# Introduction to Chiral Algebras

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1 Introduction to Chiral Algebras Nick Rozenblyum Our goal will be to prove the fact that the algebra End(V ac) is commutative. The proof itself will be very easy - a version of the Eckmann Hilton argument - once the machinery of chiral algebras is set up. 1 Chiral Algebras Let X be a smooth curve over C. A non-unital chiral algebra on X is a D-module A along with a chiral bracket map µ : j j (A A)! (A) where j : U X 2 Z : are the inclusion of the complement of the diagonal and the diagonal respectively. We require that µ be antisymmetric and satisfy a version of the Jacobi identity: Antisymmetry: µ = σ 1,2 µ σ 1,2, where σ 1,2 is induced action on A by permuting the variables of X 2. Jacobi Identity: we have three maps µ 1(23), µ (12)3 and µ 2(13) : j j (A A A)! (A) where (somewhat abusing notation) j is the inclusion of the open in X 3 which is the complement of all the diagonals and is the inclusion of X as the diagonal. We have that µ 1(23) is defined as the composition j j (A A A) (x2 =x 3 )!j j (A A)! A and the others are defined similarly. We then demand that µ 1(23) = µ (12)3 + µ 2(13) 1

2 Example 1. Let A = ω X. We then have the canonical exact sequence 0 ω X ω X j j (ω X ω X )! (ω X ) 0 which gives ω X a chiral bracket. It is clearly antisymmetric. We need to check that it satisfies the Jacobi identity. To do that, consider the Cousin complex for ωx 3 on X3 for the stratification given by the diagonals. It gives the exact sequence 0 ω 3 X j j (ω 3 X ) The three maps in the complex x1 =x 2!j j (ωx 2) x1 =x 3!j j (ωx 2) x2 =x 3!j j (ωx 2)! (ω X ) 0 j j (ω 3 X )! (ω X ) are exactly the maps in the Jacobi identity and the fact that the above Cousin complex is a complex at the term between those is exactly the condition that the Jacobi identity is satisfied. We can now finish defining a chiral algebra. Definition 2. A (unital) chiral algebra A is a non-unital chiral algebra together with a map of chiral algebras ω X A such that the restriction of the chiral bracket µ : j j (ω X A)! (A) is the canonical map coming from the complex 0 ω X A j j (ω X A)! (A). As with any kind of algebra, given a chiral algebra A, we can consider modules over it. 2

3 Definition 3. Let A be a chiral algebra. A chiral A module is a D-module M on X together with an action map satisfying the unit and Lie identity: ρ : j j (A M)! (M) Unit: we require that the restriction of ρ to ω X be the canonical map. Lie action: ρ : j j (ω X M)! (M) ρ(µ id) = ρ(id ρ) σ 12 ρ((id ρ) σ 12 ) as maps j j (A A M)! (M). Example 4. Let A be a chiral algebra. Then A is canonically a chiral A module. Let M be a D-module on X. Then the canonical map j j (ω M)! (M) makes M into a chiral ω module. In fact, because of the unit axiom, this is the unique structure of a chiral ω module on M. Thus we have an equivalence of categories {D-modules} {chiral ω modules}. For our purposes, we will assume that A is flat as an O X module. However, we will not make a similar assumption on M. For instance, we will often be interested in modules supported at a point x X. A typical example of this is the vacuum module M = i x! i! x(a)[1]. Recall that for D-modules we have the derham functor h : D-mods {sheaves} 3

4 given by modding out by the action of vector fields: h(m) = M/M Θ. Now, for a chiral algebra A, consider the composition Applying the derham functor, we get which by adjunction gives the map A A j j (A A)! (A). h(a) h(a) (h(a)) h(a) h(a) h(a) which makes h(a) into a sheaf of Lie algebras. Now suppose that M is a chiral A module supported at a point x X. Let M = i! x(m) be the underlying vector space. Pushing forward the action map j j (A M)! (M) along the first projection we get DR(X x, A) M M which is an action of the Lie algebra DR(X x, A) on M. In fact, we can shrink the curve X to get an action of the topological Lie algebra DR(D x, A) on M where DR(D x, A) = lim i! x(j x j x(a)/a ξ ) where the inverse limit is taken over submodules A ξ j x j x(a) such that the quotient j x j x(a)/a ξ is supported at x and j x is the inclusion the open set X x. 2 Factorization There is another equivalent description of chiral algebras in terms of factorization which given what we ve been doing in the seminar might be more 4

