Outward compactness
arxiv(2024)
摘要
We introduce and study a new type of compactness principle for strong logics
that, roughly speaking, infers the consistency of a theory from the consistency
of its small fragments in certain outer models of the set-theoretic universe.
We refer to this type of compactness property as outward compactness, and we
show that instances of this type of principle for second-order logic can be
used to characterize various large cardinal notions between measurability and
extendibility, directly generalizing a classical result of Magidor that
characterizes extendible cardinals as the strong compactness cardinals of
second-order logic. In addition, we generalize a result of Makowsky that shows
that Vopěnka's Principle is equivalent to the existence of compactness
cardinals for all abstract logics by characterizing the principle "Ord is
Woodin" through outward compactness properties of abstract logics.
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