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    Σ¹₁-validity is itself not recursively enumerable, as est... — Carmelics
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    Challenges→Checking the validity of an arbitrary second-order sentence φ can be recursively reduced to checking the validity of a Σ¹₁-sentence.

    Σ¹₁-validity is itself not recursively enumerable, as established by results tracing to Gödel and elaborated by Kreisel, so the reduction preserves undecidability rather than achieving any effective proof-theoretic gain.

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    Key Terms

    Gödel(as a historical figure in mathematical logic)
    Kurt Gödel was a 20th-century mathematician and logician who proved that any consistent formal system (a set of logical rules) is incomplete—meaning there are true statements it can't prove.
    Kreisel(as a historical figure in logic and philosophy of mathematics)
    Georg Kreisel (1923–2015), a mathematical logician who studied how mathematical reasoning works and whether we can be certain about mathematical truths.
    Recursively enumerable(Consequence of first-order completeness under constructive Gödel numbering)
    A set is recursively enumerable if it can be generated by an effective procedure; here applied to the set of codes of valid sentences when formulas are coded as natural numbers constructively.
    Undecidability(Wittgenstein's usage in PR §174, alluding to algorithmic decidability)
    The condition in which a bridge (connection) between symbols cannot be made with symbols.

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    proof-theoretic gain(what the reduction fails to achieve)
    An improvement in our ability to actually prove things or find answers more efficiently using formal logical methods.
    reduction(Lambda calculus or term-rewriting systems)
    A computational process analogous to computing the value of a function, proceeding through a series of discrete calculation steps applied to a term
    Σ¹₁-validity(the main subject being discussed)
    A technical category in mathematical logic that describes a certain type of logical statement or formula. It's a way of classifying statements based on their complexity and how they're structured.

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    2 topics

    Proof of definition segments1 linkedTruth & Knowledge1 linked

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