arXiv subjects

Jonathan Weinberger

Publications and source records attributed to Jonathan Weinberger.

14 recordsLinked to original sources

arXiv2026

The $\infty$-category of $\infty$-categories in simplicial type theory

Simplicial type theory (STT) was introduced by Riehl and Shulman to leverage homotopy type theory to prove results about $(\infty,1)$-categories. Initial work on simplicial type theory focused on "formal" arguments in higher category theory and, in particular, no non-trivial examples of $\infty$-category theory were constructible within STT. More recent work has changed this state of affairs by applying techniques developed initial for cubical type theory to construct the $\infty$-category of spaces. We complete this process by constructing the $\infty$-category of $\infty$-categories, recovering one of the main foundational results of $\infty$-category theory (straightening--unstraightening) purely type-theoretically. We also show how this construction enables new examples of the directed version of the structure identity principle, the structure homomorphism principle.

cs.LO↗

arXiv2025

The Yoneda embedding in simplicial type theory

Riehl and Shulman introduced simplicial type theory (STT), a variant of homotopy type theory which aimed to study not just homotopy theory, but its fusion with category theory: $(\infty,1)$-category theory. While notoriously technical, manipulating $\infty$-categories in simplicial type theory is often easier than working with ordinary categories, with the type theory handling infinite stacks of coherences in the background. We capitalize on recent work by Gratzer et al. defining the $(\infty,1)$-category of $\infty$-groupoids in STT to define presheaf categories within STT and systematically develop their theory. In particular, we construct the Yoneda embedding, prove the universal property of presheaf categories, refine the theory of adjunctions in STT, introduce the theory of Kan extensions, and prove Quillen's Theorem A.

cs.LO↗

arXiv2024

Skolem, Gödel, and Hilbert fibrations

Grothendieck fibrations are fundamental in capturing the concept of dependency, notably in categorical semantics of type theory and programming languages. A relevant instance are Dialectica fibrations which generalise Gödel's Dialectica proof interpretation and have been widely studied in recent years. We characterise when a given fibration is a generalised, dependent Dialectica fibration, namely an iterated completion of a fibration by dependent products and sums (along a given class of display maps). From a technical perspective, we complement the work of Hofstra on Dialectica fibrations by an internal viewpoint, categorifying the classical notion of quantifier-freeness. We also generalise both Hofstra's and Trotta et al.'s work on Gödel fibrations to the dependent case, replacing the class of cartesian projections in the base category by arbitrary display maps. We discuss how this recovers a range of relevant examples in categorical logic and proof theory. Moreover, as another instance, we introduce Hilbert fibrations, providing a categorical understanding of Hilbert's $ε$- and $τ$-operators well-known from proof theory.

math.CT↗

arXiv2024

Directed univalence in simplicial homotopy type theory

Simplicial type theory extends homotopy type theory with a directed path type which internalizes the notion of a homomorphism within a type. This concept has significant applications both within mathematics -- where it allows for synthetic (higher) category theory -- and programming languages -- where it leads to a directed version of the structure identity principle. In this work, we construct the first types in simplicial type theory with non-trivial homomorphisms. We extend simplicial type theory with modalities and new reasoning principles to obtain triangulated type theory in order to construct the universe of discrete types $\mathcal{S}$. We prove that homomorphisms in this type correspond to ordinary functions of types i.e., that $\mathcal{S}$ is directed univalent. The construction of $\mathcal{S}$ is foundational for both of the aforementioned applications of simplicial type theory. We are able to define several crucial examples of categories and to recover important results from category theory. Using $\mathcal{S}$, we are also able to define various types whose usage is guaranteed to be functorial. These provide the first complete examples of the proposed directed structure identity principle.

cs.LO↗

arXiv2024

On a fibrational construction for optics, lenses, and Dialectica categories

Categories of lenses/optics and Dialectica categories are both comprised of bidirectional morphisms of basically the same form. In this work we show how they can be considered a special case of an overarching fibrational construction, generalizing Hofstra's construction of Dialectica fibrations and Spivak's construction of generalized lenses. This construction turns a tower of Grothendieck fibrations into another tower of fibrations by iteratively twisting each of the components, using the opposite fibration construction.

math.CT↗

arXiv2024

Generalized Chevalley criteria in simplicial homotopy type theory

We provide a generalized treatment of (co)cartesian arrows, fibrations, and functors. Compared to the classical conditions, the endpoint inclusions get replaced by arbitrary shape inclusions. Our framework is Riehl--Shulman's simplicial homotopy type theory which supports the development of synthetic internal $(\infty,1)$-category theory.

math.CT↗

arXiv2024

Two-sided cartesian fibrations of synthetic $(\infty,1)$-categories

Within the framework of Riehl-Shulman's synthetic $(\infty,1)$-category theory, we present a theory of two-sided cartesian fibrations. Central results are several characterizations of the two-sidedness condition à la Chevalley, Gray, Street, and Riehl-Verity, a two-sided Yoneda Lemma, as well as the proof of several closure properties. Along the way, we also define and investigate a notion of fibered or sliced fibration which is used later to develop the two-sided case in a modular fashion. We also briefly discuss discrete two-sided cartesian fibrations in this setting, corresponding to $(\infty,1)$-distributors. The systematics of our definitions and results closely follows Riehl-Verity's $\infty$-cosmos theory, but formulated internally to Riehl-Shulman's simplicial extension of homotopy type theory. All the constructions and proofs in this framework are by design invariant under homotopy equivalence. Semantically, the synthetic $(\infty,1)$-categories correspond to internal $(\infty,1)$-categories implemented as Rezk objects in an arbitrary given $(\infty,1)$-topos.