5 familiar. For the moment, let X be a topological space. We can then consider the Ran space of X defined as Ran(X) = {nonempty finite subsets of X} It is topologized so that the maps X n Ran(X) are continuous. There is a very important fact which will not be relevant for now, but is very important when dealing with homology of chiral algebas. Theorem 5. If X is connected, the topological space Ran(X) is weakly contractible. We will be interested in doing algebraic geometry on Ran(X) for X an algebraic curve. Unfortunately, it is not possible to define Ran(X) as any kind of algebraic space but we will be able to make sense of quasi-coherent sheaves on Ran(X). So let s return to X being an algebraic curve over C. Definition 6. A quasi-coherent sheaf F on Ran(X) is a collection of quasi coherent sheaves F I for each finite set I together with isomorphisms ν (π) : (J/I) F J F I for every surjection π : J I, where J/I : X I X J is the corresponding diagonal. We require that the ν (π) be compatible with composition of surjections. Moreover, we demand that the F (I) have no sections supported on the diagonals. Remark 7. Because of the condition requiring no sections supported on the diagonals, quasi-coherent sheaves on Ran(X) do not form an abelian category. Definition 8. A non-unital factorization algebra B is a quasi-coherent sheaf on Ran(X) along with isomorphisms c α : j α( B (I i) ) j α(b (I) ) for a partition α : I = I 1... I n a partition of I and j α is the inclusion of the open set U = {x i x j if i and j are in different I j }. We require that the c α be compatible with subpartitions and with the ν (π). 5

6 Example 9. Let O be the non-unital factorization algebra given by O (I) = O X I. This is a factorizable algebra in the obvious way. It is the unit factorization algebra. Definition 10. A (unital) factorization algebra B is a non-unital factorization algebra equipped with a map of non-unital factorization algebras O B such that locally for every section b B (1), 1 b j j (B (1) B (1) ) lies in B (2) j j (B (1) B (1) ) and (1 b) = b. Remark 11. In the definition of a factorization algebra, we required that the unit give a map B (1) O X B (2) compatible with restriction to the diagonal. In fact, we leave it to the reader to check that this implies that we have canonical maps B (I 1) O X I 2 B (I 1 I 2 ) compatible with factorization and restrictions to the diagonals. This follows from the condition requiring no sections supported on the diagonals for quasi-coherent sheaves on Ran(X). If we consider dg-factorization algebras, then we need to specify all these maps as part of the data of a unital dgfactorization algebra. Theorem 12. There is an equivalence of categories {factorization algebras} {chiral algebras} given by B B (1) ω X Proof. Let B be a factorization algebra. Let s show that each B (I) has a canonical structure of a left D-module. Giving B (I) such a structure is equivalent to giving an isomorphism between B (I) O X I and O X I B (I) on the formal completion of the diagonal in X I X I. The unit gives maps B (I) O X I B (I I) O X I B (I) 6

7 which are isomorphisms on the formal neighborhood of the diagonal X I giving the canonical connection. Now, let A (I) = B (I) ω X I be the corresponding right D-modules and let A = A (1). We then have the Cousin complex for A (2) : 0 A (2) j j (A A)! (A) 0 which gives the chiral bracket. It is clearly antisymmetric and the unit axiom is satisfied. The Cousin complex for A (3) with stratification given by the diagonals in X 3 gives the Jacobi identity. Thus, we have a functor from factorization algebras to chiral algebras. Let us now construct the inverse functor. Let A be a chiral algebra. On X I consider the Chevalley-Cousin complex: C I = j j (A} {{ A} ) α! (j j (A A))...! (A) I-times α P art I 1 (I) where P art n (I) is the set of partitions of I into n subsets and α for α P art n (I) is the corresponding n dimensional diagonal. The terms with n copies of A are in degree n and the differentials are given by the various chiral brackets. This complex is called the Chevalley-Cousin complex because it is the Chevalley complex from the point of view of the chiral algebra, and it is the Cousin complex (for the stratification given by the diagonals) from the point of view of the factorization algebra. Evidently, these complexes are factorize and are compatible with restriction to diagonals, i.e. we have isomorphisms for surjections π : J I and for partitions I = I 1... I n. ν (π) : (J/I)! C J C I c α : j α( C I i ) j α(c I ) We will show that H n (C I ) = 0 unless n = I by induction on I. For 7

