at least one girl gets crossed off the master list

• Therefore, the number of days is bounded by the original size of the master list In fact, since each list never drops below 1, the number of days is bounded by n(n-1) <= n2.


CorollaryCorollary: Each girl will marry : Each girl will marry her absolute favorite of the boys her absolute favorite of the boys

who visit her during the TMAwho visit her during the TMA


Great! We know that TMA will Great! We know that TMA will terminate and produce a pairing.terminate and produce a pairing.

But is it stable?But is it stable?


TheoremTheorem: Let T be the pairing : Let T be the pairing produced by TMA. T is stable.produced by TMA. T is stable.

gg bbgg**



gg bb

I rejected you when you I rejected you when you came to my balcony, came to my balcony, now I got someone now I got someone

better.better.

gg**



• Let b and g be any couple in T.• Suppose b prefers g* to g. We will argue

that g* prefers her husband to b.• During TMA, b proposed to g* before he

proposed to g. Hence, at some point g* rejected b for someone she preferred. By the Improvement lemma, the person she married was also preferable to b.

• Thus, every boy will be rejected by any girl he prefers to his wife. T is stable.


Opinion PollOpinion Poll

Who is better off

in traditional

dating, the boys

or the girls?


Forget TMA for a momentForget TMA for a moment

How should we define what How should we define what we mean when we say “the we mean when we say “the optimal girl for boy b”?optimal girl for boy b”?

Flawed Attempt:Flawed Attempt: “The girl at the top of b’s “The girl at the top of b’s

list”list”


The Optimal GirlThe Optimal Girl

A boy’s A boy’s optimal girloptimal girl is the highest is the highest ranked girl for whom there is ranked girl for whom there is somesome stable pairing in which the boy gets stable pairing in which the boy gets

her.her.

She is the She is the bestbest girl he can conceivably girl he can conceivably get in a stable world. Presumably, she get in a stable world. Presumably, she might be better than the girl he gets might be better than the girl he gets in the stable pairing output by TMA.in the stable pairing output by TMA.


The Pessimal GirlThe Pessimal Girl

A boy’s A boy’s pessimal girlpessimal girl is the lowest is the lowest ranked girl for whom there is ranked girl for whom there is somesome stable pairing in which the boy gets stable pairing in which the boy gets

her.her.

She is the She is the worstworst girl he can girl he can conceivably get in a stable world. conceivably get in a stable world.


Dating Heaven and HellDating Heaven and Hell

A pairing is A pairing is male-optimalmale-optimal if if every every boy gets boy gets his his optimaloptimal mate. This is the best of all mate. This is the best of all possible stable worlds for every boy possible stable worlds for every boy simultaneously.simultaneously.

A pairing is A pairing is male-pessimalmale-pessimal if if every every boy boy gets his gets his pessimalpessimal mate. This is the worst mate. This is the worst of all possible stable worlds for every boy of all possible stable worlds for every boy simultaneously.simultaneously.


Dating Heaven and HellDating Heaven and Hell

Does a male-optimal pairing always Does a male-optimal pairing always exist? exist?

If so, is it stable?If so, is it stable?


The Naked The Naked Mathematical Mathematical

Truth!Truth!The Traditional Marriage The Traditional Marriage

Algorithm always Algorithm always produces a produces a male-optimalmale-optimal, , female-pessimalfemale-pessimal pairing. pairing.

Other issues…Other issues…

• What if the lists are partial?What if the lists are partial?• What if people lie?What if people lie?• What about same-sex couples, What about same-sex couples,

or pairing roommates?or pairing roommates?



Advice to femalesAdvice to females

Learn to make the first move. Learn to make the first move.

Conclusions…Conclusions…


REFERENCESREFERENCES

D. Gale and L. S. Shapley, D. Gale and L. S. Shapley, College College admissions and the stability of admissions and the stability of marriagemarriage, American Mathematical , American Mathematical Monthly 69 (1962), 9-15 Monthly 69 (1962), 9-15

Dan Gusfield and Robert W. Irving, Dan Gusfield and Robert W. Irving, The The Stable Marriage Problem: Structures Stable Marriage Problem: Structures and Algorithmsand Algorithms, MIT Press, 1989, MIT Press, 1989

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