1.8 qtcat-2026 2026-08-12 2026-08-14 3 00:05 https://pretalx.c3voc.de https://pretalx.c3voc.de/media/qtcat-2026/img/logo_XGKS1kh.webp Europe/Berlin R. 221 Opening 2026-08-12T09:45:00+02:00 09:45 00:15 Opening of QTCat! qtcat-2026-110775-opening Tessa Kammermeier en false https://pretalx.c3voc.de/qtcat-2026/talk/G8V7E7/ https://pretalx.c3voc.de/qtcat-2026/talk/G8V7E7/feedback/ R. 221 Talk (Speaker) 2026-08-12T10:00:00+02:00 10:00 01:00 Formalisation is the process of expressing mathematical ideas in the language understood by a proof assistant, a computer program that enables the interactive construction of verified mathematical definitions, theorems, and proofs. The stereotypical understanding of formalisation is as the rote translation of pre-existing mathematics to a cumbersome formal language, done primarily as a means of certifying the correctness of an argument. This memetic conception as a chore standing in the way of a coveted result (guaranteed correctness) has long allowed the aesthetics of formalisation to be appropriated by adversarial actors to further their financial interests ("get paid for proving lemmas on the blockchain"/"our new LLM will totally solve All Of Maths, and we have the Lean to prove it"). I aim to challenge this understanding, presenting the process of formalisation, in itself, as a force for good. I will share some of my own experiences with free-and-libre, community-supported proof assistants as a tool for independent study; genuine mathematical insights revealed by developing category theory within formal univalent type theory; and a few challenges that come with maintaining a library of formalised mathematics. qtcat-2026-110771-category-theory-formalised-humanely Amélia Liao en false https://pretalx.c3voc.de/qtcat-2026/talk/9F8KNH/ https://pretalx.c3voc.de/qtcat-2026/talk/9F8KNH/feedback/ R. 221 Talk 2026-08-12T11:30:00+02:00 11:30 00:30 String diagrams are often traced back to Richard Penrose's work in mathematical physics, but they also have an early and largely overlooked history in computer science through Günter Hotz. Following in the footsteps of Hotz, this talk will provide an introduction to string diagrams through the lens of circuit design. Rather than beginning with category theory, we start from circuits and their computational properties, and then work backwards to the categorical structures they describe. For example, we will see how modelling feedback motivates compact closed categories and reversibility suggests the structure of a monoidal dagger category. We will then see how the ability to copy data gives rise to Cartesian categories, with the no-cloning theorem identifying precisely why quantum circuits lack this structure. When circuits are no longer Cartesian, basic operations such as conditional statements become harder to reason about graphically. We conclude with a look at ongoing work on controlled monoidal categories and controlled string diagrams, which aim to restore graphical reasoning for conditional statements in non-Cartesian circuits. qtcat-2026-110765-the-road-from-circuits-to-category-theory Scott Wesley en false https://pretalx.c3voc.de/qtcat-2026/talk/X8NKSX/ https://pretalx.c3voc.de/qtcat-2026/talk/X8NKSX/feedback/ R. 221 Talk 2026-08-12T12:00:00+02:00 12:00 00:30 Consider the category of groups with its forgetful functor down to Set. This functor is monadic, which tells us that groups are sets equipped with an algebraic structure. Now consider the category of finite sets, with its inclusion into Set. This functor is not monadic for many reasons, but what monadic functor best approximates it? In other words, finite sets are most like sets equipped with what algebraic structure? The answer to this question is given my the codensity monad of FinSet -> Set. Quite surprisingly, this is the ultrafilter monad, whose algebras are compact Hausdorff spaces! That's all well an good, but why should you care? Every monad is trivially the codensity monad of some functor. Does it matter if it is the codensity monad of a specially nice one, say a fully faithful one? It does! For example, if a polynomial monad T on Set restricts to FinSet, then a general theory guarantees that there is a unique distributive law of the ultrafilter monad over T, i.e. a unique monad on CHaus which lifts T. In this talk I will tell you about this recent development