Leonardo Pacheco

The \(\mu\)-calculus is obtained by adding to modal logic the least and greatest fixed-point operators \(\mu\) and \(\nu\). The alternation depth of a formula measures the entanglement of its least and greatest fixed-point operators. Bradfield showed that, for all \(n\in\mathbb{N}\), there is a formula \(W_n\) such that \(W_n\) has alternation depth \(n\) and, over all Kripke frames, \(W_n\) is not equivalent to any formula with alternation depth smaller than \(n\).

The same may not happen over restricted classes of frames: Alberucci and Facchini showed that, over frames of \(\mathsf{S5}\), every \(\mu\)-formula is equivalent to a formula without fixed point operators. In this case, we say the \(\mu\)-calculus collapses to modal logic over frames of \(\mathsf{S5}\).

We show how Alberucci and Facchini’s proof generalize to the \(\mu\)-calculus’s collapse over frames of intuitionistic \(\mathsf{S5}\). This generalization can also be done for some non-normal logics and for graded modal logics. We also show that, on the other hand, the \(\mu\)-calculus does not collapse over the bimodal logic \(\mathsf{S5}_2\).

Slides. Full abstract.

We study some consequences of the \(\mu\)-calculus' alternation hierarchy collapse on some variations of epistemic logic with only one agent; and sketch how to prove that the alternation hierarchy is strict if we have more than one agent. Slides.

The \(\mu\)-calculus is obtained by adding fixed-point operators to modal logic. Alberucci and Facchini showed that every \(\mu\)-formula is equivalent to some modal formula over frames of \(\mathsf{S5}\). The modal logic \(\mathsf{IS5}\) is an intuitionistic variation of \(\mathsf{S5}\), first defined by Prior in 1957. We show that, with slight changes, Alberucci and Facchini's proof also works in frames of \(\mathsf{IS5}\). That is, every \(\mu\)-formula is equivalent to some modal formula over frames of \(\mathsf{IS5}\). This is joint work with Kazuyuki Tanaka. Slides.

It is known that the \(\mu\)-calculus collapses to its alternation-free fragment over transitive frames and to modal logic over equivalence relations. We adapt a proof by D'Agostino and Lenzi to show that the \(\mu\)-calculus collapses to its alternation-free fragment over weakly transitive frames. As a consequence, we show that the \(\mu\)-calculus with derivative topological semantics collapses to its alternation-free fragment. We also study the collapse over frames of \(\mathsf{S4.2}\), \(\mathsf{S4.3}\), \(\mathsf{S4.3.2}\), \(\mathsf{S4.4}\) and \(\mathsf{KD45}\), logics important for Epistemic Logic. At last, we use the \(\mu\)-calculus to define degrees of ignorance on Epistemic Logic and study the implications of \(\mu\)-calculus's collapse over the logics above. This is joint work with Kazuyuki Tanaka. Slides.

It is well-known that several variations of the axiom of determinacy play important roles in the study of reverse mathematics, and the relation between the hierarchy of determinacy (especially the level of \(\Sigma^0_2\) and \(\Sigma^0_3\)) and comprehension axioms are revealed by Tanaka, Nemoto, Montalbán, Shore, and others. In this talk, we show variations of a result by Kołodziejczyk and Michalewski relating determinacy and reflection in second-order arithmetic based on a model-theoretic characterization of the reflection principles. Slides. Video.

Working in second order arithmetic, we characterize determinacy of finite differences of open and \(\Sigma^0_2\) sets as reflection principles. In order to do so we study sequences of coded \(\beta\)-models. More precisely, we prove that: over \(\mathsf{ACA}_0\), \(\Pi^1_2\)-\(\mathsf{Ref}(\mathsf{ACA}_0)\) is equivalent to \(\forall n.(\Sigma^0_1)_n\)-\(\mathsf{Det}^*_0\), and \(\Pi^1_3\)-\(\mathsf{Ref}(\Pi^1_1\)-\(\mathsf{CA}_0)\) is equivalent to \(\forall n.(\Sigma^0_1)_n\)-\(\mathsf{Det}\); over \(\mathsf{ATR}_0\), \(\Pi^1_3\)-\(\mathsf{Ref}(\Pi^1_2\)-\(\mathsf{CA}_0)\) is equivalent to \(\forall n.(\Sigma^0_2)_n\)-\(\mathsf{Det}\). This is joint work with Keita Yokoyama.

This is joint work with Kazuyuki Tanaka. The extended abstract is available in the booklet here.

