Symplectic Flatness and Twisted Primitive Cohomology

We introduce the notion of symplectic flatness for connections and fibre bundles over symplectic manifolds. Given an A∞\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$A_\infty $$\end{document}-algebra, we present a flatness condition that enables the twisting of the differential complex associated with the A∞\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$A_\infty $$\end{document}-algebra. The symplectic flatness condition arises from twisting the A∞\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$A_\infty $$\end{document}-algebra of differential forms constructed by Tsai, Tseng and Yau. When the symplectic manifold is equipped with a compatible metric, the symplectic flat connections represent a special subclass of Yang–Mills connections. We further study the cohomologies of the twisted differential complex and give a simple vanishing theorem for them.

For a vector bundle E over a smooth manifold M of dimension d, we can study differential forms Ω * (M, E) taking values in the fiber space of E. For these forms, we can write a de Rham-type complex for Ω * (M, E): where locally d A = d + A and A ∈ Ω 1 (M, End E) is the connection acting on E. The above complex is only a differential complex if the curvature of the connection, F = (d A ) 2 = dA + A ∧ A = 0.This is the well-known fact that the de Rham complex can be twisted by a flat bundle, i.e. a bundle E that allows for a connection whose curvature vanishes.Now suppose M is a complex manifold of dimension d = 2n and E is a complex vector bundle over M .On complex manifolds, the differential forms can be decomposed into (p, q) components, that is and the exterior derivative decomposes into the Dolbeault operators, d = ∂ + ∂.The decompositions of forms and exterior derivative allow us to consider a more refined complex, the Dolbeault complex, which can be twisted as well: 0 / / A 0,0 (M, E) ∂A / / A 0,1 (M, E) ∂A / / . . .∂A / / A 0,n (M, E) ∂A / / 0 (1.2) where locally ∂A = ∂ + A 0,1 and A 0,1 is the (0, 1) component of the connection form A. That the above complex is actually a differential complex imposes the condition Notice that this is a weakening of the smooth flatness condition F = 0 in that (1.3) requires only that the F 0,2 component of the curvature vanishes.But a complex vector bundle with a connection such that F 0,2 = 0 is well-known to be equivalent to the bundle E having the structure of a holomorphic vector bundle.And so in the complex case, complex flat bundles which can twist the Dolbeault complex are just holomorphic vector bundles.
Flat bundles and holomorphic vector bundles are basic and important objects on smooth and complex manifolds, respectively.In this paper, we are interested in exploring special vector bundles on symplectic manifolds.Specifically, we ask a simple question: is there a symplectic flatness condition for a vector bundle E over a symplectic manifold, (M 2n , ω)?
We can answer this question by proceeding in a similar way as in the smooth and complex cases described above.Let us first recall that on a symplectic manifold, (M 2n , ω), the differential forms can be expressed in a polynomial expansion in powers of ω.This decomposition of forms is commonly called the Lefschetz decomposition and given by Ω k (M ) = k=2r+s ω r ∧ P s (M ) , where P s (M ) for s = 0, 1, . . ., n, denotes the space of primitive forms, i.e. forms that we can not extract an ω from them.More precisely, a form β ∈ P s (M ) if there does not exist an ξ ∈ Ω s−2 (M ) such that β = ω ∧ ξ.And besides forms, the exterior derivative, like in the complex case, also has a decomposition into two linear differential operators (∂ + , ∂ − ) dependent on the symplectic structure [7] and with desirable properties: (∂ + ) 2 = (∂ − ) 2 = 0 and ω ∧ ∂ + ∂ − = −ω ∧ ∂ − ∂ + .Together, they lead to a differential complex that is elliptic (see [7] and references therein) 0 / / P 0 (M ) ∂ + / / P 1 (M ) Hence, we can try to extend this complex to one acting on P * (M, E), the space of primitive forms with values in E, and also twist the operators (∂ + , ∂ − , ∂ + ∂ − ) with a connection form as in (1.1) and (1.2).However, the peculiar definitions of the differentials in (1.5) raise immediate issues.In particular, note that both ∂ − : P s → P s−1 and ∂ + ∂ − : P n → P n do not increase the degree of the form by one.Can we simply twist these operators in the primitive complex above by a connection one-form?
The twisting procedure in the symplectic case turns out to be a bit more subtle.Nevertheless, twisting the primitive complex can still be achieved because the complex has an A ∞ -algebra structure [6].We will show in this paper, that given any A ∞ -module structure of differential forms taking values in E, there is a natural twisting of the differentials of the complex by a connection one-form.The new twisted differentials however do not represent a deformation of the A ∞ -module and hence does not lead to a new A ∞ -module structure.
Our result in the symplectic case is the following.We can write down a twisted primitive complex of forms taking values in E, if the connection one-form on E satisfies the following symplectic flatness condition: Definition 1.1.For (M 2n , ω) a symplectic manifold, let π : E → M be a vector bundle with a connection, d A the corresponding covariant derivative and F the curvature two-form.We call the connection symplectically flat if where Φ ∈ Ω 0 (M, End E).If such a connection exists on E, we say E is a symplectically flat bundle.
Remark 1.2.For a principal bundle P over (M 2n , ω), we say P is symplectically flat if there exists a connection form on P whose curvature satisfies (1.6).
Though written as two equations, condition (1.6) for symplectically flat is effectively just a single equation in all 2n dimensions.For when n = 1, the first equation, F = Φ ω , is trivial and gives no condition.On the other hand, when n ≥ 2, the second equation, d A Φ = 0 , becomes unnecessary as it is implied by the first equation of (1.6) and the Bianchi identity.
To arrive at the symplectic flatness condition, we shall begin in Section 2 by reviewing the A ∞ -algebra structure of the primitive cochain complex of Tsai, Tseng, and Yau [6].We will present a general procedure to twist the differential complex associated with any A ∞ -algebra.
This general procedure gives the flatness conditions for de Rham, Dolbeault and the primitive symplectic complex and results in the twisted primitive complex given in (2.15).
With the symplectic flatness condition established, it is helpful to have examples of symplectic flat bundles and understand some of their properties.We will do this in Section 3. Indeed, we will show that symplectically flat connections are a special type of Yang-Mills connections Smooth M Complex (M 2n , J) Table 1: A comparison of smooth, complex and symplectic flat bundles.
in the presence of a compatible metric on (M 2n , ω).Also, as shown in Table 1, one can choose special local frames such that the local connection form takes the form A = Φ λ, where Φ is a constant matrix and dλ = ω.We also point out an interesting relationship: when ω is an integral class and we can define a circle bundle X over M whose Euler class is given by ω, (i.e. the prequantum circle bundle of M ), the symplectic flat bundle lifts to a flat bundle on X.
Finally, having twisted the primitive elliptic complex, we analyze the resulting cohomologies on P * (M, E) in Section 4. We will calculate these twisted primitive cohomologies on R 2n and also prove a simple vanishing theorem for the twisted cohomologies when Φ is invertible.
In this paper, we will mainly focus on symplectically flat vector bundles.An extensive discussion of symplectically flat principal bundles including their classification will be given in a companion work [8].
2 Twisting the differential complex of an A ∞ -algebra On symplectic manifolds, Tsai-Tseng-Yau [6] showed that the primitive cochain complex in (1.5) can be extended to an A ∞ -algebra.(More precisely, it is a commutative A 3 algebra.)We will first describe this algebra below.For ease, we will call this algebra of differential forms on symplectic manifolds the TTY algebra.We then proceed to give a heuristic description of how to twist the differential of the primitive TTY algebra.Going further, we show in generality how the differential of any A ∞ -module can be twisted.The twisted differential of the TTY algebra is just a special case of this general A ∞ -module twisting.

