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Matteo Viale

Publications and source records attributed to Matteo Viale.

28 records · Page 2Linked to original sources
arXiv2016

Generic absoluteness and boolean names for elements of a Polish space

It is common knowledge in the set theory community that there exists a duality relating the commutative $C^*$-algebras with the family of $B$-names for complex numbers in a boolean valued model for set theory $V^B$. Several aspects of this correlation have been considered in works of the late $1970$'s and early $1980$'s, for example by Takeuti, and by Jech. Generalizing Jech's results, we extend this duality so as to be able to describe the family of boolean names for elements of any given Polish space $Y$ (such as the complex numbers) in a boolean valued model for set theory $V^B$ as a space $C^+(X,Y)$ consisting of functions $f$ whose domain $X$ is the Stone space of $B$, and whose range is contained in $Y$ modulo a meager set. We also outline how this duality can be combined with generic absoluteness results in order to analyze, by means of forcing arguments, the theory of $C^+(X,Y)$.

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arXiv2015

Category forcings, $MM^{+++}$, and generic absoluteness for the theory of strong forcing axioms

We introduce a category whose objects are stationary set preserving complete boolean algebras and whose arrows are complete homomorphisms with a stationary set preserving quotient. We show that the cut of this category at a rank initial segment of the universe of height a super compact which is a limit of super compact cardinals is a stationary set preserving partial order which forces $MM^{++}$ and collapses its size to become the second uncountable cardinal. Next we argue that any of the known methods to produce a model of $MM^{++}$ collapsing a superhuge cardinal to become the second uncountable cardinal produces a model in which the cutoff of the category of stationary set preserving forcings at any rank initial segment of the universe of large enough height is forcing equivalent to a presaturated tower of normal filters. We let $MM^{+++}$ denote this statement and we prove that the theory of $L(Ord^{ω_1})$ with parameters in $P(ω_1)$ is generically invariant for stationary set preserving forcings that preserve $MM^{+++}$. Finally we argue that the work of Larson and Asperó shows that this is a next to optimal generalization to the Chang model $L(Ord^{ω_1})$ of Woodin's generic absoluteness results for the Chang model $L(Ord^ω)$. It remains open whether $MM^{+++}$ and $MM^{++}$ are equivalent axioms modulo large cardinals and whether $MM^{++}$ suffices to prove the same generic absoluteness results for the Chang model $L(Ord^{ω_1})$.

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arXiv2014

A Boolean Algebraic Approach to Semiproper Iterations

These notes present a compact and self-contained approach to iterated forcing with a particular emphasis on semiproper forcing. We tried to make our presentation accessible to any scholar who has some familiarity with forcing and boolean valued models and full details of all proofs are given. We focus our presentation using the boolean algebra language and defining an iteration system as a directed and commutative system of complete and injective homomorphisms between complete and atomless boolean algebras. It is well known that the boolean algebra approach to forcing and iterations is fully equivalent to the standard one. While there are several monographs where forcing is introduced by means of boolean valued models, to our knowledge no detailed account of iterated forcing following a boolean algebraic approach has yet appeared. We believe that this different approach is fruitful since the richness of the algebraic language simplifies many calculations and definitions, among which that of RCS-limits. Some of the advantages of this approach have been already outlined by Donder and Fuchs in http://arxiv.org/abs/math/9207204. The first part of these notes present the general framework needed to develop the notion of limit of an iterated system of forcings in the boolean algebraic language. The second part contains a proof of the main result of Shelah on semiproper iterations, i.e. that RCS-limit of semiproper iterations are semiproper.

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arXiv2012

Martin's maximum revisited

We present several results relating the general theory of the stationary tower forcing developed by Woodin with forcing axioms. The main results is that the forcing axiom MM^{++} (also known as MM^{+ω_1}) decides the Π_2-theory of H_{ω_2} with respect to stationary set preserving forcings. We argue that this is a close to optimal generalization to H_{ω_2} of Woodin's absoluteness results for L(R).