about distributive laws involving codensity monads, and how they relate to the main subject of my thesis: pushforward monads. We will use the ulftrafilter monad as our central example. qtcat-2026-110760-who-cares-about-codensity-monads Adrián Doña Mateo en false https://pretalx.c3voc.de/qtcat-2026/talk/SBZJVC/ https://pretalx.c3voc.de/qtcat-2026/talk/SBZJVC/feedback/ R. 221 Talk (Speaker) 2026-08-12T14:00:00+02:00 14:00 01:00 Higher algebra is the algebra of (connective) spectra. Deeper algebra is the algebra of (coconnective) categorical spectra. Deeper algebra organizes and simplifies many phenomena, from Galois theory to the classification of phases of matter. I will introduce these areas, and I will report on joint work in progress with David Reutter. qtcat-2026-110773-the-fundamental-theorem-of-deeper-algebra Theo Johnson-Freyd en false https://pretalx.c3voc.de/qtcat-2026/talk/9QMSKT/ https://pretalx.c3voc.de/qtcat-2026/talk/9QMSKT/feedback/ Workshop 1 Mini-Workshop 2026-08-12T15:30:00+02:00 15:30 01:30 We will meet in a cozy group where I will teach you how to cross stitch the QTCat logo as a colorful souvenir or a nice little project between talks or on the ride home. No prior cross stitch knowledge required! I will bring enough kits with cloth, yarn and needles for everyone. There will be many more colors if you want to create your own designs :3 qtcat-2026-110766-let-s-cross-stitch-a-qtcat-beginner-friendly /media/qtcat-2026/submissions/S8VGEV/image_C6HW2K3.webp rhandos en true https://pretalx.c3voc.de/qtcat-2026/talk/S8VGEV/ https://pretalx.c3voc.de/qtcat-2026/talk/S8VGEV/feedback/ Workshop 2 Mini-Workshop 2026-08-12T15:30:00+02:00 15:30 01:30 As a non binary transmasc, I always felt like my community was missing. I'm not a sociologist, but as far as I know, there are almost no articles on this subject. However, the lack of a transmasc community is commonly discussed by trans folk, and we agree that there is no unified transmasc community akin to the transfem one. I will give some insight about the missing community and my experience as a transmasc, and I will follow up with some hypotheses about this lack of community. The talk will be followed by a discussion about isolation and communities of queer folks. qtcat-2026-110776-where-is-the-transmasc-community Heliadore Johann Depauw en true https://pretalx.c3voc.de/qtcat-2026/talk/EUJTX8/ https://pretalx.c3voc.de/qtcat-2026/talk/EUJTX8/feedback/ Outside Event 2026-08-12T18:30:00+02:00 18:30 03:00 We will watch the partial solar eclipse together at the Gorch-Fock-Park. qtcat-2026-110778-solar-eclipse en true https://pretalx.c3voc.de/qtcat-2026/talk/MTZCHA/ https://pretalx.c3voc.de/qtcat-2026/talk/MTZCHA/feedback/ R. 221 Talk (Speaker) 2026-08-13T10:00:00+02:00 10:00 01:00 Invertibility is a crucial notion in category theory, providing the correct notion of sameness for objects within a category and equivalences of categories. This notion readily generalises to finite-dimensional higher categories inductively by replacing equalities with higher dimensional isomorphisms. The situation becomes more subtle with infinite-dimensional categories where there are different notions of invertibility. In this talk, we will give an introduction to weak ω-categories and we will study coinductively invertible cells within them. We will then describe computads with invertible generators as data for freely generating ω-categories. qtcat-2026-110772-coinductive-invertibility-in-higher-categories Ioannis Markakis en false https://pretalx.c3voc.de/qtcat-2026/talk/LARUFT/ https://pretalx.c3voc.de/qtcat-2026/talk/LARUFT/feedback/ R. 221 Talk 2026-08-13T11:30:00+02:00 11:30 00:30 While there are several formalisms for quantum field theory, a rigorous mathematical description is still not available. In some areas, as for example in conformal field theory, one is better equipped and proper definitions like vertex algebras can be made. By introducing the concept of a chiral algebra on an algebraic curve, Beilinson and Drinfeld have provided a reformulation of vertex algebras that centers around the similarity between the operator product expansion and the Jacobi identity for Lie algebras. Concretely, a chiral algebra is a Lie algebra in the category of D-modules. While for vertex algebras a generalisation to algebraic varieties of higher dimension seems very