math.CT↗

arXiv2024

Internal sums for synthetic fibered $(\infty,1)$-categories

We give structural results about bifibrations of (internal) $(\infty,1)$-categories with internal sums. This includes a higher version of Moens' Theorem, characterizing cartesian bifibrations with extensive aka stable and disjoint internal sums over lex bases as Artin gluings of lex functors. We also treat a generalized version of Moens' Theorem due to Streicher which does not require the Beck--Chevalley condition. Furthermore, we show that also in this setting the Moens fibrations can be characterized via a condition due to Zawadowski. Our account overall follows Streicher's presentation of fibered category theory à la Bénabou, generalizing the results to the internal, higher-categorical case, formulated in a synthetic setting. Namely, we work inside simplicial homotopy type theory, which has been introduced by Riehl and Shulman as a logical system to reason about internal $(\infty,1)$-categories, interpreted as Rezk objects in any given Grothendieck--Rezk--Lurie $(\infty,1)$-topos.

math.CT↗

arXiv2024

Smooth and Proper Maps

This is an expository note explaining how the geometric notions of local connectedness and properness are related to the $\Sigma$-type and $\Pi$-type constructors of dependent type theory.

math.CT↗

arXiv2023

Formalizing the $\infty$-Categorical Yoneda Lemma

Formalized $1$-category theory forms a core component of various libraries of mathematical proofs. However, more sophisticated results in fields from algebraic topology to theoretical physics, where objects have "higher structure," rely on infinite-dimensional categories in place of $1$-dimensional categories, and $\infty$-category theory has thusfar proved unamenable to computer formalization. Using a new proof assistant called Rzk, which is designed to support Riehl-Shulman's simplicial extension of homotopy type theory for synthetic $\infty$-category theory, we provide the first formalizations of results from $\infty$-category theory. This includes in particular a formalization of the Yoneda lemma, often regarded as the fundamental theorem of category theory, a theorem which roughly states that an object of a given category is determined by its relationship to all of the other objects of the category. A key feature of our framework is that, thanks to the synthetic theory, many constructions are automatically natural or functorial. We plan to use Rzk to formalize further results from $\infty$-category theory, such as the theory of limits and colimits and adjunctions.

math.CT↗

arXiv2022

Synthetic fibered $(\infty,1)$-category theory

We study cocartesian fibrations in the setting of the synthetic $(\infty,1)$-category theory developed in the simplicial type theory introduced by Riehl and Shulman. Our development culminates in a Yoneda Lemma for cocartesian fibrations.

math.CT↗

arXiv2022

Strict stability of extension types

The theory of $(\infty,1)$-categories can be developed synthetically in an augmentation of homotopy type theory introduced by Riehl--Shulman. Central to their development is an additional type forming operation called extensions. The original article sketches the semantics of this formal system, explaining how the simplicial homotopy theory can be used to reason about $(\infty,1)$-categories presented using the Segal space model. However, they leave it open to demonstrate the strict stability of extension types. We prove this using the splitting method of Voevodsky, later generalized by Lumsdaine--Warren to local universes. The practical upshot is that this system has semantics in simplicial objects of an $\infty$-topos, and thus can be used to prove theorems about internal $\infty$-categories in the sense of Martini--Wolf.

math.CT↗

arXiv2022

A Synthetic Perspective on $(\infty,1)$-Category Theory: Fibrational and Semantic Aspects

Reasoning about weak higher categorical structures constitutes a challenging task, even to the experts. One principal reason is that the language of set theory is not invariant under the weaker notions of equivalence at play, such as homotopy equivalence. From this point of view, it is natural to ask for a different foundational setting which more natively supports these notions. Our work takes up on suggestions in the original article arXiv:1705.07442 by Riehl--Shulman to further develop synthetic $(\infty,1)$-category theory in simplicial homotopy type theory, including in particular the study of cocartesian fibrations. Together with a collection of analytic results, notably due to Riehl--Verity and Rasekh, it follows that our type-theoretic account constitutes a synthetic theory of fibrations of internal $(\infty,1)$-categories, w.r.t. to an arbitrary Grothendieck--Rezk--Lurie-$(\infty,1)$-topos via Shulman's major result arXiv:1904.07004 about strictification of univalent universes.

math.CT↗

arXiv2021

Simplicial sets inside cubical sets

As observed recently by various people the topos $\mathbf{sSet}$ of simplicial sets appears as essential subtopos of a topos $\mathbf{cSet}$ of cubical sets, namely presheaves over the category $\mathbf{FL}$ of finite lattices and monotone maps between them. The latter is a variant of the cubical model of type theory due to Cohen et al. for the purpose of providing a model for a variant of type theory which validates Voevodsky's Univalence Axiom and has computational meaning. Our contribution consists in constructing in $\mathbf{cSet}$ a fibrant univalent universe for those types that are sheaves. This makes it possible to consider $\mathbf{sSet}$ as a submodel of $\mathbf{cSet}$ for univalent Martin-Löf type theory. Furthermore, we address the question whether the type-theoretic Cisinski model structure considered on $\mathbf{cSet}$ coincides with the test model structure, the latter of which models the homotopy theory of spaces. We do not provide an answer to this open problem, but instead give a reformulation in terms of the adjoint functors at hand.

math.CT↗