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Mathematics · unit version 1

Set specifications and language-expression limits

Turn informal requirements into quantified membership rules, construct languages, and prove the limits of a precisely stated expression grammar.

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Readiness and preparation

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Does a separate witness for each set give one witness shared by every set?

Membership specifications depend on the order and scope of their quantifiers.

Compare reasoning and find a refresher

No. Two disjoint nonempty sets each have a member but have no common member. State which choices a witness may depend on.

Review quantifier order and witnesses

What must a structural-induction proof check?

An expression-limit theorem must cover every permitted syntax constructor.

Compare reasoning and find a refresher

Prove the property for each base expression, then prove every constructor preserves it assuming the property for each arbitrary child. Checking finitely many expressions cannot replace these cases.

Review constructor cases and induction

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Specify the filter and prove its expressive limits

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Lesson sequence

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  1. Specify membership and construct language expressions

    Lesson details and objectives

    Translate nested-set requirements into predicates and justify language constructions through word membership and complete split cases.

    • Write typed membership-only specifications and distinguish separate existential witnesses from one uniform witness.
    • Prove set equality by reversible membership conditions and handle empty collections and empty words explicitly.
    • Construct and verify finite language expressions using only a declared alphabet, primitives and operations.

    6 practice tasks.

  2. Prove a limit of a language-expression grammar

    Lesson details and objectives

    Use finite and cofinite length sets as a structural invariant of a small log-filter language, then distinguish expressibility from decidability.

    • Prove closure of finite/cofinite unary languages under every permitted constructor, with all concatenation cases.
    • Use structural induction to establish nondefinability rather than extrapolating from finite tests.
    • Prove a converse expressibility result separately and analyze how adding star changes the grammar.

    6 practice tasks.