2
$\begingroup$

Definition: A poset P is $\mathbf{stationary}$ $\kappa^{+}$-$\mathbf{linked}$ if for every sequence of conditions $(p_{\gamma} | \gamma < \kappa^{+})$, there is a regerssive function $f: \kappa^{+} \to \kappa^{+}$ (i.e. $f(\gamma) < \gamma$, for $\gamma \neq 0$), such that for some club $C \subseteq \kappa^{+}$ and for all $\gamma, \delta \in C$ with cofinality $\kappa$, $f(\gamma) = f(\delta)$ implies $p_{\gamma}, p_{\delta}$ are compatible.

Construction of the poset: Let V be a model of $SMP_{2}(\omega_{\alpha + 1})$ ($\alpha$ is some fixed countable ordinal). That implies (We do not need the exact definition of SMP, it just gives us the next statements):

  1. $GMA_{\omega_{\alpha + 1}}$ (I will not give definition for $GMA_{\omega_{\alpha + 1}}$, since we do not need it. In fact we need to prove the stationary $\aleph_{\alpha + 2}$-linked in order to use GMA)

  2. $2^{\aleph_{\alpha}} = \aleph_{\alpha + 1}$

  3. $2^{\aleph_{\alpha + 1}}$ is weakly inaccessible

Now let $\aleph_{\alpha + 2} \leq \lambda < 2^{\aleph_{\alpha + 1}}$ and P be the poset consists of conditions of the form $(t, f)$, where

  1. $t$ is a tree of height $\beta + 1$ for some $\beta < \omega_{\alpha + 1}$ and levels at most of size $\aleph_{\alpha}$

  2. $f$ is a function with $dom(f) \subseteq \lambda$, $|dom(f)| = \aleph_{\alpha}$ and $ran(f) = t_{\beta}$, where $t_{\beta}$ is the $\beta$th level of the tree $t$.

The order is defined as follows: $(u, g) \leq (t, f)$ iff

  1. $t$ is an initial segment of $u$, $dom(f) \subseteq dom(g)$,

  2. for every $\delta \in dom(f)$, either $f(\delta) = g(\delta)$ (if $t$ and $u$ have the same height) or $f(\delta) <_{u} g(\delta)$ otherwise.

This is a forcing notion that adds an $\aleph_{\alpha + 1}$-Kurepa tree with $\lambda$ cofinal branches.

I have already proved that it is $< \aleph_{\alpha + 1}$-closed and has the $\aleph_{\alpha + 2}$ chain condition. In order to prove the $\aleph_{\alpha + 2}$ chain condition I noticed that we have only $2^{\aleph_{\alpha}} = \aleph_{\alpha + 1}$-many possibilities for the first coordinate and then applied a $\Delta$-system Lemma on the domains of the second coordinate.

Question: Is P stationary $\aleph_{\alpha + 2}$-linked?

There is a Theorem for the case that $\alpha = 0$, which claims that poset is stationary $\aleph_{2}$-linked, but tells nothing more in the proof. So, I believe the answer is yes, but I cannot see how to prove it

$\endgroup$
12
  • $\begingroup$ You should define SMP and GMA. $\endgroup$ Commented Oct 15 at 13:56
  • $\begingroup$ I think it is not necessary. SMP gives us the three statements I have written. This is what we need and the exact definition is complex. As for GMA, we won't use it. In fact we need to prove the stationary $\aleph_{\alpha + 2}$-linked, in order to use GMA. $\endgroup$ Commented Oct 15 at 14:05
  • $\begingroup$ Pretend for a second that $\lambda = \aleph_{\alpha+2}$. Is the claim clear? $\endgroup$ Commented Oct 15 at 16:46
  • $\begingroup$ No, I cannot see why the claim holds if $\lambda = \aleph_{\alpha + 2}$. I would appreciate if you could explain it to me. $\endgroup$ Commented Oct 15 at 17:02
  • 1
    $\begingroup$ So you take a club $C$ on which for $\alpha<\beta$ from $C$, $\beta$ is above the domain of $p_\alpha$. This is how you reduce compatibility to the lower parts. For the actual range, you need to enumerate your tree forcing and relevant subsets like I said. Sorry I don’t feel like giving more details. Please look up a proof of the stationary chain condition for Cohen forcing to get an idea. $\endgroup$ Commented Oct 17 at 9:57

0

You must log in to answer this question.

Browse other questions tagged .