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El Problema del Matrimonio Estable El Problema del Matrimonio Estable Roger Z. Ríos Programa de Posgrado en Ing. de Sistemas Facultad de Ing. Mecánica y Eléctrica Universidad Autónoma de Nuevo León Seminario de Clase “Optimización de Flujo en Redes” PISIS – FIME - UANL Cd. Universitaria24 Agosto 2007

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Agenda Problem definition Solution algorithm Variations and extensions Real-world application

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Problem Definition 2 disjoint sets of size n (women, men) Men’s preferencesWomen’s preferences Matching M ={(1,1), (2,3), (3,2), (4,4)} Blocking pair (4,1)

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Problem Definition 2 disjoint sets of size n (women, men) Men’s preferencesWomen’s preferences Matching M ={(1,1), (2,3), (3,2), (4,4)} Blocking pair (4,1)

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Problem Definition 2 disjoint sets of size n (women, men) Men’s preferencesWomen’s preferences Matching M ={(1,1), (2,3), (3,2), (4,4)} Blocking pair (4,1)

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Problem Definition (SMP) Instance of size n (n women, n men) and strictly ordered preference list Matching: 1-1 correspondence between men and women If woman w and man m matched in M w and m are partners, w=p M (m), m=p M (w) (w,m) block a match M if w and m are not partners, but w prefers m to p M (w) and m prefers w to p M (m) A match for which there is at least one blocking pair is unstable

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Solution: Properties Stability checking (for a given matching M) is easy Stable matching existence (not obvious) due to Gale and Shapley (1962)

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Solution: Stability for (m:=1 to n) do for (each w such that m prefers w to p M (m)) do if (w prefers m to p M (w)) then report matching unstable stop endif Report matching stable for (m:=1 to n) do for (each w such that m prefers w to p M (m)) do if (w prefers m to p M (w)) then report matching unstable stop endif Report matching stable Simple stability checking algorithm O(n 2 )

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Solution assign each person to be free while (some man m is free) do { w:=first woman on m’s list to whom m has not yet proposed if (w is free) then assign m and w to be engaged {to each other} else if (w prefers m to her fiance m’) then assign m and w to be engaged and m’ to be free else w rejects m {and m remains free} } end while assign each person to be free while (some man m is free) do { w:=first woman on m’s list to whom m has not yet proposed if (w is free) then assign m and w to be engaged {to each other} else if (w prefers m to her fiance m’) then assign m and w to be engaged and m’ to be free else w rejects m {and m remains free} } end while Basic Gale-Shapley man-oriented algorithm

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Solution: Proof Theorem: For any given SMP instance the G-S algorithm terminates with a stable matching Proof: No man can be rejected by all woman Termination in O(n 2 ) No blocking pairs If m prefers w to p M (m), w must have rejected m at some point for a man she prefers better Theorem: For any given SMP instance the G-S algorithm terminates with a stable matching Proof: No man can be rejected by all woman Termination in O(n 2 ) No blocking pairs If m prefers w to p M (m), w must have rejected m at some point for a man she prefers better

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Solution: Example Men’s preferencesWomen’s preferences Man 1 proposes to woman 4 (accepted) Partial matching M ={(4,1)}

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Solution: Example Men’s preferencesWomen’s preferences Man 1 proposes to woman 4 (accepted) Partial matching M ={(4,1)} Man 2 proposes to woman 2 (accepted) Partial matching M ={(2,2), (4,1)}

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Solution: Example Men’s preferencesWomen’s preferences Man 1 proposes to woman 4 (accepted) Partial matching M ={(4,1)} Man 2 proposes to woman 2 (accepted) Partial matching M ={(2,2), (4,1)}

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Solution: Example Men’s preferencesWomen’s preferences Man 3 proposes to woman 2 (accepted and women 2 rejects man 2) Partial matching M ={(2,3), (4,1)}

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Solution: Example Men’s preferencesWomen’s preferences Man 3 proposes to woman 2 (accepted and women 2 rejects man 2) Partial matching M ={(2,3), (4,1)} Man 2 proposes to woman 3 (accepted) Partial matching M ={(2,3), (3,2), (4,1)}

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Solution: Example Men’s preferencesWomen’s preferences Man 3 proposes to woman 2 (accepted and women 2 rejects man 2) Partial matching M ={(2,3), (4,1)} Man 2 proposes to woman 3 (accepted) Partial matching M ={(2,3), (3,2), (4,1)}

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Solution: Example Men’s preferencesWomen’s preferences Man 4 proposes to woman 3 (rejected, woman 3 prefers man 2) Partial matching M ={(2,3), (3,2), (4,1)}

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Solution: Example Men’s preferencesWomen’s preferences Man 4 proposes to woman 3 (rejected, woman 3 prefers man 2) Partial matching M ={(2,3), (3,2), (4,1)} Man 4 proposes to woman 1 (accepted) Final Matching M ={(1,4), (2,3), (3,2), (4,1)} (Stable matching)

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Solution: Example Men’s preferencesWomen’s preferences Man 4 proposes to woman 3 (rejected, woman 3 prefers man 2) Partial matching M ={(2,3), (3,2), (4,1)} Man 4 proposes to woman 1 (accepted) Final Matching M ={(1,4), (2,3), (3,2), (4,1)} (Stable matching)

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Extensions/Variations Stable Marriage Problem Sets of unequal size Unnacceptable partners Indifference Stable Roommate Problem Stable Resident/Hospital Problem

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Real-World Application Stable Resident/Hospital Problem (SRHP) Largest and best-known application of SMP Used by the National Resident Matching Program (NRMP)

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SRHP Application R (residents), H (hospitals), q i := # of available spots in hospital i (r,h) is a blocking pair if r prefers h to his/her current hospital and h prefers r to at least one of its assigned residents Solution: Transformation into a SMP Specialized algorithm

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SRHP Application History 1 st dilemma: Early proposals 2 nd dilemma: Tight acceptance start -2

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SRHP Application Solution: Transformation into a SMP Specialized algorithm SRHP Instance Residents r 1 (h 1, h 2, h 3, …) … Hospitals h 1 (r 1, r 2, r 3, …), q 1 =3 … SMP Instance Residents r 1 (h 11, h 12, h 13, h 2, h 3, …) … Hospitals h 11 (r 1, r 2, r 3, …) h 12 (r 1, r 2, r 3, …) h 13 (r 1, r 2, r 3, …) …

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Related Hard Problems Finding all different stable matchings Maximum number of stable matchings Parallelization

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Conclusions Stable matchings Gale-Shapley algorithm Math/Computer Science/Economics Scientific support to decision-making

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Questions? Acknowledgements: Racing - Barça El Sardinero This Sunday 12:00 CDT

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