difficult, higher chiral algebras can be defined within the theory of quasi-categories. Via a polysimplicial model of the derived sections on configuration spaces, in joint work with Zhengping Gui and Charles Young, we have shown that the fundamental example reassembles a strong homotopy Lie algebra. Lurie introduced a construction in the topological setting that links En-algebras to factorisation algebras, the Koszul dual of chiral algebras. Together with Keyou Zeng, our aim is to unify both pictures with the purpose of describing mixed topological/holomorphic theories in analogy to the notions of raviolo vertex algebras and configuration spaces as described in Alfonsi, Kim, Young. Link to my slides: [https://cloud.lauraolivia.eu/index.php/s/mWnZ7HKbizK4ick](https://cloud.lauraolivia.eu/index.php/s/mWnZ7HKbizK4ick) qtcat-2026-110754-from-higher-to-raviolo-chiral-algebras /media/qtcat-2026/submissions/DRCAUD/image_Lyh5V8Y.webp Laura Olivia Felder en true https://pretalx.c3voc.de/qtcat-2026/talk/DRCAUD/ https://pretalx.c3voc.de/qtcat-2026/talk/DRCAUD/feedback/ R. 221 Talk 2026-08-13T12:00:00+02:00 12:00 00:30 Given a category C and a class of "weak equivalences" W in C, the morphisms in the localization Ho(C) of C at W generally consist of "zigzags" of maps in C and W of arbitrary length. In certain good cases, however, any morphism in Ho(C) is represented by a span of a leftward pointing weak equivalence followed by a rightward pointing map in C. It is known that these good cases yield a similar calculus of fractions in the context of infinity-categories and infinity-categorical localization. We aim to describe an alternative proof of this fact using a special kind of cocartesian fibration. If time permits, we describe how this approach gives a quick proof that the underlying infinity-category of a fibration category (resp. model of homotopy type theory with appropriate type constructors) has finite limits (resp. is locally cartesian closed). qtcat-2026-110764-the-calculus-of-fractions-via-cocartesian-fibrations Daniël Apol (they/them) en true https://pretalx.c3voc.de/qtcat-2026/talk/M8E3PD/ https://pretalx.c3voc.de/qtcat-2026/talk/M8E3PD/feedback/ R. 221 Talk (Speaker) 2026-08-13T14:00:00+02:00 14:00 01:00 As a type theorist who hangs out with category theorists, I am always fascinated by what we share and by how we differ. What do some of us fuss over that others just take for granted? I gave a talk at Topos Institute with some observations in this area five years ago. In this talk, I'll look at what has changed, including me, in those five years. In particular, I have experienced the need to have more to say about monoidal structure - juxtaposition - as well as composition. I'll talk about opportunities and obstacles for using appropriate types to work with these cats. qtcat-2026-110774-cats-and-types-side-by-side Conor Titania Mc Bride en false https://pretalx.c3voc.de/qtcat-2026/talk/WDTZ3Q/ https://pretalx.c3voc.de/qtcat-2026/talk/WDTZ3Q/feedback/ Workshop 1 Mini-Workshop 2026-08-13T15:30:00+02:00 15:30 01:30 **Does everyone have their own visual grammar?** Across disciplines, researchers rely on diagrams to convey ideas, explore concepts, and illustrate relationships. In this mini-workshop, we will have fun creating, interpreting, and discussing our own diagrams together. Through a series of team challenges, we will translate concepts into diagrams and experiment with different ways of representing ideas visually. Along the way, we will compare and discuss our approaches. As we will see, there are no right or wrong answers. The workshop is designed to bring together people from different backgrounds, encourage us to learn from each other, and enjoy exchanging ideas about the many ways in which diagrams can be constructed and interpreted. No artistic talent or prior knowledge is required. Whether you are a philosopher, mathematician, linguist, computer scientist, or biologist, this workshop is for you if you love structures. **Format:** flexible (60, 90, or 120 minutes) **Group size:** 3-12 participants (max 16) qtcat-2026-110756-diagrammar-drawing-meaning-together /media/qtcat-2026/submissions/7HUMDD/image_bYNqVDn.webp Kerstin S. Raab en true https://pretalx.c3voc.de/qtcat-2026/talk/7HUMDD/ https://pretalx.c3voc.de/qtcat-2026/talk/7HUMDD/feedback/ Workshop 2 Mini-Workshop 2026-08-13T15:30:00+02:00 