Difference between revisions of "Worldly"
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{{DISPLAYTITLE: Worldly cardinal}} | {{DISPLAYTITLE: Worldly cardinal}} | ||
| − | + | A cardinal $\kappa$ is ''worldly'' if $V_\kappa$ is a model of $\text{ZFC}$. It follows that $\kappa$ is a [[strong limit]], a [[beth fixed point]] and a fixed point of the enumeration of these, and more. | |
| − | A cardinal $\kappa$ is ''worldly'' if $V_\kappa$ is a model of ZFC. It follows that $\kappa$ is a [[strong limit]], a [[beth fixed point]] and a fixed point of the enumeration of these, and more. | + | |
* Every [[inaccessible]] cardinal is worldly. | * Every [[inaccessible]] cardinal is worldly. | ||
Revision as of 13:57, 11 November 2017
A cardinal $\kappa$ is worldly if $V_\kappa$ is a model of $\text{ZFC}$. It follows that $\kappa$ is a strong limit, a beth fixed point and a fixed point of the enumeration of these, and more.
- Every inaccessible cardinal is worldly.
- Nevertheless, the least worldly cardinal is singular and hence not inaccessible.
- The least worldly cardinal has cofinality $\omega$.
- Indeed, the next worldly cardinal above any ordinal, if any exist, has cofinality $\omega$.
- Any worldly cardinal $\kappa$ of uncountable cofinality is a limit of $\kappa$ many worldly cardinals.
Degrees of worldliness
A cardinal $\kappa$ is $1$-worldly if it is worldly and a limit of worldly cardinals. More generally, $\kappa$ is $\alpha$-worldly if it is worldly and for every $\beta\lt\alpha$, the $\beta$-worldly cardinals are unbounded in $\kappa$. The cardinal $\kappa$ is hyper-worldly if it is $\kappa$-worldly. One may proceed to define notions of $\alpha$-hyper-worldly and $\alpha$-hyper${}^\beta$-worldly in analogy with the hyper-inaccessible cardinals. Every inaccessible cardinal $\kappa$ is hyper${}^\kappa$-worldly, and a limit of such kinds of cardinals.
The worldly cardinal terminology was introduced in lectures of J. D. Hamkins at the CUNY Graduate Center and at NYU.