8 I = 1, the complex is just given by A and there is nothing to prove. Now, for a general I, consider a codimension one diagonal i : X I X I U : v and a given decomposition I = I [1], with U the complement of X I. The open set U is affine and we have a short exact sequence of complexes 0 i! (C I ) C I v v (C I ) 0. By induction, H n (i! (C I )) = 0 unless n = I = I + 1 and by induction and factorization H n (v v (C I )) = 0 unless n = I since U is a union of complements of diagonals. Thus we need to show that the map vanishes. Let H I +1 (i! (C I )) H I +1 (C I ) Z := H I +1 (C I ) = Ker(C I I C I +1 I ) We have a canonical exact sequence with respect to the decomposition X I = X I X 0 Z ω X v v (Z ω X ) i! (Z) 0. Furthermore, we have a commutative diagram v v (Z ω) i! (Z) C I I d C I +1 I where the top map is given by the unit. It follows that the map is indeed zero. H I +1 (i! (C I )) H I +1 (C I ) Now, let B (I) := H I (C I ) ω 1 X I. By above, it is a factorization algebra. Furthermore, it gives the inverse functor {chiral algebras} {factorization algebras}. 8

9 The factorization perspective on chiral algebras is a very useful one. Suppose that B 1 and B 2 are factorization algebras. Then B (I) = B (I) 1 B (I) 2 is also a factorization algebra in the obvious way. In this way, we can consider tensor products of chiral algebras. Another important aspect of factorization algebra is that they admit a nonlinear analogue of factorization spaces. This will allow us to construct chiral algebras from geometry. Definition 13. A factorization space G is a collection of (ind-) schemes G I over X I for each finite set I along with isomorphisms (G J ) X I G I for every diagonal embedding X I X J and for every partition I = I 1... I n factorization ismorphisms G I U ( G Ii ) U where U is the open set corresponding to the partition. We require that these isomorphisms be compatible in the usual way. A factorization space is unital if in addition we have maps X I 1 G I2 G I2 I 2 compatible with factorization and restrictions to diagonals. We have already seen an example of a unital factorization space, namely the Beilinson-Drinfeld Grassmannian. Let G be an algebraic group. Recall that the Beilinson-Drinfeld Grassmannian Gr I (G) X I is defined as the moduli space of the following triples (in what follows we omit reference to G in the notation) Gr I = {P Bun G (X), (x 1,..., x I ) X I, φ : P X {(xi )} P triv X {(xi )}} where P triv Bun G (X) is the trivial G-bundle. Now, suppose we are given a unital factorization space G and a linearization functor, i.e. a rule for obtaining a sheaf on X I from each G I which preserves factorization then we can obtain a factorization algebra. Examples 9

10 of such linearization functors are global sections and pushforward of some canonically defined sheaf. In the case of the Beilinson-Drinfeld Grassmannian, consider for each I, S I the sheaf of D-module δ functions along the unit section X I Gr I. Now let V I be the pushforward of S I to X I as a quasi-coherent sheaf. These form a factorization algebra with fibers given by the vacuum module for every x X. We could also consider δ functions as a twisted D-module and in the same way obtain a factorization algebra with fibers given by the vacuum module at the corresponding level. In addition to describing chiral algebras in factorization terms, we can also describe modules. Definition 14. Let B be a factorization algebra. A factorization B module M is a collection of D-modules M Ĩ on XĨ for each finite set I, where Ĩ = I { }. For every surjection π : J Ĩ such that π( ) =, we have the corresponding diagonal XĨ X J and we are given isomorphisms M ( J) X Ĩ M (Ĩ) and for every partition I = I 1... I n, we are given factorization isomorphisms MĨ U ( B I i I M n ) U 0<i<n where U XĨ is the open set corresponding to the partition Ĩ = I 1... I n 1 Ĩn. We require that the isomorphisms be mutually compatible. Theorem 15. Let A be a chiral algebra and B be the corresponding factorization algebra. There is an equivalence of categories {factorization B-modules} {chiral A-modules} As a consequence, we can consider tensor products of chiral modules: let B 1 and B 2 be factorization algebras and M 1 and M 2 respective factorization modules. Then M = M 1 M 2 is a factorization module for the factorization algebra B = B 1 B 2. 10