15:30 01:30 Claiming that big tech companies have been our allies would be quite a stretch. The good news are you don't have to rely on their servers to bring your activism (or whatever else you might want to host) online. In times of looming censorship, privacy violation, right wing propaganda & lobbying digital sovereignty becomes increasingly relevant. In this mini workshop I will set up a web server (based on Debian, Apache, PHP & MariaDB) on an old raspberry pi from scratch. With that you can run smaller websites and even open source cloud platforms like Nextcloud from your living room with energy consumption under 30kWh per year. I will assume only basic IT skills, explain the fundamentals on the go and share the code I use. You can also bring your own device and follow along! The equipment you need: - Raspberry Pi 2B or newer - empty microSD card with minimum 8GB - micro USB phone charger (USB type C for newer models) - ethernet cable - to go public: a rooter that you have access to Link to my notes: [https://cloud.lauraolivia.eu/index.php/s/mWnZ7HKbizK4ick](https://cloud.lauraolivia.eu/index.php/s/BWqbjW6X7LFCpq6) qtcat-2026-110755-self-hosting-for-beginners-your-very-own-web-server-in-less-than-an-hour-for-under-50 Laura Olivia Felder en true https://pretalx.c3voc.de/qtcat-2026/talk/X8JHS9/ https://pretalx.c3voc.de/qtcat-2026/talk/X8JHS9/feedback/ Outside Event 2026-08-13T18:30:00+02:00 18:30 03:00 We will have a BBQ at the main campus at Von-Melle-Park 5. qtcat-2026-110777-bbq en true https://pretalx.c3voc.de/qtcat-2026/talk/7FZSD8/ https://pretalx.c3voc.de/qtcat-2026/talk/7FZSD8/feedback/ R. 221 Talk 2026-08-14T10:00:00+02:00 10:00 00:30 The first part of this talk will be an overview of my personal mathematical history. This history is typical, until the point where Academia asked me to make a compromise I couldn't accept. At this point, I decided to go independent. The second part of this talk will explain the pros and cons of this kind of independence, as well as a how-to-guide for starting your own non-profit research institute. This talk will include several fun mathematical puzzles. qtcat-2026-110763-how-to-be-a-mathematician-on-your-own-terms-or-at-least-how-i-did-it Jade Master en true https://pretalx.c3voc.de/qtcat-2026/talk/VZRU3N/ https://pretalx.c3voc.de/qtcat-2026/talk/VZRU3N/feedback/ R. 221 Talk 2026-08-14T10:30:00+02:00 10:30 00:30 Each algebraic theory has some free algebras. Which algebraic theories have the property that all their algebras are free? It turns out that it is not hard to state precisely which theories have this property; proving that they are the only ones is a much more difficult task. Proofs were given in the 1970s in the language of logic by Steven Givant[1], and in the 2010s in the language of classical universal algebra by Keith A. Kearnes, Emil W. Kiss, and Ágnes Szendrei[2]. Tom Leinster and I investigated this result from a category theoretic point of view[3]: I will talk about the theories in question, highlighting some of their interesting properties. I will also spend some time asking what it might mean for us to live in a world where everyone is free, and in particular, how we might get there. [1] Steven Givant. Universal Horn classes categorical or free in power. *Annals of Mathematical Logic*, 15(1):1–53, 1978. [2] Keith A. Kearnes, Emil W. Kiss, and Ágnes Szendrei. Varieties whose finitely generated members are free. *Algebra Universalis*, 79, 2018. [3] Maia Woolf. *Algebraic theories all of whose algebras are free*. MScR Thesis, University of Edinburgh, 2025. qtcat-2026-110757-worlds-where-all-are-free Maia Woolf (she/her) en true https://pretalx.c3voc.de/qtcat-2026/talk/VSBM8C/ https://pretalx.c3voc.de/qtcat-2026/talk/VSBM8C/feedback/ R. 221 Talk (Speaker) 2026-08-14T11:45:00+02:00 11:45 01:00 I will talk about a famous category variously known by its objects as the category of containers, by its morphisms as the category of lenses, and via an equivalence as the category of polynomial functors Poly (which is less practical to work with but has better puns). As well as being theoretically interesting this category has a frankly ridiculous number of practical applications, which I will do my best to give an impression of. After being blocked for years by the difficulty of programming with lenses, last year I quit academia to