### Huybrechts on Branes in K3, I

#### Posted by Urs Schreiber

Yesterday we had Daniel Huybrechts from Bonn talking in the ZMP seminar about stability conditions on derived categories of coherent sheaves over K3 surfaces.

Daniel Huybrechts
*Derived and Abelian Equivalence of K3 Surfaces*

math.AG/0604150

which is based on work by Bridgeland, who defined and studied stability conditions on triangulated categories in

Tom Bridgeland
*Stability Conditions on Triangulated Categories*

math.AG/0212237

and applied that to the case of derived categories of coherent sheaves on K3 surfaces in

Tom Bridgeland
*Stability conditions on K3 surfaces*

math.AG/0307164 .

This is pure math, but there is a nice dictionary which maps it 1-1 to the physics of D-branes. For a good overview of how this works see

Paul S. Aspinwall
*D-Branes on Calabi-Yau Manifolds
*

hep-th/0403166,

which I once tried to summarize here.

Roughly, this dictionary is as follows:

A “geometric brane” for a B-model topological string on $X$ (i.e. a brane which can be regarded as a submanifold of $X$ with a vector bundle on it) is encoded in a coherent sheaf on $X$.

The general brane for the B-model string is obtained by stacking a collection of geometric branes and anti-branes on top of each other and turning on tachyon condensates between them (which in part mutually annihilates them, the remaining piece being a general brane). In the formalism, this corresponds to a bounded complex of coherent sheaves on $X$ (with the differential of the complex encoding the tachyonic string condensates stretching between the branes).

But ultimately we are interested not in the boundary conditions (= branes) of the topological B-model string, but of the physical type II string (i.e. of a CFT instead of a TQFT).

It turns out that all the branes of the topological B-string correspond to branes of the physical string - but not all of them are “stable” for the physical string. Instead, every brane for the topological string is supposed to decay into a collection of stable branes - the physical BPS branes.

More precisely, the derived category of coherent sheaves is what is called “triangulated”, which means that it contains lots of certain triangle diagrams (i.e. collections of three of its objects with certain morphism between them, satisfying some conditions). These triangles precisely encode the “brane chemistry”. Roughly, a triangle

says (as familiar from exaxt sequences), that the brane $B$ is an “extension” of the brane $C$ by the brane $A$. In other words, the existence of this triangle encodes the potential brane reaction

This is very much like in chemistry. (And of course oversimplified, see section 6.2 of Aspinwall’s review for the details.)

Which branes are stable and which are not is encoded by a “stability condition”, which, on the physics side, is called $\Pi $-stability. As far as I am aware, the entire motivation for Bridgeland to define stability conditions on triangulated categories comes from the desire to axiomatize this piece of physical input. The abstract definition of a stability condition on a triangulated category may look completely ad hoc, its natural meaning becomes manifest once you think of distinguished triangles as describing reaction processes of fusion and decay of branes.

Hence, whether one is interested in D-branes or not, when reasoning about stability condtions on derived categories of coherent sheaves it helps a lot to keep the above dictionary in mind. It makes many of the constructions and results better memorizable. (For more on the physics side see Eric Sharpe’s encyclopedia entry $\to $).

All, right, below I reproduce a transcript of the talk.

The talk consisted of two parts:

I) Introduction to the concept of derived categories of coherent sheaves.

II) Equivalences of categories versus isomorphism of target spaces.

The **main result** presented (theorem 0.1 in Huybrechts’ paper mentioned above) says, in terms of the physics side of the dictionary, that two K3 surfaces are indistinguishable as target spaces for the topological string if and only if there are complexified Kähler classes on them which make them indistinguishable for the physical string.

The following are the notes that I took in the talk (I have included some links and some personal comments, set in italics). The notes on part II) are given in a seperate entry.

**I)** Introduction to the concept of derived categories of coherent sheaves.

In the following, $X$ denotes an algebraic variety ($\to $), more precisely, a smooth projective variety sitting in some projective space. Later we restrict attention to $X$ being a K3 surface ($\to $).

Denote by $\mathrm{Coh}(X)$ the category of coherent sheaves ($\to $) on $X$.