Preliminaries: TTY algebra
We mostly follow the notations of [6].
As mentioned, differential forms on a symplectic manifold (M 2n , ω) has a Lefschetz decomposition.Any η k ∈ Ω k (M ) can be expressed as a polynomial in ω: where {β k , β k−2 , . . ., β k−2p , . ..} are all primitive forms in P * (M ) and are determined uniquely by η k and ω.The non-degeneracy of ω also allows us to define the following three operators on differential forms [6]: from a form.For example, acting on the Lefschetz component ω r ∧ β s for β s ∈ P s , 3. Π p : By Lefschetz decomposition, every η ∈ Ω k can be uniquely written as β + ω p+1 ∧ γ, The third operator, Π p , is a projection operator and defines the space of p-filtered forms: Note that the Lefschetz decomposition of p-filtered forms has at most terms of order ω p .
For each p = 0, . . ., n, there is a TTY algebra consisting of forms in F p Ω * (M ).In this paper, we are mainly concerned with the p = 0 filtered case F 0 Ω * (M ) = P * (M ) which consist of just primitive forms.To simplify notation, we will write Π = Π 0 , i.e. the projection onto the primitive component of the Lefschetz decomposition.
The TTY algebra is an A ∞ -algebra.Let us recall the definition of an A ∞ -algebra (for a reference, see [3]).
Definition 2.1.An A ∞ -algebra is a Z-graded vector space A = k∈Z A k endowed with graded linear maps The first three relations of (2.2) are the following: By the third relation, if m 3 = 0, then m 2 , which acts as a product, is associative.An A ∞algebra with only {m 1 , m 2 } non-zero is simply a differential graded algebra (DGA).The TTY algebra is however generally non-associative with m 3 = 0, but m k for k ≥ 4 = 0 can be set to zero.Hence, it can be more precisely called an A 3 algebra.
First, the elements of the primitive TTY-algebra are those of the differential complex (1.5).The grading follows that of the complex, and to help distinguish the two sets of primitive forms, we use the ± subscript where for k = 0, 1, . . ., n, P k + = P k have grading k, and P k − = P k have grading 2n + 1 − k.As for the m k maps, the first map, m 1 , is just the differential of the complex (1.5).
The m 1 map.
The m 2 map is the product operation and is dependent on the pair of primitive spaces that it acts on.At times, we will denote it by the product symbol and write m 2 (β, γ) as β × γ.This × operation is graded commutative.
The m 2 map.
Note that when j + k ≤ n, the second term is trivial, and when j + k > n, the first term is trivial.