We study the reflection axioms of the form: if a \(\Pi^1_n\) sentence \(\varphi\) is provable in \(T\), then \(\varphi\) is true (for a fixed theory \(T\)). We prove the relation between the existence of sequence of \(\beta_k\) models and reflection axioms for theories of dependent choices. We also comment on the consequences of these results for determinacy in second-order arithmetic. This is joint work with Keita Yokoyama.

We investigate the alternation hierarchy of the \(\mu\)-calculus in some modal logics commonly used for epistemic logic.

Epistemic logic is the logic of knowledge, belief and related notions. In this presentation I define and motivate three different modal semantics for epistemic logic: relational semantics, topological semantics and neighborhood semantics. I also comment on the relation between these semantics and common knowledge. I assume no knowledge of modal logic.

Abstract: We survey recent results on reflection in second-order arithmetic. The reflection principles we consider can be roughly divided into two categories: semantic reflection and syntactic reflection. (Survey.)

Abstract: It is known that several variations of the axiom of determinacy play important roles in the study of reverse mathematics, and the relation between the hierarchy of determinacy and comprehension are revealed by Tanaka, Nemoto, Montalbán, Shore, and others. We prove variations of a result by Kołodziejczyk and Michalewski relating determinacy of arbitrary boolean combinations of \(\Sigma^0_2\) sets and reflection in second-order arithmetic. Specifically, we prove that: over \(\mathsf{ACA}_0\), \(\Pi^1_2\)-\(\mathsf{Ref}(\mathsf{ACA}_0)\) is equivalent to \(\forall n.(\Sigma^0_1)_n\)-\(\mathsf{Det}^*_0\); \(\Pi^1_3\)-\(\mathsf{Ref}(\Pi^1_1\)-\(\mathsf{CA}_0)\) is equivalent to \(\forall n.(\Sigma^0_1)_n\)-\(\mathsf{Det}\); and \(\Pi^1_3\)-\(\mathsf{Ref}(\Pi^1_2\)-\(\mathsf{CA}_0)\) is equivalent to \(\forall n.(\Sigma^0_2)_n\)-\(\mathsf{Det}\). We also restate results by Montalbán and Shore to show that \(\Pi^1_3\)-\(\mathsf{Ref}(\mathsf{Z}_2)\) is equivalent to \(\forall n.(\Sigma^0_3)_n\)-\(\mathsf{Det}\) over \(\mathsf{ACA}_0\). Preprint.

Abstract: It is known that the \(\mu\)-calculus collapses to its alternation-free fragment over transitive frames and to modal logic over equivalence relations. We adapt a proof by D'Agostino and Lenzi to show that the \(\mu\)-calculus collapses to its alternation-free fragment over weakly transitive frames. As a consequence, we show that the \(\mu\)-calculus with derivative topological semantics collapses to its alternation-free fragment. We also study the collapse over frames of \(\mathsf{S4.2}\), \(\mathsf{S4.3}\), \(\mathsf{S4.3.2}\), \(\mathsf{S4.4}\) and \(\mathsf{KD45}\), logics important for Epistemic Logic. At last, we use the \(\mu\)-calculus to define degrees of ignorance on Epistemic Logic and study the implications of \(\mu\)-calculus's collapse over the logics above. DOI: 10.1007/978-3-031-15298-6_13

Abstract: Epistemic Logic normally discourses on knowledge, belief, and related concepts. We here study ignorance instead. With the help of the \(\mu\)-calculus, we analyze the degrees of ignorance in which an agent doesn't know whether or not a given proposition is true. Building on the study by Stalnaker, we argue that logics "closer" to \(\mathsf{S4.2}\) allow greater degrees of ignorance, compared to logics "closer" to \(\mathsf{S5}\). Extended abstract available here.

Abstract: In this paper, we study one-variable fragments of modal \(\mu\)-calculus and their relations to parity games. We first introduce the weak modal \(\mu\)-calculus as an extension of the one-variable modal \(\mu\)-calculus. We apply weak parity games to show the strictness of the one-variable hierarchy as well as its extension. We also consider games with infinitely many priorities and show that their winning positions can be expressed by both \(\Sigma^\mu_2\) and \(\Pi^\mu_2\) formulas with two variables, but requires a transfinite extension of the \(L_\mu\)-formulas to be expressed with only one variable. At last, we define the \(\mu\)-arithmetic and show that a set of natural numbers is definable by both a \(\Sigma^\mu_2\) and a \(\Pi^\mu_2\) formula of \(\mu\)-arithmetic if and only if it is definable by a formula of the one-variable transfinite \(\mu\)-arithmetic. DOI: 10.1142/9789811259296_0002