15th SLALM

May 21, 2012

[Edit: Aug. 22, 2012]

The 15th SLALM (Latin American Symposium in Mathematical Logic) was held in Bogotá, June 4-8, 2012. There were also three tutorials preceding the main event, on May 30-June 2. I give one of the tutorials, on Determinacy and Inner model theory, 10:40-12 each day, at the Universidad Nacional. Here is the abstract:

Since the invention of forcing, we know of many statements that are independent of the usual axioms of set theory, and even more that we know are consistent with the axioms (but we do not yet know whether they are actually provable).

These proofs of consistency typically make use of large cardinal assumptions. Inner model theory is the most powerful technique we have developed to analyze the structure of large cardinals. It also allows us to show that the use of large cardinals is in many cases indispensable. For years, the main tool in the development of this area was fine structure theory.

Determinacy (in suitable inner models) is a consequence of large cardinals, and recent work has revealed deep interconnections between determinacy assumptions and the existence of inner models with large cardinals, thus showing that descriptive set theory is also a key tool.

The development of these connections started in earnest with Woodin’s core model induction technique, and has led to what we now call Descriptive inner model theory.

The goal of the mini-course is to give a rough overview of these developments.

I have written a set of notes based on these talks, and will be making it available soon.

In addition, I gave one of the invited talks during the set theory session: Forcing with {\mathbb P}_{\rm max} over models of strong versions of determinacy. Here is the abstract:

Hugh Woodin introduced {\mathbb P}_{\rm max}, a definable poset, and showed that, when forcing with it over L({\mathbb R}) (in the presence of determinacy), one recovers choice, and obtains a model of many combinatorial assertions for which simultaneous consistency was not known by traditional forcing techniques. {\mathbb P}_{\rm max} can be applied to larger models of determinacy. As part of joint work with Larson, Sargsyan, Schindler, Steel, and Zeman, we show how this allows us to calibrate the strength of different square principles.

This is of course related to the paper I discussed recently.

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Square principles in Pmax extensions

May 21, 2012

As mentioned previously, I am part of a SQuaRE (Structured Quartet Research Ensemble), “Descriptive aspects of inner model theory”. The other members of the group are Paul Larson, Grigor Sargsyan, Ralf Schindler, John Steel, and Martin Zeman. We have just submitted our paper Square principles in {\mathbb P}_{\rm max} extensions to the Israel Journal of Mathematics. The paper can be downloaded form my papers page, or from the arXiv.

The forcing {\mathbb P}_{\rm max} was developed by Hugh Woodin in his book The axiom of determinacy, forcing axioms, and the nonstationary ideal. {\mathbb P}_{\rm max} belongs to L({\mathbb R}) and, if determinacy holds, the theory that it forces is combinatorially rich, and we do not currently know how to replicate it with traditional forcing methods.

In particular, Woodin showed that, starting with a model of {\sf AD}_{\mathbb R}+“\Theta is regular”, a strong form of determinacy, the {\mathbb P}_{\rm max} extension satisfies {\sf MM}({\mathfrak c}), the restriction of Martin’s maximum to posets of size at most {}|{\mathbb R}|. It is natural to wonder to what extent this can be extended. In this paper, we study the effect of {\mathbb P}_{\rm max} on square principles, centering on those that would be decided by {\sf MM}({\mathfrak c}^+).

These square principles are combinatorial statements stating that a specific version of compactness fails in the universe, namely, there is a certain tree without branches. They were introduced by Ronald Jensen in his paper on The fine structure of the constructible universe. The most well known is the principle \square_\kappa:

Definition. Given a cardinal \kappa, the principle \square_{\kappa} holds iff there exists a sequence \langle C_{\alpha} \mid \alpha < \kappa^{+} \rangle such that for each \alpha < \kappa^{+},

  1. Each C_{\alpha} is club in \alpha;
  2. For each limit point \beta of C_{\alpha}, C_{\beta} = C_{\alpha} \cap \beta; and
  3. The order type of each C_\alpha is at most \kappa.

For \kappa=\omega this is true, but uninteresting. The principle holds in Gödel’s L, for all uncountable \kappa. It is consistent, relative to a supercompact cardinal, that it fails for all uncountable \kappa. For example, this is a consequence of Martin’s maximum.

Recently, the principle \square(\kappa) has been receiving some attention.

Definition. Given an ordinal \gamma, the principle \square(\gamma) holds iff there exists a sequence \langle C_{\alpha} \mid \alpha <\gamma \rangle such that

  1. For each \alpha < \gamma, each C_{\alpha} is club in \alpha;
  2. For each \alpha<\gamma, and each limit point \beta of C_{\alpha}, C_{\beta} = C_{\alpha} \cap \beta; and
  3. There is no thread through the sequence, i.e., there is no club E\subseteq \gamma such that C_{\alpha} = E \cap \alpha for each limit point \alpha of E.