For β
The m 3 maps measures the non-associativity of the product ×.The non-associativity only arises when all three forms in the input come from P * + .The m 3 map.

Twisting the differential of the primitive TTY algebra
We now give a heuristic description of the twisting of the primitive elliptic complex (1.5).Let E be a vector bundle and consider Ω * (M, E) and P * (M, E), the space of differential forms and primitive forms, respectively, taking values in E. We start with the relation of (∂ + , ∂ − ) with d in (1.4).In particular, acting on primitive forms, ∂ + = Π d and ∂ − = L −1 d.Now, we can decompose the twisted exterior derivative into two components when acting on primitive forms where we have noted in the second line that a primitive form wedge a one-form has a Lefschetz decomposition into two terms, and in the third line, that the decomposition is independent of the choice of local frames.This allows us to define the global twisted operators: which gives a twisted version of (1.4).Locally, we have the expressions (2.9) Now we can express the action of (d A ) 2 on a primitive form in two ways.First, note that the commutator Therefore, we have Alternatively, we can also write where in the second line, we have Lefschetz decomposed the curvature F = F 0 + ω Φ with (2.12) and matching the Lefschetz components, we find This suggests the following twisted primitive complex This is a differential complex if F satisfies the symplectically flat condition of F = Φω (i.e. F 0 = 0) and d A Φ = 0 given in Definition 1.6.In particular, we write out the composition of the differential operators in the middle of the complex: In the next subsection, we will give a more systematic description of how to obtain the twisted differentials.We will show how all A ∞ -algebras can be twisted and that the symplectic flat condition needed above matches exactly the required condition for general twisting.

Twisting the differential of an A ∞ -module
We are interested to twist the primitive elliptic complex (1.5) in a similar manner to how the de Rham complex is twisted in (1.1).Prior to twisting, the untwisted de Rham complex together with the wedge product gives the de Rham DGA: In the presence of a vector bundle E with fiber V , twisting the complex consists of locally tensoring by V , i.e.
Ω * (U ) ⊗ V and modifying the differential In order that the twisted complex remains a differential complex, we obtain the condition Let us make two observations.First, modifying . Indeed, we have not and do not need to define a new product m ′ 2 on Ω * (M ) ⊗ V .So twisting the de Rham complex does not represent a deformation preserving the DGA structure.Second, without modifying the maps (m 1 , m 2 ), we can tensor the de Rham DGA by matrices: Note that in the context of this tensored de Rham DGA, the flatness condition can be written simply in terms of the deformed m ′ 1 map: Now to twist the primitive elliptic complex, we first observe that any A ∞ -algebra also has a natural tensor product with matrices.Let Ω be an A ∞ -algebra.We twist A = Ω ⊗ End V where V , a vector space, is the fiber of E. m k acting on Ω ⊗k can be extended to m k acting on Elements of the complex are vector-valued.They are elements of B = Ω ⊗ V which to be precise is an A ∞ -module over A. (See for example [3] for the definition of an A ∞ module.)The m k acting on B is given by The above twisting for differential graded algebras can be generalized to twist any A ∞module B. In [2], Gibson described the case for A 3 -algebra (i.e.m k = 0 for all k ≥ 4) and Here, we give the general statement for any A ∞ -module B. Definition 2.2.Let {A, m k } be an A ∞ -algebra, B an A ∞ -module over A, and A an element of A of grading one.We define the operator m ′ 1 as where Here, we use the notation m ′ 1 since it is obtained by twisting m 1 , but m ′ 1 is generally not the differential of an A ∞ -module.As mentioned above, we do not need morphisms m ′ k for k ≥ 2 as we are not aiming to obtain a deformed A ∞ -structure.The theorem below will give a sufficient condition that ensures that m We first note a simple relation for the δ k 's.
When we take the sum over s first, we get By assumption The above prescription for A ∞ twisting motivated by twisting the de Rham DGA gives the standard twisting for the Dolbeault complex and the primitive TTY-algebra.
Example 2.6 (Twisted primitive TTY-algebra).Let E be a vector bundle over a symplectic manifold (M 2n , ω).Given a local trivialization over U ⊂ M , a connection can be represented locally as d + A where A ∈ Ω 1 (U, End E) = P 1 (U, End E).The primitive complex P * (U, E) is an A ∞ -module over the TTY-algebra P * (U, End E).Hence, we can define the operator m ′ 1 as in Definition 2.2 and is explicitly given by (2.16) In details for the β ∈ P n + (U, E) case, we have Furthermore, Therefore, m ′ 1 (A) = 0 if and only if the curvature has no primitive component, i.e.F = Φ ω, and also dΦ + [A, Φ] = 0, having noted that L −1 F = Φ.The second equation means that Φ is covariantly constant which implies the global condition d A Φ = 0 .Remark 2.7.For the higher p-filtered TTY algebra (i.e.p > 0), if we define m ′ 1 as in Definition 2.2, then m ′ 1 (A) = 0 is just the the usual flat connection condition.

