# Lecture series: Hirzebruch-Riemann-Roch as a categorical trace.

## Professor Dennis Gaitsgory (Harvard University)

Location: Bonn, Germany.
Dates: January 10-13, 2017.

• Tuesday January 10:
• 04:30 PM - 06:30 PM.
• Vivatsgasse 7, Hörsaal MPIM
• Wednesday January 11:
• 02:00 PM - 04:00 PM.
• Endenicher Allee 60, Room 1.016 (Lipschitz hall)
• Friday January 13:
• 12:30 PM - 02:30 PM.
• Vivatsgasse 7, Hörsaal MPIM

### Abstract

Let $$X$$ be a smooth proper scheme over a field of characteristic 0, and let $$E$$ be a vector bundle on $$X$$. The classical Hirzebruch-Riemann-Roch says that the Euler characteristic of the cohomology $$H^*(X,E)$$ equals $$\int_X \text{ch}(E) \; \text{Td}(X)$$. Thus, HRR is an equality of numbers, i.e., elements of a set. In these talks, we will explain a proof of HRR that uses the hierarchy $\{2-\text{categories}\} \rightarrow \{1-\text{categories}\} \rightarrow \{\text{Vector spaces}\} \rightarrow \{\text{Numbers}\}.$ I.e., the origin of HRR will be 2-categorical. The procedure by which we go down from 2-categories to numbers is that of *categorical trace*.

However, in order to carry out our program, we will need to venture into the world of higher categories: the 2-category we will be working with consists of DG-categories, the latter being higher categorical objects. And the process of calculation of the categorical trace will involve derived algebraic geometry: the key geometric player will be the self-intersection of the diagonal of $$X$$, a.k.a. the inertia (derived) scheme of $$X$$.

So, this series of talks can be regarded as providing a motivation for studying higher category theory and derived algebraic geometry: we will use them in order to prove an equality of numbers. That said, we will try to make these talks self-contained, and so some necessary background will be supplied.

Video recordings are available at https://www.mpim-bonn.mpg.de/node/7032