arXiv subjects

Ryuya Hora

Publications and source records attributed to Ryuya Hora.

8 recordsLinked to original sources

arXiv2025

Normalization of a subgroup, in a topos, and of a word-congruence

This paper provides a new categorical definition of a normalization operator motivated by topos theory and its applications to algebraic language theory. We first define a normalization operator $\Xi \to \Xi$ in any category that admits a colimit of all monomorphisms $\Xi$, which we call a local state classifier. In the category of group actions for a group $G$, this operator coincides with the usual normalization operator, which takes a subgroup $H\subset G$ and returns its normalizer subgroup $\mathrm{Nor}_G(H)\subset G$. Using this generalized normalization operator, we prove a topos-theoretic proposition that provides an explicit description of a local state classifier of a hyperconnected quotient of a given topos. We also briefly explain how these results serve as preparation for a topos-theoretic study of regular languages, congruences of words, and syntactic monoids.

math.CT↗

arXiv2025

Games as recursive coalgebras: A categorical view on the Nim-sum

In 1901, Bouton proved that a winning strategy of the game of Nim is given by the bitwise XOR, called the nim-sum. But, why does such a weird binary operation work? Led by this question, this paper introduces a categorical reinterpretation of combinatorial games and the nim-sum. The main categorical gadget used here is recursive coalgebras, which allow us to redefine games as ``graphs on which we can conduct recursive calculation'' in a concise and precise way. For game-theorists, we provide a systematic framework to decompose an impartial game into simpler games and synthesize the quantities on them, which generalizes the nim-sum rule for the Conway addition. To read the first half of this paper, the categorical preliminaries are limited to the definitions of categories and functors. For category theorists, this paper offers a nicely behaved category of games $\mathbf{Game}$, which is a locally finitely presentable symmetric monoidal closed category comonadic over $\mathbf{Set}$ admitting a subobject classifier! As this paper has several ways to be developed, we list seven open questions in the final section.

math.CO↗

arXiv2025

Lawvere's fourth open problem: Levels in the topos of symmetric simplicial sets

In the topos of simplicial sets, it makes sense to ask the following question about a given natural number $n$: what is the minimum value $m$ such that $n$-skeletality implies $m$-coskeletality? This is an instance of the Aufhebung relation in the sense of Lawvere, who introduced this notion for an arbitrary Grothendieck topos $\mathcal{E}$ in place of $\mathbf{sSet}$, and levels/essential subtopoi in place of dimensions. We compute this Aufhebung relation for the topos of symmetric simplicial sets. In particular, we show that it is given by $2l-1$ for the level labelled by $l\geq 3$, which coincides with the previously known case of simplicial sets. This result provides a solution to the fourth of the seven open problems in topos theory posed by Lawvere in 2009.

math.CT↗

arXiv2025

Grothendieck topoi with a left adjoint to a left adjoint to a left adjoint to the global sections functor

This paper introduces the notion of complete connectedness of a Grothendieck topos, defined as the existence of a left adjoint to a left adjoint to a left adjoint to the global sections functor, and provides many examples. Typical examples include presheaf topoi over a category with an initial object, such as the topos of sets, the Sierpi\'nski topos, the topos of trees, the object classifier, the topos of augmented simplicial sets, and the classifying topos of many algebraic theories, such as groups, rings, and vector spaces. We first develop a general theory on the length of adjunctions between a Grothendieck topos and the topos of sets. We provide a site characterisation of complete connectedness, which turns out to be dual to that of local topoi. We also prove that every Grothendieck topos is a closed subtopos of a completely connected Grothendieck topos.

math.CT↗

arXiv2024

Topoi of automata I: Four topoi of automata and regular languages

Both topos theory and automata theory are known for their multi-faceted nature and relationship with topology, algebra, logic, and category theory. This paper aims to clarify the topos-theoretic aspects of automata theory, particularly demonstrating through two main theorems how regular (and non-regular) languages arise in topos-theoretic calculation. First, it is shown that the four different notions of automata form four types of Grothendieck topoi, illustrating how the technical details of automata theory are described by topos theory. Second, we observe that the four characterizations of regular languages (DFA, Myhill-Nerode theorem, finite monoids, profinite words) provide Morita-equivalent definitions of a single Boolean-ringed topos, situating this within the context of Olivia Caramello's 'Toposes as Bridges.' This paper also serves as a preparation for follow-up papers, which deal with the relationship between hyperconnected geometric morphisms and algebraic/geometric aspects of formal language theory.

cs.FL↗

arXiv2024

Solution to Lawvere's first problem: a Grothendieck topos that has proper class many quotient topoi

This paper solves the first of the open problems in topos theory posted by William Lawvere, concerning the existence of a Grothendieck topos that has proper class many quotient topoi. This paper concretely constructs such Grothendieck topoi, including the presheaf topos on the free monoid generated by countably infinitely many elements PSh(M_{\omega}). Utilizing the combinatorics of the classifying topos of the theory of inhabited objects and with the help of a system of pairing functions, the problem is reduced to a theorem of Vopenka, Pultr, and Hedrlin, which states that any set admits a rigid relational structure.

math.CT↗

arXiv2023

Quotient toposes of discrete dynamical systems

This paper gives a classification of classes of discrete dynamical systems (a set equipped with an endofunction) closed under finite limits and small colimits. The conclusion is simple: they bijectively correspond to the ideals of the product poset $\mathbb{N} \times \mathbb{N}$, where the first $\mathbb{N}$ is ordered by the usual order and the second is by the divisibility. Our method is based on a detailed analysis of the behaviors of states, especially non-periodic behaviors, in discrete dynamical systems. Specifically, extending the fundamental quantity, time until entering a loop, to even those states that do not enter a loop plays a crucial role. Our classification is closely related to epimorphisms from $\mathbb{N}$ in the category of monoids. There are countably many injective epimorphisms from $\mathbb{N}$, including $\mathbb{N} \to \mathbb{Z}$. Those injective epimorphisms correspond to the non-periodic behaviors of states of discrete dynamical systems. We discuss this point at the end of the paper. This fun puzzle is motivated by an open problem in topos theory. Lawvere left open problems in topos theory on his webpage, and the first problem is called quotient toposes. The main theorem of this paper provides a non-trivial example of this problem, which is not implied by any known results. This paper also provides a theoretical framework to address the open problem. We define a preorder among the objects of a Grothendieck topos (which we have named generative order), which enables us to reduce calculations of quotient toposes to calculations of objects. Our method is its application to the topos of discrete dynamical systems.

math.CT↗

arXiv2023

Internal Parameterization of Hyperconnected Quotients

One of the most fundamental facts in topos theory is the internal parameterization of subtoposes: the bijective correspondence between subtoposes and Lawvere-Tierney topologies. In this paper, we introduce a new but elementary concept, "a local state classifier," and give an analogous internal parameterization of hyperconnected quotients (i.e., hyperconnected geometric morphisms from a topos). As a corollary, we obtain a solution to the Boolean case of the first problem of Lawvere's open problems.

math.CT↗