11 3 Lie-* algebras A Lie-* algebra L is a D-module on X along with a Lie-* bracket µ : L L! (L) satisfying the Jacobi identity (in the same way as in the definition of a chiral algebra). Remark 16. Suppose that a Lie-* algebra L is holonomic as a D-module. In this case, we have the functor which is left adjoint to! =. In this case, we can consider the *-tensor product L L := (L L) and being a Lie-* algebra is equivalent to being a Lie algebra µ : L L L in the tensor category of holonomic D-modules with the *-tensor product. An advantage of considering Lie-* algebras is that it is relatively easy to construct examples. Suppose g is a Lie algebra. Then g D X is a Lie-* algebra. In fact, we could implement this construction for any Lie algebra in the category of quasi-coherent sheaves on X by instead tensoring over O X. Let A be a chiral algebra. Then the composition A A j j (A A)! (A) makes A into a Lie-* algebra. In fact this functor has a left adjoint. Theorem 17. The above functor {chiral algebras} {Lie-* algebras} has a left adjoint L A(L) called the chiral envelope. Proof (Sketch). The statement is local on the curve so we can assume without loss of generality that X is affine. Given a Lie-* algebra, we will construct a factorization algebra using auxiliary Lie algebras. For a finite set I, consider the space X I X and let 11

12 p i for i = 1, 2 be the two projection maps to X I and X respectively. Let j : U X I X be the open subset given by Now, consider U = {((x i ), x) X I X x i x for i I}. L (I) 0 = p 1 p 2(L) ω 1 X I and L(I) = p 1 j j p 2(L) ω 1 X I. These are Lie algebras in the category of left D-modules on X I. We have (I) that the fibers of L 0 and L (I) are given by L (I) 0 (x 1,...,x I ) = H dr(x, L) and L(I) (x 1,...,x I ) = H dr(x {x 1,..., x I }, L). By construction, we have that B(L) (I) = U( L (I) )/U( L (I) ) = Ind L (I) O L (I) X I 0 is a factorization algebra (here U(L) denotes the universal enveloping algebra of the Lie algebra L). Let A(L) be the corresponding chiral algebra. Note that we have an exact sequence of D-modules on X 0 L (1) 0 L (1) L ω 1 X 0 which gives a map L A(L). The fact that it s a map of Lie-* algebras follows from a similar exact sequence on X 2. Now suppose A is a chiral algebra and we have a map of Lie-* algebras L A. Taking derham cohomology of the action map j j (L A)! (A) along the first component makes B (1) := A ω 1 X into a Lie L (1) module. The unit section of B (1) gives a map of left D-modules U( L (1) ) B (1). Furthermore, the following diagram commutes L (1) Ã (1) L ω 1 X A ω 1 X 12

13 where Ã(1) = p 1 j j p 2(A) ω 1 (1). It follows that L X I 0 kills the unit section in B (1). This gives us the desired map A(L) A. A similar argument on X 2 shows that it s a map of chiral algebras. Consider the case where L = g D X. In this case, we have that the fibers A(L) x at x X are given by Since we have a Cartesian square A(L) x = Ind H0 dr (X x,g D X) H 0 dr (X,g D X) C. H 0 dr (X, g D X) H 0 dr (X x, g D X) H 0 dr (D x, g D X ) = g(o x ) H 0 dr (D x, g D X ) = g(k x ) it follows that A(L) x = Ind g(kx) g(o C = V ac x) x. Thus in this case the fibers are given by the vacuum module for g. In fact, this chiral algebra agrees with the one we constructed before using the Beilinson- Drinfeld Grassmannian. For Lie-* algebras, we can consider two types of modules: Definition 18. Let L be a Lie-* algebra. A Lie-* L module is a D-module M on X along with an action map satisfying the Lie action identity. ρ : L M! (M) A chiral L module is a D-module M on X along with a chiral action map ρ : j j (L M)! (M) satisfying the following condition. For j : U X 2 X the complement of the diagonals {x 1 = x} and {x 2 = x}, we have (similarly to the case of chiral modules over a chiral algebra) the maps ρ 1(23) : j j (L L M) id ρ 23! j j (L M)! (M) 13

