Constructible hierarchy: Difference between revisions
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How is the reflection principle stated for general cumulative hierarchies?
RhubarbJayde (talk | contribs) |
(How is the reflection principle stated for general cumulative hierarchies?) |
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* For limit \(\alpha\), \(L_\alpha\) is the union of \(L_\beta\) for \(\beta < \alpha\).
Note that this is a cumulative hierarchy, and thus the [[reflection principle]] applies.{{citation needed}}
This is always contained in the respective rank of the von Neumann hierarchy: \(L_\alpha \subseteq V_\alpha\). This can be shown by a transfinite induction argument. It initially completely actually agrees with \(V\): all subsets of a finite set are definable, therefore \(L_\alpha = V_\alpha\) for \(\alpha \leq \omega\). However, while \(V_{\omega+1}\) is uncountable, there are (as we mentioned) only countably many subsets of a countable subset, and thus \(L_{\omega+1}\) is countable and a proper subset of \(V_{\omega+1}\). In general, \(|L_\alpha| = |\alpha|\) for \(\alpha \geq \omega\).
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