*Examples* for coherent sheaves on $X$ are

i) Holomorphic vector bundles ($\to $) on $X$ are coherent sheaves (or rather, their sheaves of sections are).
(*This corresponds to stacks of space-filling geometric branes.*)

ii) If $C\subset X$ is a holomorphic curve, then its structure sheaf ${O}_{C}$ is a coherent sheaf.
(*This would be a lower-dimensional geometric brane.*)

iii) For every point $x\in X$, the skyscraper sheaf $k(x)$ is a coherent sheaf. (This is the sheaf whose fiber over $U$ is nonempty precisely if $x\in U$).
(*These skyscrapersheaves encode 0-branes, localized at points (we do not consider a temporal deriction in this sort of game).)*)

iv) Let $E$ be a vector bundle on $X$, or rather its sheaf of sections, and let $E\to k(x)$ be the obvious projection onto the skyscraper sheaf at $x$, then

is a torsion free coherent sheaf.

For $E$ trivial this is the “ideal sheaf” (with “ideal” in the sense of ideal of a ring) ${I}_{x}$, namely the sheaf of holomorphic functions vanishing at the point $x$.

(*This corresponds to a space filling stack of D-branes having reacted with an anti-0-brane at position $x$.*)

Once we understand these examples, we understand all coherent sheaves, in the sense that every coherent sheaf can be constructed by performing direct sums, quotients, kernels, etc. (*etc?*) of the sheaves in the above examples.

Next, we pass from $\mathrm{Coh}(X)$ to ${D}^{b}(\mathrm{Coh}(X))$, its derived category ($\to $, $\to $).

This works as follows.

Denote by ${K}^{b}(X):={K}^{\u2022}(\mathrm{Coh}(X))$ the category of bounded complexes of coherent sheaves. Its objects are bounded complexes, its morphisms are chain homotopy classes of chain maps between morphisms.

(*BTW, what would happen if we did not divide out by chain homotopies? If we even considered the full 2-category of complexes, chain maps and chain homotopies?*)

(*Given any complex, we can compute its cohomology in every degree. We can regard the collection of the cohomologies as a complex themselves, with all differentials being trivial. Given a chain map between two complexes, we get a chain map between the corresponding trivial complexes of cohomologies. Hence cohomology is really a functor from the category of complexes of coherent sheaves to to the category of complexes of abelian groups. If a morphism in ${K}^{b}(X)$ is not an isomorphism, but maps to an isomorphism under this functor, then we call it a “quasi isomorphism”*.)

The derived category ${D}^{b}(X):={D}^{b}(\mathrm{Coh}(X))$ is like ${K}^{b}(X)$, but with all quasi-isomorphisms regarded as true isomorphisms.

This is a special case of “localization of a category” ($\to $).

Now, ${D}^{b}(X)$ is no longer abelian ($\to $) - but it is triangulated ($\to $)!

In fact, every derived category of any abelian category is triangulated. This is hence not a special property of coherent sheaves. On the other hand ${D}^{b}(X)$ is actually $\u2102$-linear triangulated (*which hopefully means the obvious thing*).

A category being triangulated means in particular that it carries the following two crucial structures.

i) There is a shift endomorphism

(*for the case of derived categories $E[1]$ is the same complex as $E$, but shifted in degree to the left, by one unit, i.e. $E[1{]}^{n}={E}^{n+1}$*)

and

ii) there are “distinguished triangles”, which are diagrams

As an example: every “mapping cone” ($\to $) gives rise to such a triangle.

(*See math.AG/0001045 for an explanation of the mapping cone and why it has the name it has. The triangle obtained from the mapping cone of the map $E\stackrel{f}{\to}F$ between “geometric branes” $E$ and $F$ corresponds to the D-brane reaction induced by $f$ when this is regarded as a tachyon condensate of strings stretching between brane $E$ and brane $F$*.)

We also need to mention something called a *t-structure* ($\to $) on a triangulated category. This is a collection of subcategories with some properties. For derived categories, such subcategories include ${D}^{\le 0}(X)$ and ${D}^{\ge 0}(X)$, the subcategories of complexes concentrated in non-positive or in non-negative degree, respectively.

The intersection of these two subcategories is called the *core* of ${D}^{b}(X)$. Evidently, this is naturally identified with the original category $\mathrm{Coh}(X)$.