Using the Core Model Induction technique developed by Woodin, work of Ernest Schimmerling, extended by Steel, has shown that the statement

2^{\aleph_1}=\aleph_2+\lnot\square(\omega_2)+\lnot\square_{\omega_2}

implies that determinacy holds in L({\mathbb R}), and the known upper bounds in consistency strength are much higher.

Here are some of our results: First, if one wants to obtain \lnot\square_{\omega_2} in a {\mathbb P}_{\rm max} extension, one needs to start from a reasonably strong determinacy assumption:

Theorem. Assume {\sf AD}_{\mathbb R}+“\Theta is regular”, and that there is no \Gamma \subseteq \mathcal{P}(\mathbb{R}) such that L(\Gamma, \mathbb{R}) \models“\Theta is Mahlo in {\sf HOD}“. Then \square_{\omega_{2}} holds in the {\mathbb P}_{\rm max} extension.

This results uses a blend of fine structure theory with the techniques developed by Sargsyan on his work on hybrid mice. The assumption cannot be improved since we also have the following, the hypothesis of which are a consequence of  {\sf AD}_{\mathbb R}+V=L({\mathcal P}({\mathbb R}))+“\Theta is Mahlo in {\sf HOD}”.

Theorem. Assume that {\sf AD}^+ holds and that \theta is a limit on the Solovay sequence such that that there are cofinally many \kappa<\theta that are limits of the Solovay sequence and are regular in {\sf HOD}. Then \square_{\omega_2} fails in the {\mathbb P}_{\rm max} extension of {\sf HOD}_{\mathcal{P}_{\theta}(\mathbb{R})}.

Here, {\mathcal P}_\alpha({\mathbb R}) denotes the collection of subsets of {\mathbb R} of Wadge rank less than \alpha. The Solovay sequence, introduced by Robert Solovay in The independence of {\sf DC} from {\sf AD}, is a refinement of the definition of \Theta, the least ordinal \alpha for which there is no surjection f:{\mathbb R}\to\alpha:

Definition. The Solovay sequence if the sequence of ordinals \langle \theta_{\alpha} \mid \alpha \leq \gamma \rangle such that

  1. \theta_{0} is the least ordinal that is not the surjective image of the reals by an ordinal definable function;
  2. For each \alpha < \gamma, \theta_{\alpha + 1} is the least ordinal that is not the surjective image of the reals by a function definable from an ordinal and a set of reals of Wadge rank \theta_{\alpha};
  3. For each limit ordinal \beta \leq \gamma, \theta_{\beta} = \sup\{\theta_{\alpha} \mid \alpha < \beta\}; and
  4. \theta_{\gamma} = \Theta.

The problem with the result just stated is that choice fails in the resulting model. To remedy this, we need to start with stronger assumptions. Still, these assumptions greatly improve the previous upper bounds for the consistency (with {\sf ZFC}) of 2^{\aleph_1}=\aleph_2+\lnot\square(\omega_2)+\lnot\square_{\omega_2}. In particular, we now know that this theory is strictly weaker than a Woodin limit of Woodin cardinals.

Theorem. Assume that {\sf AD}_{\mathbb R} holds, that V = L(\mathcal{P}(\mathbb{R})), and that stationarily many elements \theta of cofinality \omega_{1} in the Solovay sequence are regular in {\sf HOD}. Then in the {\mathbb P}_{\rm max} * {\rm Add}(\omega_{3},1)-extension, \square_{\omega_2} fails.

(The forcing {\rm Add}(\omega_3,1) adds a Cohen subset of \omega_3. This suffices to well-order {\mathcal P}({\mathbb R}), and therefore to force choice. That in the resulting model we also have 2^{\aleph_1}=\aleph_2+\lnot\square(\omega_2) follows from prior work of Woodin.)

Finally, I think I should mention a bit of notation. In the paper, we say that \kappa^+ is square inaccessible iff \square_\kappa fails. We also say that \gamma is threadable iff \square(\gamma) fails. This serves to put the emphasis on the negations of the square principles, which we feel is where the interest resides. It also solves the slight notational inconvenience of calling \square_\kappa a principle that is actually a statement about \kappa^+.

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305 – Projects

May 9, 2012

As mentioned before, I asked my 305 students to write a short paper as a final project. I am posting them here, with their permission; it is my hope that people will find them useful. There are some very nice papers here.

  1. The 17 plane symmetry groups. By Samantha Burns, Courtney Fletcher, and Aubray Zell.
  2. The Banach-Tarski paradox. By Josh Giudicelli, Chantel Kelly, and James Kunz.
  3. The quaternions & octonions. By Kyle McAllister.
  4. The pocket cube. By Mike Mesenbrink, and Nicole Stevenson.
  5. 17 plane symmetry groups. By Anna Nelson, Holly Newman, and Molly Shipley.
  6. The Banach-Tarski paradox and amenability. By Kameryn Williams.

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