14 with ρ (12)3 and ρ 2(13) defined similarly. We demand that ρ (12)3 = ρ 1(23) ρ 2(13). We have the following important fact about chiral modules over a Lie-* algebra. Theorem 19. Let L be a Lie-* algebra. Then there is an equivalence of categories {chiral L-modules} {chiral A(L)-modules}. Now, suppose M is a chiral L-module. We have a forgetful functor to Lie-* modules given by the composition This functor has a left adjoint L M j j (L M)! (M). Ind : {Lie-* L-modules} {chiral L-modules} given as follows. Let M be a Lie-* L-module. Taking derham cohomology of the action map L M! (M) along the first component, we see that M := M ω 1 X Now, let Ind (M) (Ĩ) = Ind LĨ LĨ0p (M ) is a Lie L (1) 0 -module. for a finite set I where p : XĨ X is the projection to the last component. We have that Ind (M) := {Ind (M)Ĩ} is a factorization module for the factorization algebra corresponding to A(L). We then define Ind(M) to be the corresponding chiral L-module. 4 Commutative Chiral Algebras Let A be a chiral algebra. We say that A is commutative if the composition vanishes. A A j j (A A)! (A) We have the following characterization of commutative chiral algebras. 14

15 Theorem 20. There is an equivalence of categories given by {commutative chiral algebras} {commutative left D X -algebras} A A ω 1 X. Proof. Recall that we have a canonical exact sequence 0 A A j j (A A)! (A! A) 0. It follows that for a commutative chiral algebra, we have a map m : A! A A. In fact, as we ll see m makes A l := A ω 1 X a commutative algebra in the category of left D-modules. Commutativity of m (on A l ) clearly follows from anti-commutativity of the chiral bracket. Furthermore, the unit gives the unit section η : O X A l. Note that we can factor the chiral bracket as j j (A A) = (A l A l ) j j (ω X ω X ) (A l A l )! (ω X ) = Thus for a section we have =! (A l A l )! (ω X )! (A l )! (ω X ) =! (A). ((a b) s 2 ) A l A l j j (ω X ω X ) µ((a b) s 2 ) = (a b) µ ω (s 2 ) where (a b) := m(a, b) and µ ω is the chiral bracket for ω. In these terms, the Jacobi identity for A becomes (a (b c)) µ ω 1(23)(s 3 ) = ((a b) c) µ ω (12)3(s 3 ) + (b (a c)) µ ω 2(13)(s 3 ) for a section (a b c) s 3 (A l A l A l ) j j (ω X ω X ω X ). 15

16 From the Jacobi identity for ω, we deduce that all three terms on the left side of the tensors are equal. Thus, the product m on A l is associative. Now, suppose that B is a commutative left D X -algebra. We can define a chiral bracket on A = B ω X by µ : j j (A A)! (A! A)! (A) where the first map is the canonical map and the second is the one given by the multiplication map on B. By a similar argument as above, this makes A into a chiral algebra. Now, suppose that B is a commutative left D X algebra. We will describe B modules in terms of the corresponding chiral algebra. Definition 21. Let A be a chiral algebra. A commutative A-module is a chiral A-module M such that the composition vanishes. A M j j (A M)! (M) Suppose B is a commutative left D X algebra and A the corresponding commutative chiral algebra. We then have an equivalence of categories given by {(D X -B)-modules} {commutative A-modules} M M := M ω X with the chiral action map given by the composition j j (A M)! (A! M)! (M) where the last map comes from the action map B M M. Now, for a commutative D X algebra B, we can describe the corresponding chiral algebra in factorization terms as follows. Let Z = Spec(B l ). Then Z is a D X -scheme. In this situation, we can construct a counital factorization space Z I X I by considering multijets. An S point of Z I is given by a map φ : S X I along with a horizontal section ˆX S Z, where ˆX S is the 16

