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Tuesday, October 28, 2014

Spencinar #3: Saturated ideals (part 2)

Now we'll construct an \aleph_2-saturated ideals on \omega_1. The construction is originally due to Kunen.

The idea is similar to the previous construction. We will look at the ideal induced by a large cardinal embedding j after a collapsing the large cardinal \kappa to become \omega_1. We know the quotient algebra is equivalent to the image of the collapse, so we hope that this is \kappa^+-c.c. To do this requires a huge cardinal, which gives a situation where where j(\kappa) is also the amount of closure of the target model in V. We want to collapse to turn j(\kappa) into \kappa^+, but in order to have a master condition to extend the embedding to the extension by this collapse, we cannot use the Levy collapse. Instead, we will use the Silver collapse. The first poset collapsing \kappa to become \omega_1 must absorb the Silver collapse and so also cannot be the Levy collapse: we will construct a poset which will fulfill this purpose by design.

(Drinking game: consume a designated amount of an alcoholic beverage every time you read the word "collapse" in this post...)

Definition: The Silver collapse S(\kappa,\lambda) is the forcing whose elements are partial functions p: \kappa\times \lambda\rightarrow\kappa with domain of size \le \kappa so that there is some \eta<\kappa so that the domain is contained in \eta\times \lambda, and for all (\alpha,\beta) in the domain, p(\alpha,\beta)<\beta.

Like the Levy collapse \mathrm{Col}(\kappa,<\lambda), the Silver collapse adds surjections from \kappa to everything less than \lambda with uniformly bounded domain, but we are allowed to have some conditions with support of size equal to \kappa. It is \lambda-c.c., <\kappa-closed, and every regular cardinal of the ground model between \kappa and \lambda is collapsed with cofinality \kappa.

Start with a huge cardinal \kappa and a huge embedding j:V\rightarrow M with critical point \kappa so that j(\kappa)=\lambda and M is closed under \lambda-sequences. We will define a "universal collapse" \mathbb{P} with the \kappa-c.c. which will turn \kappa into \omega_1, and then use the Silver collapse in the extension to turn \lambda into \omega_2.

The crucial property of \mathbb{P} is the following: if \mathbb{Q} completely embeds into \mathbb{P} and has inaccessible cardinality \alpha, then there is a complete embedding i:\mathbb{Q}\ast S(\alpha,\kappa)\rightarrow \mathbb{P} extending the embedding of \mathbb{Q}. Thus, we are preparing \mathbb{P} to absorb the Silver collapse.

The construction of \mathbb{P} is an iteration with finite support starting with 0th stage \mathrm{Col}(\omega,<\kappa), and at inaccessible stages \alpha, forcing with the Silver collapse of V^\mathbb{Q} for some poset \mathbb{Q} which completely embeds into \mathbb{P}_\alpha. By a suitable bookkeeping, we can achieve the crucial property. By the chain condition of the Silver collapse, it is easy to see that \mathbb{P} has the \kappa-c.c.

Since \mathbb{P} is \kappa-c.c., \mathbb{P} completely embeds into j(\mathbb{P}) via j (any maximal antichains of \mathbb{P} have size <\kappa, so they are fixed by j and remain maximal in j(\mathbb{P})).

So, letting G be generic for \mathbb{P} over V, we can force further to obtain \hat{G} which is V[G]-generic for j(\mathbb{P}) and extend j to j^+:V[G]\rightarrow M[\hat{G}]. Note that M[\hat{G}] is \lambda-closed in V[\hat{G}].

Therefore using the crucial property of j(\mathbb{P}), there is a complete embedding i:\mathbb{P}\ast S(\kappa,\lambda)\rightarrow j(\mathbb{P}) extending j restricted to \mathbb{P}. Let \mathbb{Q} be this Silver collapse, and let H\in M[\hat{G}] be the generic for \mathbb{Q} over V[G] induced by \hat{G} via the complete embedding (using closure of M[\hat{G}]. We can extend the embedding j^+ to V[G\ast H], since \bigcup j``H is a condition in j(\mathbb{Q}). This is where we used the Silver collapse--for the Levy collapse, \bigcup j``H would have too large of a domain, it does not interact well with the huge embedding. The usual large cardinal techniques allow us to extend to \hat{j}:V[G\ast H]\rightarrow M[\hat{G}\ast\hat{H}].

Now consider the V[G\ast H]-ultrafilter U(\hat{j},\kappa) (defined in V[\hat{G}\ast\hat{H}]). We can use the <\lambda-closure of j(\mathbb{Q}) to find a \tilde{U} in V[\hat{G}] which is a V[G\ast H]-ultrafilter, normal, and V[G\ast H]-\kappa-complete. The construction works by taking a decreasing \lambda-sequence of conditions which together decide \kappa\in \hat{j}(X) for every X in V[G\ast H], and additionally meeting dense sets which ensure that V[G\ast H]-sequences of fewer than \kappa-many of such X which are forced in to be U(\hat{j},\kappa) will also be in the ultrafilter.

In V[G\ast H], \kappa=\omega_1 and \lambda=\omega_2. Let I=\{X\subseteq \kappa: 1\Vdash X\in \dot{\tilde{U}}\}. Then it can be checked that I is normal and \kappa-complete, and since (using similar arguments as in Part 1) j(\mathbb{P})/G\ast H is \lambda-c.c. and equivalent to the quotient algebra of I, I is \aleph_2-saturated.

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