Supercompact cardinal

In set theory, a supercompact cardinal is a type of large cardinal. They display a variety of reflection properties.

Formal definition

If λ is any ordinal, κ is λ-supercompact means that there exists an elementary embedding j from the universe V into a transitive inner model M with critical point κ, j(κ)>λ and

${}^{\lambda }M\subseteq M\,.$

That is, M contains all of its λ-sequences. Then κ is supercompact means that it is λ-supercompact for all ordinals λ.

Alternatively, an uncountable cardinal κ is supercompact if for every A such that |A| ≥ κ there exists a normal measure over [A]^{< κ}, in the following sense.

[A]^{< κ} is defined as follows:

$[A]^{{<\kappa }}:=\{x\subseteq A||x|<\kappa \}\,.$

An ultrafilter U over [A]^{< κ} is fine if it is κ-complete and $\{x\in [A]^{{<\kappa }}|a\in x\}\in U$, for every $a\in A$. A normal measure over [A]^{< κ} is a fine ultrafilter U over [A]^{< κ} with the additional property that every function $f:[A]^{{<\kappa }}\to A$ such that $\{x\in [A]^{{<\kappa }}|f(x)\in x\}\in U$ is constant on a set in $U$. Here "constant on a set in U" means that there is $a\in A$ such that $\{x\in [A]^{{<\kappa }}|f(x)=a\}\in U$.

Properties

Supercompact cardinals have reflection properties. If a cardinal with some property (say a 3-huge cardinal) that is witnessed by a structure of limited rank exists above a supercompact cardinal κ, then a cardinal with that property exists below κ. For example, if κ is supercompact and the Generalized Continuum Hypothesis holds below κ then it holds everywhere because a bijection between the powerset of ν and a cardinal at least ν⁺⁺ would be a witness of limited rank for the failure of GCH at ν so it would also have to exist below κ.

Finding a canonical inner model for supercompact cardinals is one of the major problems of inner model theory.

See also

Strongly compact cardinal

References

• Drake, F. R. (1974). Set Theory: An Introduction to Large Cardinals (Studies in Logic and the Foundations of Mathematics ; V. 76). Elsevier Science Ltd. ISBN 0-444-10535-2.
• Jech, Thomas (2002). Set theory, third millennium edition (revised and expanded). Springer. ISBN 3-540-44085-2.
• Kanamori, Akihiro (2003). The Higher Infinite : Large Cardinals in Set Theory from Their Beginnings (2nd ed.). Springer. ISBN 3-540-00384-3.