17 completion of X S along the subscheme given by the union of the graphs of φ. When Z is an affine D X -scheme, each Z I is an affine D X I-scheme. Let (B) I be the left D X I-module of global sections of Z I. As quasi-coherent sheaves, these form a factorization algebra. Let A be the corresponding chiral algebra. Claim 22. The chiral algebra A is commutative and the chiral bracket is given by the multiplication map on B. To prove the claim, we will make use of the Eckmann-Hilton argument. We will write down a formal algebraic proof, but here s the basic idea of the proof, which is extremely simple. Suppose you wanted to explain to someone that addition was commutative. For instance, say you wanted to show that = Well, is the number of marbles you have if you have two piles of marbles - 7 marbles in the left pile and 15 marbles in the right pile. Well, is what you would get by moving one pile of marbles past the other. This is essentially the argument. A slightly more sophisticated version of this argument says that a monoid in the category of monoids is a commutative monoid. In this case, one can denote one multiplication as vertical composition and the other as horizontal and essentially carry out the same argument as with the marbles, moving one multiplication past the other. This is the Eckmann-Hilton argument and it shows for instance that higher homotopy groups of a topological space are commutative. In our context, we have the following theorem. Theorem 23. Let A be a chiral algebra with a compatible unital binary operation m : A! A A Then A is a commutative chiral algebra, m makes A l into a commutative algebra and the chiral bracket on A factors through m. Proof. Clearly, it suffices to show that the chiral bracket factors through m. Compatibility of chiral bracket with the binary operation means that the following diagram is commutative (i.e. m is a map of chiral algebras): j j ((A! A) (A! A)) j j (A A)! (A! A)! (A) 17

18 We have two unit maps α, β : ω X A for the chiral bracket and the binary operation respectively. Let s show that these agree. From the commutativity of the above diagram and the unit axioms, we have the following commutative diagram j j ((ω α! ω β ) (ω β! ω α ))! (ω β! ω β ) =! (ω β )! (A) j j (ω α ω α )! (ω α ) It follows that the two units agree. Now, we can construct a section given by s : j j (A A) j j ((A! A) (A! A)) s : j j (A A) = j j ((A! ω) (ω! A) j j ((A! A) (A! A)) It follows that we the chiral bracket factors as j j (A A)! (A! A)! (A) where the last map is given by! (m). Remark 24. We can actually strengthen the above theorem slightly by not imposing the condition that A is a chiral algebra. All that is necessary is that there is a unital chiral operation and a unital binary operation µ : j j (A A)! (A) m : A! A A which are compatible. In this case, µ makes A into a commutative chiral algebra, m makes A l into a commutative left D X -algebra and one determines the other. We leave the proof as an exercise for the reader. Let us now return to considering multijets. In this case, B is a commutative left D X algebra, and A is the chiral algebra corresponding to the factorization space given by multijets of Spec(B). As a right D X -module, we clearly have A = B ω X, and the multiplication on B is compatible with the chiral bracket. It follows by Eckmann-Hilton that A is a commutative chiral algebra and the chiral bracket is given by the multiplication map. 18

19 5 Factorization Modules on Higher Powers Let A be a chiral algebra. Thus far, we have considered chiral modules supported at a point and chiral modules on the curve. We can also define modules on higher powers of X. We do so in factorization terms. Let B be the corresponding factorization algebra. Definition 25. Let I 0 be a finite set. A factorization B module M on X I 0 is a collection D X Ĩ-modules {MĨ} for finite sets I with Ĩ := I I 0. For every surjection π : J Ĩ such that π I 0 = id, we have the corresponding diagonal XĨ X J are we are given isomorphisms M ( J) X Ĩ M (Ĩ) and for every partition I = I 1... I n, we are given factorization isomorphisms MĨ U ( B (Ii) MĨn ) U 0<i<n where U XĨ is the open set corresponding to the partition Ĩ = I 1... Ĩn. We require that the isomorphisms be mutually compatible. One can also give a definition of factorization modules on X I 0 in terms of chiral operations similarly to the definition of chiral modules on X. In fact as in the case of chiral modules on X, we have that the forgetful functor {factorization B-modules on X I 0 } {D X I 0 -modules} if faithful. Furthermore, the category of factorization modules on X I 0 abelian category. is an Example For any I 0, B (I 0) is a factorization module on X I Suppose M is a module on X I 0, and for a finite set J 0 = J I 0, we have that N = M (J 0) is a factorization module on X J Suppose M is a factorization module on X I 0 then for any surjection π : J 0 I 0, we have the corresponding diagonal i : X I 0 X J 0. We then have i (M) is a factorization module on X J 0. Similarly, if N is a factorization module on X J 0, i (N ) is a factorization module on X I 0. 19