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arXiv2011

Martin's Maximum and tower forcing

There are several examples in the literature showing that compactness-like properties of a cardinal $κ$ cause poor behavior of some generic ultrapowers which have critical point $κ$ (Burke \cite{MR1472122} when $κ$ is a supercompact cardinal; Foreman-Magidor \cite{MR1359154} when $κ= ω_2$ in the presence of strong forcing axioms). We prove more instances of this phenomenon. First, the Reflection Principle (RP) implies that if $\vec{\mathcal{I}}$ is a tower of ideals which concentrates on the class $GIC_{ω_1}$ of $ω_1$-guessing, internally club sets, then $\vec{\mathcal{I}}$ is not presaturated (a set is $ω_1$-guessing iff its transitive collapse has the $ω_1$-approximation property as defined in Hamkins \cite{MR2540935}). This theorem, combined with work from \cite{VW_ISP}, shows that if $PFA^+$ or $MM$ holds and there is an inaccessible cardinal, then there is a tower with critical point $ω_2$ which is not presaturated; moreover this tower is significantly different from the non-presaturated tower already known (by Foreman-Magidor \cite{MR1359154}) to exist in all models of Martin's Maximum. The conjunction of the Strong Reflection Principle (SRP) and the Tree Property at $ω_2$ has similar implications for towers of ideals which concentrate on the wider class $GIS_{ω_1}$ of $ω_1$-guessing, internally stationary sets. Finally, we show that the word "presaturated" cannot be replaced by "precipitous" in the theorems above: Martin's Maximum (which implies SRP and the Tree Property at $ω_2$) is consistent with a precipitous tower on $GIC_{ω_1}$.

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arXiv2011

Guessing models and generalized Laver diamond

We analyze the notion of guessing model, a way to assign combinatorial properties to arbitrary regular cardinals. Guessing models can be used, in combination with inaccessibility, to characterize various large cardinals axioms, ranging from supercompactness to rank-to-rank embeddings. The majority of these large cardinals properties can be defined in terms of suitable elementary embeddings j\colon V_γ\to V_λ. One key observation is that such embeddings are uniquely determined by the image structures j [ V_γ]\prec V_λ. These structures will be the prototypes guessing models. We shall show, using guessing models M, how to prove for the ordinal κ_M=j_M (\crit(j_M)) (where π_M is the transitive collapse of M and j_M is its inverse) many of the combinatorial properties that we can prove for the cardinal j(\crit(j)) using the structure j[V_γ]\prec V_{j(γ)}. κ_M will always be a regular cardinal, but consistently can be a successor. Guessing models M with κ_M=\aleph_2 exist assuming the proper forcing axiom PFA. By means of these models we shall introduce a new structural property of models of PFA: the existence of a "Laver function" f : \aleph_2 \to H_{\aleph_2} sharing the same features of the usual Laver functions f :κ\to H_κprovided by a supercompact cardinal κ. Further applications of our analysis will be proofs of the singular cardinal hypothesis and of the failure of the square principle assuming the existence of guessing models. In particular the failure of square shows that the existence of guessing models is a very strong assumption in terms of large cardinal strength.

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arXiv2010

On the consistency strength of the proper forcing axiom

Recently the second author introduced combinatorial principles that characterize supercompactness for inaccessible cardinals but can also hold true for small cardinals. We prove that the proper forcing axiom PFA implies these principles hold for $ω_2$. Using this, we argue to show that any of the known methods for forcing models of PFA from a large cardinal assumption requires a strongly compact cardinal. If one forces PFA using a proper forcing, then we get the optimal result that a supercompact cardinal is necessary.

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arXiv2008

Some consequences of reflection on the approachability ideal

We study the approachability ideal I[κ^+] in the context of large cardinals properties of the regular cardinals below a singular κ. As a guiding example consider the approachability ideal I[\aleph_{ω+1}] assuming that \aleph_ωis strong limit. In this case we obtain that club many points in \aleph_{ω+1} of cofinality \aleph_n for some n>1 are approachable assuming the joint reflection of countable families of stationary subsets of \aleph_n. This reflection principle holds under Martin's maximum for all n>1 and for each n>1 is equiconsistent with \aleph_n being weakly compact in L. This characterizes the structure of the approachability ideal I[\aleph_{ω+1}] in models of Martin's maximum.

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arXiv2007

A family of covering properties for forcing axioms and strongly compact cardinals

This paper presents the main results in my Ph.D. thesis. In what follows several proofs of SCH are presented introducing a family of covering properties which implies both SCH and the failure of various forms of square. These covering properties are also applied to investigate models of strongly compact cardinals or of strong forcing axioms like MM or PFA.

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