Large cardinals

Weakly compact cardinal

In mathematics, a weakly compact cardinal is a certain kind of cardinal number introduced by ; weakly compact cardinals are large cardinals, meaning that their existence cannot be proven from the standard axioms of set theory. (Tarski originally called them "not strongly incompact" cardinals.) Formally, a cardinal κ is defined to be weakly compact if it is uncountable and for every function f: [κ] 2 → {0, 1} there is a set of cardinality κ that is homogeneous for f. In this context, [κ] 2 means the set of 2-element subsets of κ, and a subset S of κ is homogeneous for f if and only if either all of [S]2 maps to 0 or all of it maps to 1. The name "weakly compact" refers to the fact that if a cardinal is weakly compact then a certain related infinitary language satisfies a version of the compactness theorem; see below. Every weakly compact cardinal is a reflecting cardinal, and is also a limit of reflecting cardinals. This means also that weakly compact cardinals are Mahlo cardinals, and the set of Mahlo cardinals less than a given weakly compact cardinal is stationary. (Wikipedia).

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Related pages

If and only if | Predicate (mathematical logic) | Mahlo cardinal | Unfoldable cardinal | Compactness theorem | Tree (set theory) | List of large cardinal properties | Large cardinal | Cardinal number | Mathematics | Set (mathematics) | Reflecting cardinal | Homogeneous (large cardinal property) | Inaccessible cardinal | Strongly compact cardinal | Critical point (set theory) | Stationary set | Cardinality | Transitive set