Difference between revisions of "High-jump"

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Latest revision as of 03:53, 13 September 2017

High-jump cardinals are a certain kind of large cardinals. A cardinal $\kappa$ is high-jump if it is the critical point of an elementary embedding $j:V\to M$ such that $M$ is closed under sequences of length $sup\{j(f)(\kappa)|f:\kappa\to\kappa\}$. This closure condition is a weakening of the definition of a huge cardinal.

Definition

Let $j:M\to N$ be a (nontrivial) elementary embedding. The clearance of $j$ is the ordinal $sup\{j(f)(\kappa)|f:\kappa\to\kappa\}$ where $\kappa$ is the critical point of $j$.

A cardinal $\kappa$ is high-jump if there exists $j:V\to M$ with critical point $\kappa$ and clearance $\theta$ such that $M^\theta\subset M$, i.e. $M$ contains all sequences of elements of $M$ of length $\theta$. $j$ is called a high-jump embedding, and a normal fine ultrafilter on some $P_\kappa\lambda$ generating an ultrapower embedding that is high-jump is high-jump ultrafilter (or high-jump measure).

$\kappa$ is called almost high-jump if $M$ is closed under sequences of length $<\theta$ instead, i.e. $M^\lambda\subset M$ for all $\lambda<\theta$. $j$ is then an almost high-jump embedding. This means that for all $f:\kappa\to\kappa$, $M^{j(f)(\kappa)}\subset M$. Shelah for supercompactness cardinals are a natural weakening of almost high-jump cardinals which allows to have one embedding per $f:\kappa\to\kappa$ rather than a single embedding for all such $f$s.

$\kappa$ is high-jump order $\eta$ (resp. almost high-jump order $\eta$) if there exists a strictly increasing sequence of ordinals $\{\theta_\alpha|\alpha<\eta\}$ such that for all $\alpha<\eta$, there exists a high-jump embedding (resp. almost high-jump embedding) with critical point $\kappa$ and clearance $\theta_\alpha$. $\kappa$ is super high-jump (resp. super almost high-jump) if there are high-jump embeddings (resp. almost high-jump embeddings) with arbtirarily large clearance (i.e. it is "(almost) high-jump order $Ord$").

A high-jump cardinal $\kappa$ has unbounded excess closure if for some clearance $\theta$, for all cardinals $\lambda\geq\theta$, there is a high-jump measure on $P_\kappa\lambda$ generating an embedding with clearance $\theta$.

Properties

Let $j:V\to M$ a nontrivial elementary embedding with critical point $\kappa$ and clearance $\theta$. Then:

  • There is no $f:\kappa\to\kappa$ such that $j(f)(\kappa)=\theta$.
  • $\kappa^{+}\leq cf(\theta)\leq 2^\kappa$ (see cofinality)
  • $\beth^M_\theta=\theta$
  • $M_\theta\prec M_{j(\kappa)}$
  • $M_\theta\models ZFC$

The following holds when $\kappa$ is almost high-jump, and holds in both $V$ and $M$:

  • $\theta^\kappa=\theta$
  • $\theta$ is singular.
  • $V_\theta\prec M_{j(\kappa)}$
  • $V_\theta\models ZFC$

The following statements also holds:

  • Suppose there is a almost high-jump cardinal. Then there are many cardinals below it that are Shelah for supercompactness. Also, in the model $V_\kappa$ there are many supercompact cardinals.
  • Every high-jump cardinal is almost high-jump, and has order $\theta$; in fact, in the models $V_\theta$, $V_\kappa$ and $M_{j(\kappa)}$ there are many super almost high-jump cardinals.
  • The existence of a high-jump cardinal with order $\eta$ implies that for every $\gamma<\eta$, there exists a model in which that cardinal is high-jump with order $\gamma$. The same statement holds for almost high-jump cardinals.
  • The existence of a high-jump cardinal with unbounded excess closure is equiconsistent with the existence of a cardinal $\kappa$ such that for all sufficiently large $\lambda$, there exists a high-jump measure on $P_\kappa\lambda$.
  • Suppose $\kappa$ is almost huge; then in the model $V_\kappa$ there are many cardinals that are high-jump with unbounded excess closure.
  • Suppose that there exists a pair of cardinals ($\kappa$, $\theta$) such that there is a high-jump embedding $j:V\to M$ with critical point $\kappa$ and clearance $\theta$ and such that $M^{2^\theta}\subset M$. Then the cardinal $\kappa$ is super high-jump in the model $V_\theta$, and the cardinal $\kappa$ has high-jump order $\theta$ in $V$. Furthermore, there are many super-high-jump cardinals in the models $V_\kappa$, $V_\theta$, and $M_{j(\kappa)}$.
  • The least high-jump cardinal is not $\Sigma_2$-reflecting. In particular, it is not supercompact and not even strong. The same is true for the least almost-huge cardinal, the least almost-high-jump cardinal, and the least Shelah-for-supercompactness cardinal.

Name origin

Read More

  • Norman Lewis Perlmutter, The large cardinals between supercompact and almost-huge [1]


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