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Mathematics · unit version 1

Matching completions, families and unequal sides

Prove stability under unknown comparisons, count independent families of stable outcomes, and adapt proposal invariants to unequal populations.

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Readiness and preparation

Check your starting point

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Does one participant preferring another partner make a blocking pair?

Unknown comparisons must be checked at both endpoints.

Compare reasoning and find a refresher

No. Both participants must prefer one another to their current partners, or to being unmatched when the model permits it.

Review blocking pairs and model assumptions

Why may a receiver replace a provisional partner?

Unequal populations still need the best-seen proposal invariant.

Compare reasoning and find a refresher

The receiver keeps its favorite proposal so far, so its held partner never gets worse. Rejected proposers then try their next untried choice.

Review deferred acceptance and its invariants

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Audit the quantifiers and matching assumptions

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Lesson sequence

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  1. Certify stability when preference comparisons are incomplete

    Lesson details and objectives

    Use reachability in consistent comparison orders to determine whether a fixed matching stays stable under every independent completion.

    • Distinguish unknown comparisons, unacceptable partners, inconsistent reports and independent strict complete extensions.
    • Prove the reachability criterion for forced preferences and the pairwise criterion for universal stability.
    • Construct blocking completions when a certificate fails and separate possible stability from necessary stability.

    6 practice tasks.

  2. Construct and count independent stable-matching choices

    Lesson details and objectives

    Use strict preference barriers to combine local outcomes, exclude cross-block matches and prove an exact exponential family count.

    • Prove local two-by-two stability and combine independent choices without cross-block blockers.
    • Prove every stable matching stays within balanced preference blocks and derive an exact count.
    • Handle odd-size extensions and diagnose failed barriers without generalizing the count to every preference profile.

    6 practice tasks.

  3. Deferred acceptance when the two sides have different sizes

    Lesson details and objectives

    Allow exhausted proposers, prove stability and saturation with complete acceptable lists, and locate the boundary when lists become incomplete.

    • Trace proposals with unequal finite sides and distinguish free, exhausted and provisionally held participants.
    • Prove the proposal bound, terminal stability and exactly min(p,r) pairs under complete acceptable strict lists.
    • Handle empty sides, reversed roles and incomplete-list counterexamples without claiming arbitrary unmatched identities.

    6 practice tasks.