20 4. Suppose M is a factorization module on X I 0 and N is a factorization module on X J 0, then j j (M N ) is a factorization module on X I 0 J 0, where j : U = {(x i ), (y j ) x i y j } X I 0 X J 0. The last example above allows us to define a pseudo-tensor structure on the category of chiral A-modules on X by setting Hom({M 1, M 2 }, N) = Hom(j j (M 1 M 2 ), (N)) for M 1, M 2, N factorization A modules on X, where the Hom on the righthand side is in the category of factorization A-modules on X 2. 6 Chiral Algebra of Endomorphisms Let A be a chiral algebra and B the corresponding factorization algebra. We have seen that each B (I) is a factorization B-module on X I. Let REnd(B) (I) = RHom(B (I), B (I) ) where RHom is in the derived category of factorization B-modules on X I. Let REnd(A) (I) = REnd(B) (I) ω X I be the corresponding right D-modules. We have that REnd(B) is a dg-factorization algebra. Namely, for a diagonal : X I X J, we have L (REnd(B) (J) ) REnd(B) (I) and for a partition I = I 1... I n and j : U X I the corresponding open set j (REnd(B) (I) ) j ( REnd(B) (I i) ). Suppose that we are given a decomposition I = I 0 J. For the factorization B module on X I 0 M = B I 0, we have that M I = B I. It follows that we have maps p (REnd(B) I 0 ) = REnd(B) (I0) O X J REnd(B) (I) which makes REnd(B) into a unital dg-factorization algebra. It is coconnective, i.e. it has no cohomology in negative degrees. 20

21 Lemma 27. Let B be a coconnective unital dg-factorization algebra. Then (B 0 ) (I) = H 0 (B (I) ) is a unital factorization algebra. Proof. Let A (I) = B (I) ω X I and A (I) 0 = B (I) 0 ω X I be the corresponding right D-module. The Cousin complex for A (2) gives the triangle A (2) j j (A (2) )! (A (1) ). The long exact sequence in cohomology gives the exact sequence 0 A (2) 0 j j (A (2) 0 )! (A (1) 0 ). The unit gives a commutative diagram 0 A (2) 0 j j (A (2) 0 )! (A (1) 0 ) 0 A (1) 0 ω X j j (A (1) 0 ω X )! (A (1) 0 ) 0 It follows that j j (A (2) 0 ) = j j (A (1) 0 A (1) 0 )! (A (1) 0 ) is surjective. Similar considerations on higher powers of X shows that A (1) 0 is a chiral algebra with B 0 the corresponding factorization algebra. For a chiral algebra A with corresponding factorization algebra B, let End(A) be the chiral algebra corresponding to the factorization algebra given by H 0 (REnd(B)). Composition of morphisms gives us an algebra map End(A)! End(A) End(A) which is compatible with the chiral algebra structure. It follows that End(A) is a commutative chiral algebra. Let A g be the chiral envelope of the Lie-* algebra D X g for a Lie algebra g. We want to show that the algebra of endomorphisms of the vacuum module of A g supported at a point is commutative. Even though the chiral algebra End(A) is commutative for any chiral algebra A, the corresponding statement for the vacuum module supported at a point is not necessarily true. However, it is true in the case of vertex operator algebras. 21

22 Definition 28. A vertex operator algebra V is an assignment X V X of a chiral algebra V X on every smooth curve X, along with compatible isomorphisms φ (V Y ) V X for etale maps φ : X Y. Example 29. The chiral algebra A g is defined for any smooth curve X and these form a vertex operator algebra. Now, let s show that in the case of a vertex operator algebra V, the algebra of endomorphisms of the vacuum module is commutative. Since the statement is etale local, we can restrict ourselves without loss of generality to the case that X = A 1. Since V is a vertex operator algebra, we have that A := V A 1 is a translation equivariant D-module on A 1. Let i : {0} A 1 be the inclusion map and V ac A := i! i! (A)[1] be the vacuum module supported at {0}. We have that the algebra of endomorphisms of V ac A is given by End(V ac A ) = End(H 0 (Ri! (A)[1])) = H 0 (REnd(Ri! (A)[1])) = H 1 (REnd(Ri! (A))). The Grothendieck spectral sequence gives the exact sequence 0 H 1 (Ri! (REnd(A))) H 1 (REnd(i! (A))) i! (H 1 (REnd(A))). Since A is translation equivariant so is H 1 (REnd(A)). In particular, H 1 (REnd(A)) is flat. It follows that i! (H 1 (REnd(A))) = 0 and therefore H 1 (Ri! (REnd(A))) H 1 (REnd(i! (A))) End(V ac A ). By commutativity of End(A), the algebra H 1 (Ri! (REnd(A))) is commutative and therefore so is End(V ac A ). 22

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