Square principle explained

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. As such they may be viewed as a kind ofincompactness phenomenon.^[1] They were introduced by Ronald Jensen in his analysis of the fine structure of the constructible universe L.

Definition

Define Sing to be the class of all limit ordinals which are not regular. Global square states that there is a system

(C _{\beta)} _{\beta} satisfying:C _{\beta} is a club set of \beta .

1. ot

(C _{\beta)} < \beta

1. If

\gamma is a limit point of C _{\beta} then \gamma \in Sing and C _{\gamma} = C _{\beta} \cap \gamma

Variant relative to a cardinal

Jensen introduced also a local version of the principle.^[2] If

\kappa is an uncountable cardinal, then \Box _{\kappa} asserts that there is a sequence (C _{\beta\mid\beta} alimitpointof \kappa ^{+)} satisfying:C _{\beta} is a club set of \beta .

1. If

cf \beta < \kappa , then |C _{\beta|} < \kappa

1. If

\gamma is a limit point of C _{\beta} then C _{\gamma} = C _{\beta} \cap \gamma

Jensen proved that this principle holds in the constructible universe for any uncountable cardinal κ.

Notes and References

1. Section 4.
2. , p. 443.