# Re: Paper and slides on indefiniteness of CH

Dear Harvey,

Maybe I can be of some help with the set theory questions (Hugh, feel free to correct or amplify what I say):

On Wed, 15 Oct 2014, Harvey Friedman wrote:

Hugh wrote:

Suppose $M$ is a ctm and $M \vDash \textsf{ZFC}$. Must $M$ have an outer model of $\textsf{ZFC}$ which is cardinal preserving and not a set forcing extension? Number Theory is full of problems which seem completely out of reach. And one can easily generate such problems in a foundational guise.

I have a couple of questions about Hugh’s question.

1. The question as formulated involves both forcing extensions and general (outer) extensions. Are there appropriate formulations that

a. Do not refer to forcing at all.

Here is one (but it is cheating, and I’ll explain why below): Does every ctm $M$ of ZFC have an outer model $N$ of ZFC with the following covering property? For some cardinal $\kappa$ of $N$, every function $f\in N$ (with ordinal domain) is covered by a multi-valued function $g \in M$ with the same domain and at most $\kappa$ values? (I.e., for each $\alpha$ in the domain of $f$, $g(\alpha)$ has size at most $\kappa$ and $f(\alpha)$ is an element of $g(\alpha)$).

This is “cheating” because I have just used a theorem of Bukovsky to reformulate “set-generic extension” in terms of a covering property!

b. Given any ctm $M$, refer only to forcing extensions.

Here is one: Does every $M$ have a cardinal-preserving forcing extension which adds a new set but no new real? This was first asked by Matt Foreman. I think it is still open.

2. What are the known natural conditions on $M$ that are known to be sufficient? Also for variants in 1 above.

If the GCH holds at some infinite cardinal $\kappa$ in M then one can add a new subset of $\kappa^+$ without adding a new subset of kappa and without collapsing cardinals. If the GCH holds at unboundedly many cardinals in $M$ then $M$ has a cardinal-preserving extension which is not a set-generic extension.

So the difficulty is with models $M$ in which the GCH fails at all sufficiently large cardinals.

Best,
Sy