## May 24, 2017

### A Type Theory for Synthetic ∞-Categories

#### Posted by Emily Riehl

One of the observations that launched homotopy type theory is that the rule of identity-elimination in Martin-Löf’s identity types automatically generates the structure of an $\infty$-groupoid. In this way, homotopy type theory can be viewed as a “synthetic theory of $\infty$-groupoids.”

It is natural to ask whether there is a similar *directed* type theory that describes a “synthetic theory of $(\infty,1)$-categories”
(or even higher categories). Interpreting types directly as (higher) categories runs into various problems, such as the fact that not all maps between categories are exponentiable (so that not all $\prod$-types exist), and that there are numerous different kinds of “fibrations” given the various possible functorialities and dimensions of categories appearing as fibers. The 2-dimensional directed type theory of Licata and Harper has semantics in 1-categories, with a syntax that distinguishes between co- and contra-variant dependencies; but since the 1-categorical structure is “put in by hand”, it’s not especially synthetic and doesn’t generalize well to higher categories.

An alternative approach was independently suggested by Mike and by Joyal, motivated by the model of homotopy type theory in the category of Reedy fibrant simplicial spaces, which contains as a full subcategory the $\infty$-cosmos of complete Segal spaces, which we call *Rezk spaces*. It is not possible to model ordinary homotopy type theory directly in the Rezk model structure, which is not right proper, but we can model it in the Reedy model structure and then identify internally some “types with composition,” which correspond to Segal spaces, and “types with composition and univalence,” which correspond to the Rezk spaces.

Almost five years later, we are finally developing this approach in more detail. In a new paper now available on the arXiv, Mike and I give definitions of *Segal* and *Rezk types* motivated by these semantics, and demonstrate that these simple definitions suffice to develop the synthetic theory of $(\infty,1)$-categories. So far this includes functors, natural transformations, co- and contravariant type families with discrete fibers ($\infty$-groupoids), the Yoneda lemma (including a “dependent” Yoneda lemma that looks like “directed identity-elimination”), and the theory of coherent adjunctions.

## May 12, 2017

### Unboxing Algebraic Theories of Generalised Arities

#### Posted by Emily Riehl

*Guest post by José Siqueira*

We began our journey in the second Kan Extension Seminar with a discussion of the classical concept of * Lawvere theory *, facilitated by Evangelia. Together with the concept of a model, this technology allows one to encapsulate the behaviour of algebraic structures defined by collections of $n$-ary operations subject to axioms (such as the ever-popular groups and rings) in a functorial setting, with the added flexibility of easily transferring such structures to arbitrary underlying categories $\mathcal{C}$ with finite products (rather than sticking with $\mathbf{Set}$), naturally leading to important notions such as that of a Lie group.

Throughout the seminar, many features of Lawvere theories and connections to other concepts were unearthed and natural questions were addressed — notably for today’s post, we have established a correspondence between Lawvere theories and finitary monads in $\mathbf{Set}$ and discussed the notion of operad, how things go in the enriched context and what changes if you tweak the definitions to allow for more general kinds of limit. We now conclude this iteration of the seminar by bringing to the table “Monads with arities and their associated theories”, by Clemens Berger, Paul-André Melliès and Mark Weber, which answers the (perhaps last) definitional “what-if”: what goes on if you allow for operations of more general arities.

At this point I would like to thank Alexander Campbell, Brendan Fong and Emily Riehl for the amazing organization and support of this seminar, as well as my fellow colleagues, whose posts, presentations and comments drafted a more user-friendly map to traverse this subject.

## May 1, 2017

### A Discussion on Notions of Lawvere Theories

#### Posted by Emily Riehl

*Guest post by Daniel Cicala*

The Kan Extension Seminar II continues with a discussion of the paper Notions of Lawvere Theory by Stephen Lack and Jirí Rosický.

In his landmark thesis, William Lawvere introduced a method to the study of universal algebra that was vastly more abstract than those previously used. This method actually turns certain mathematical stuff, structure, and properties into a mathematical object! This is achieved with a *Lawvere theory*: a bijective-on-objects product preserving functor $T \colon \aleph^{\text{op}}_0 \to \mathbf{L}$ where $\aleph_0$ is a skeleton of the category $\mathbf{FinSet}$ and $\mathbf{L}$ is a category with finite products. The analogy between algebraic gadgets and Lawvere theories reads as: stuff, structure, and properties correspond respectively to 1, morphisms, and commuting diagrams.

To get an actual instance, or a *model*, of an algebraic gadget from a Lawvere theory, we take a product preserving functor $m \colon \mathbf{T} \to \mathbf{Set}$. A model picks out a set $m(1)$ and $n$-ary operations $m(f) \colon m(1)^n \to m(1)$ for every $T$-morphism $f \colon n \to 1$.

To read more about classical Lawvere theories, you can read Evangelia Aleiferi’s discussion of Hyland and Power’s paper on the topic.

With this elegant perspective on universal algebra, we do what mathematicians are wont to do: generalize it. However, there is much to consider undertaking such a project. Firstly, what elements of the theory ought to be generalized? Lack and Rosický provide a clear answer to this question. They generalize along the following three tracks:

consider a class of limits besides finite products,

replace the base category $\mathbf{Set}$ with some other suitable category, and

enrich everything.

Another important consideration is to determine exactly how far to generalize. Why not just go as far as possible? Here are two reasons. First, there are a number of results in this paper that stand up to further generalization if one doesn’t care about constructibility. A second limiting factor of generalization is that one should ensure that central properties still hold. In *Notions of Lawvere Theory*, the properties lifted from classical Lawvere theories are

the correspondence between Lawvere theories and monads,

that algebraic functors have left adjoints, and

models form reflective subcategories of certain functor categories.

Before starting the discussion of the paper, I would like to take a moment to thank Alexander, Brendan and Emily for running this seminar. I have truly learned a lot and have enjoyed wonderful conversations with everyone involved.