# Berkeley

A cardinal $\kappa$ is a **Berkeley** cardinal, if for any transitive set $M$ with $\kappa\in M$ and any ordinal $\alpha<\kappa$ there is an elementary embedding $j:M\to M$ with $\alpha<\text{crit }j<\kappa$. These cardinals are defined in the context of ZF set theory without the axiom of choice.

The Berkeley cardinals were defined by W. Hugh Woodin in about 1992 at his set-theory seminar in Berkeley, with J. D. Hamkins, A. Lewis, D. Seabold, G. Hjorth and perhaps R. Solovay in the audience, among others, issued as a challenge to refute a seemingly over-strong large cardinal axiom. Nevertheless, the existence of these cardinals remains unrefuted in ZF.

If there is a Berkeley cardinal, then there is a forcing extension that forces that the least Berkeley cardinal has cofinality $ω$. It seems that various strengthenings of the Berkeley property can be obtained by imposing conditions on the cofinality of $\kappa$ (the larger cofinality, the stronger theory, up to regular $\kappa$).[1]

We call $\kappa$ a **club Berkeley** cardinal if $\kappa$ is regular and for all clubs $C ⊆ \kappa$ and all transitive sets $M$ with $\kappa ∈ M$ there is $j ∈ \mathcal{E}(M)$ with
$\mathrm{crit}(j) ∈ C$.[1]

We call $\kappa$ a **limit club Berkeley** cardinal if it is a club Berkeley cardinal and a limit of Berkeley cardinals.[1]

## Relations

If $\kappa$ is the least Berkeley cardinal, then there is $\gamma\lt\kappa$ such that $(V_\gamma , V_{\gamma+1})\vDash\mathrm{ZF}_2 + \text{“There is a Reinhardt cardinal witnessed by $j$ and an $\omega$-huge above $\kappa_\omega(j)”$}$.[1]

Each club Berkeley cardinal is totally Reinhardt.[1]. The relation between Berkeley cardinals and club Berkeley cardinals is unknown.[1]

If $\kappa$ is a limit club Berkeley cardinal, then $(V_\kappa , V_{\kappa+1})\vDash\text{“There is a Berkeley cardinal that is super Reinhardt”}$.[1] Moreover, the class of such cardinals are stationary.

## References

Main library