# Learning roadmap to 'Differential cohomology in a cohesive $\infty$ topos'

I am very curious to study arXiv:1310.7930 (henceforth:DCCT) but am not sure if I have the pre-requisites. I am familiar with basic algebraic topology (singular cohomology, classifying spaces, characteristic classes), differential geometry (bundles, connections, Chern--Weil theory) and some elementary homotopy theory (model categories, spectra, generalised cohomology theories). I am comfortable with the Physics aspects (classical and quantum field theories).

However it appears that in order to understand DCCT, I should know the basics of

• Homotopy Type Theory
• Toposes
• Higher Toposes
• Infinity categories

I have no clue about these subjects. A detailed study of each of these subjects appears to be a humongous task. So I wish to study these subjects with a view to learn as much is necessary for DCCT (and possibly slightly more).

My questions are:

1. What are the subjects which I should familiarise myself with, in order to be able to read DCCT?

(I am not sure if my listing above is accurate, I am just guessing by a glance at the TOC.)

and

2. Could you please suggest a roadmap to learning the required basics for a person with my background?

As far as possible, I prefer to try to learn stuff linearly and minimise back-and-forth. So a roadmap of pre-requisites would be of great help.

It would be very kind and helpful if you could give pointed references to specific chapters instead of whole books. Thanks a lot!

• I don't think Homotopy Type Theory is going to be necessary, a cursory glance make its usage seem more a presentation choice than anything else. Oct 13 at 21:33
• @DenisNardin Thanks! It is helpful to know that I can skip HoTT for now. 2 days ago

I haven't read all of DCCT so take this with a grain of salt, but after having spent a lot of time with it, this is how I would recommend getting started on the abstract stuff.

First one must learn classical Grothendieck Topos Theory. Chapter 1.2 of DCCT gives a pretty good motivation and some nice examples of sheaves on $$\mathsf{Cart}$$, but I would recommend David Carchedi's course on Topos theory, which is the quickest course that I could find that covered most of the relevant material.

After learning Grothendieck topos theory one must then learn how to combine homotopy theory with topos theory, this was originally achieved using simplicial sheaves or presheaves, and then was abstracted to the definition of a model topos. For this there is

Now one must learn Infinity Topos Theory. This is harder to recommend resources for as there are much fewer. There are many places to find recommendations for resources on learning infinity category theory, but honestly you don't need to delve into too much of the details to understand much for DCCT, you can really take much of infinity category theory as a black box that just works like usual category theory but with equivalences instead of isomorphisms and homotopy type mapping spaces instead of hom sets. I'd recommend Rezk's notes on quasicategories, get comfortable with the basics, and then watch Rezk's lectures on Youtube, as well as Joyal's.

I'm not saying you need to look at all of these resources, but what you need to know is contained in them. I would start off by first looking at some of Schreiber's previous papers that he mixed into DCCT, like Cech Cocycles for Differential Characterstic Classes and the Principal Infinity Bundles papers 1 and 2.

Addendum: Morally, the use of higher topos theory in DCCT is as a generalization of the nonabelian cohomology of Grothendieck and Giraud. In DCCT, these generalized cohomology classes are given by higher principal bundles. The use of higher category theory makes the formulation of this theory very elegant, but ultimately it is grounded in the theory of stacks and gerbes. Knowing this theory is not necessary for understanding DCCT or HTT, but it is a great way to build motivation and see what a lot of this theory is actually being used for. Here are some references in the differential geometry setting:

• Wow! Thanks so much for the detailed and helpful answer! A follow up question along the same lines: I find 'stacks' mentioned a lot in DCCT. Will the references you suggested cover this topic? If not, could you please suggest how to go about learning it? (I have zero knowledge of algebraic geometry, so references which do not use AG would be very helpful.) Thanks! 2 days ago
• Ah I forgot to mention stacks. See the addendum in the edit above. 2 days ago

I would say that understanding traditional differential cohomology is a reasonable prerequisite.

There are multiple good sources:

Further suggestions can be found in this answer:

References for differential cohomology and differential characters

For higher topos theory (as it is relevant for DCCT) one could start with