In mathematical set theory, a square principle is a combinatorial principle asserting the existence of a cohering sequence of short closed unbounded (club) sets so that no one (long) club set coheres with them all.
[1] They were introduced by Ronald Jensen in his analysis of the fine structure of the constructible universe L. Define Sing to be the class of all limit ordinals which are not regular.
Global square states that there is a system
satisfying: Jensen introduced also a local version of the principle.
satisfying: Jensen proved that this principle holds in the constructible universe for any uncountable cardinal
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