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Laura Fontanella

Publications and source records attributed to Laura Fontanella.

7 recordsLinked to original sources
arXiv2026

Bar-recursion and Preservation of Cardinals

This work presents a transfinite version of the bar-recursion in the context of classical realizability models for set theory. Bar-recursion has been previously used to obtain realizability interpretations of countable choice and dependent choice, and was employed by Krivine to realize the continuum hypothesis in classical realizability. In this paper, we introduce a transfinite variant of bar-recursion and use it to construct realizability models validating uncountable fragments of the Axiom of Choice. Moreover, our construction reveals that this generalized bar-recursion is related to preservation of cardinals. To show this, we define an analogue of the forcing notion of $\kappa$-closure for classical realizability algebras that we call $\kappa$-fully-closed. We show that, in realizability algebras satisfying the $\kappa$-full-closure property, generalized bar-recursion realizes that any cardinal up to $\kappa$ admits a representative in the realizability model which remains a cardinal.

math.LO↗
arXiv2025

Realizing the totally unordered structure of ordinals

We present tools for analysing ordinals in realizability models of classical set theory built using Krivine's technique for realizability. This method uses a conservative extension of $ZF$ known as $ZF_{\varepsilon}$, where two membership relations co-exist, the usual one denoted $\in$ and a stricter one denoted $\varepsilon$ that does not satisfy the axiom of extensionality; accordingly we have two equality relations, the extensional one $\simeq$ and the strict identity $=$ referring to sets that satisfy the same formulas. We define recursive names using an operator that we call reish, and we show that the class of recursive names for ordinals coincides extensionally with the class of ordinals of realizability models. We show that reish$(ω)$ is extensionally equal to omega in any realizability model, thus recursive names provide a useful tool for computing $ω$ in realizability models. We show that on the contrary $\varepsilon$-totally ordered sets do not form a proper class and therefore cannot be used to fully represent the ordinals in realizability models. Finally we present some tools for preserving cardinals in realizability models, including an analogue for realizability algebras of the forcing property known as the $κ$-chain condition.

math.LO↗
arXiv2016

Square and Delta reflection

Starting from infinitely many supercompact cardinals, we force a model of ZFC where $\aleph_{ω^2+1}$ satisfies simultaneously a strong principle of reflection, called $Δ$-reflection, and a version of the square principle, denoted $\square(\aleph_{ω^2+1}).$ Thus we show that $\aleph_{ω^2+1}$ can satisfy simultaneously a strong reflection principle and an anti-reflection principle.

math.LO↗
arXiv2012

Strong Tree Properties for Small Cardinals

An inaccessible cardinal kappa is supercompact when (kappa, lambda)-ITP holds for all lambda greater than or equal to kappa. We prove that if there is a model of ZFC with infinitely many supercompact cardinals, then there is a model of ZFC where for every natural number n greater than 1 and for every ordinal mu greater than or equal to aleph_n, we have (aleph_n, mu)-ITP.

math.LO↗
arXiv2011

Strong tree Properties for two successive cardinals

An inaccessible cardinal $κ$ is supercompact when $(κ, λ)$-ITP holds for all $λ\geq κ.$ We prove that if there is a model of $\ZFC$ with two supercompact cardinals, then there is a model of \ZFC where simultaneously $(\aleph_2, μ)$-ITP and $(\aleph_3, μ')$-ITP hold, for all $μ\geq \aleph_2$ and $μ'\geq \aleph_3.$

math.LO↗