# Math Help - Set theory question.

1. ## Set theory question.

Hi all,

In Kunen's book, one of the exercises is the following:

Show that for $\kappa > \omega$, |H( $\kappa$)| = $2^{< \kappa}$.

But, H( $\omega$) = V( $\omega$) and |V( $\omega$)| = $\omega$. Also, $2^{< \omega}= \omega$ (because $2^{< \omega}$ is the cardinality of the union of omega many finite sets (the characteristic functions for each n)).

Is this wrong?

Sam

2. The notation used in this post is so non-standard, I think that you need to define what each symbol means or how it is used.

3. Ok, so:

$\kappa$ is used as a variable over cardinals.
| $x$| denotes the cardinality of $x$.
$\alpha^{<\beta}$ is the cardinality of the union of $\alpha^{\delta}$ for $\delta < \beta$.
H( $\kappa$) denotes the set of sets which are hereditarily of cardinality $< \kappa$.
V( $\alpha$) is the $\alpha^{th}$ level in the cumulative hierarchy.

Hope this helps.

Please say if I've left anything important out.

Regards
Sam

4. Just a tangential point: the notation used is not non-standard as far as I am aware. Kunen's book is a standard text, which contains all of this notation except V(\alpha). In Kunen R is used instead of V. In most modern treatments, though, V is used. See Von Neumann universe - Wikipedia, the free encyclopedia, for an example.

Regards
Sam