get more research time and now I am building a programming language in which all types denote containers (aka polynomial functors) and functions denote lenses. This language, which I also call Poly, is half way through development and can hold together just barely enough to give a working demo. I will talk about the challenges of designing a syntax around an unfamiliar semantics, and the interplay of syntax, semantics and pragmatics in programming language development. This is joint work with Zanzi Mihejevs (my primary partner, who introduced me to gender, polyamory and programming language theory) and Dylan Braithwaite (my PhD student and also my boss) qtcat-2026-110770-poly-is-difficult-but-rewarding Jules Hedges en true https://pretalx.c3voc.de/qtcat-2026/talk/NFAZFU/ https://pretalx.c3voc.de/qtcat-2026/talk/NFAZFU/feedback/ R. 221 Talk 2026-08-14T14:30:00+02:00 14:30 00:30 Often we define functors F where each object F(A) satisfies a universal property; these could be products, limits, colimits, (co)ends, (co)free objects, kan extensions, exponential objects... Usually, existence and uniqueness of the mediating morphisms allow you to define the functorial action and verify functoriality, respectively. But as these constructions get more complicated, the diagram-chasing gets more and more tedious, and one gets the feeling that there should be some universal theorem giving functoriality for free. This talk will show such a silver bullet _does_ exist! In these situations of a **parametrised family of universal properties** there is an economical, "maximally lazy" way to define your functors on objects, and deduce the action on morphisms. qtcat-2026-110762-parametrised-representability Ruby Khondaker en false https://pretalx.c3voc.de/qtcat-2026/talk/TB7KBX/ https://pretalx.c3voc.de/qtcat-2026/talk/TB7KBX/feedback/ R. 221 Talk 2026-08-14T15:00:00+02:00 15:00 00:30 A closing for QTCat, some closure for myself and maybe a bit about categorical completions. qtcat-2026-110779-closing-closure-completion Tessa Kammermeier en false https://pretalx.c3voc.de/qtcat-2026/talk/8FPG9V/ https://pretalx.c3voc.de/qtcat-2026/talk/8FPG9V/feedback/ R. 221 Mini-Workshop 2026-08-14T16:00:00+02:00 16:00 01:30 QTCat was inspired by QTMART (Queer and Trans Mathematicians in Algebra and Representation Theory) and TACT (Trans-Actions in Category Theory). If you feel inspired by QTCat or just want to hear me rant about university bureaucracy, come and join me as I talk about the work that went into making QTCat possible. qtcat-2026-110780-how-to-qt Tessa Kammermeier en true https://pretalx.c3voc.de/qtcat-2026/talk/F7RS9D/ https://pretalx.c3voc.de/qtcat-2026/talk/F7RS9D/feedback/ Workshop 1 Mini-Workshop 2026-08-14T16:00:00+02:00 16:00 01:30 Queer tango reimagines Argentine tango as an inclusive practice where everyone is welcome to dance regardless of gender, sexuality, body, age, or experience. By challenging the heteronormative traditions and fixed roles of tango, it creates a welcoming space to explore new ways of moving and connecting, fostering community and celebrating queer joy. In this introductory workshop, we will explore the basic principles of movement, as well as connection with our partners, the music, and the dancers around us. The workshop will last 1 hour followed by 30 minutes of free dancing. It can accommodate any number of participants depending on the available room size. No partner, special shoes, or prior dance experience are required. qtcat-2026-110767-introduction-to-queer-tango Ioannis Markakis / Peter Camporeale en true https://pretalx.c3voc.de/qtcat-2026/talk/Y8H3CM/ https://pretalx.c3voc.de/qtcat-2026/talk/Y8H3CM/feedback/ Workshop 2 Mini-Workshop 2026-08-14T16:00:00+02:00 16:00 01:30 In this mini-workshop, I would like to initiate a discussion on the following question: To what extent does being queer influence how a person practices mathematics, and what kind of mathematics emerges as a result? My starting point is similar questions and lines of inquiry concerning women and mathematics: How might gender influence the practice of doing mathematics? Is there such a thing as “female mathematics”? (Time: 1h. Participants: no limit.) qtcat-2026-110761-queer-mathematics Andrea Blunck en true https://pretalx.c3voc.de/qtcat-2026/talk/J9MHPJ/ https://pretalx.c3voc.de/qtcat-2026/talk/J9MHPJ/feedback/