Examples and properties of symplectically flat bundles
We first give some simple examples of symplectically flat bundles.Clearly here, the symplectically flat condition F = Φ ω is a stronger condition than the self-dual condition.
Below we give some properties of symplectically flat bundles.where r is the rank of vector bundle E. Then d A can be written as d + A, and Φ can be represented by a matrix Φ s with respect to this frame.Take λ ∈ Ω 1 (U ) such that dλ = ω.
Since d + A is a symplectically-flat covariant derivative, a straightforward calculation shows that By Lemma 3.6 there exists some invertible g such that Thus, Then we have On the other hand, be another local frame.Then Φ will be represented by gΦ s g −1 with respect to this new frame.
And the local covariant derivative becomes As an application of Proposition 3.5, suppose (M, ω) is a Kähler manifold and its Levi-Civita connection is symplectically flat.Then the Ricci curvature Ric(u, v) satisfies By Theorem 3.5 around every point in M , we can find some g such that gΦg −1 is a constant.
That implies that tr Φ = tr gΦg −1 is a constant.For the same reason, tr JΦ is also a constant.
Therefore, the Ricci tensor r(u, v) has the following property: where c = 1 2 (tr JΦ + i tr Φ).In other words, we have Proposition 3.7.If the Levi-Civita connection of a Kähler manifold is symplectically flat, then the manifold is Kähler-Einstein.
Symplectically flat bundles also have a simple alternative description.When ω is integral, it induces a flat connection on the prequantum circle bundle, i.e. the circle bundle over M with Euler class given by ω.
To show this, we recall a result of Tanaka-Tseng [5] that the primitive TTY algebra is A ∞quasi-isomorphic to the cone algebra C * (M ) = Ω * (M ) ⊕ θ Ω * (M ) where dθ = ω.Furthermore, when ω is integral and we can consider a circle bundle X over M whose Euler class is ω, then the de Rham DGA of the circle bundle Ω * (X) is quasi-isomorphic to both the C * (M ) algebra and the primitive TTY algebra.
We can extend these quasi-isomorphism relations between algebras to include symplectically flat connections.Let E be a vector bundle over M with a connection, and d A the corresponding covariant derivative.On the twisted cone algebra C * (M, E) = Ω * (M, E) ⊕ θ Ω * (M, E) with dθ = ω, we define the operator Proposition 3.8.The above connection is symplectically flat if and only if Proof.Let Φ −1 be the inverse of Φ.We note that Therefore, for arbitrary α ∈ Ω * (M, E), we have that It follows that both ∂ +A and ∂ −A also commute with Φ −1 .So for any

Local cohomologies
For arbitrary x ∈ M , we have a neighborhood U of x isomorphic to R 2n such that E| U ≃ U × V .
There exist λ ∈ Ω 1 (U ) such that dλ = ω.According the proof of Proposition 3.5, we can find a frame {e i } on E| U such that Ae i = Φλe i and Φ ∈ End V is a constant.Locally, we have A = Φλ and we obtain the following result regarding the twisted primitive cohomologies: Theorem 4.4.
Here, ker Φ and coker Φ are subspaces of V and can be represented by constant sections.
We will make use for the proof of the theorem the local Poincare' lemmas for the {∂ + , ∂ + ∂ − , ∂ − } operators in [7].Before giving the proof, let us first write down some expressions that are m 1closed, where m 1 refers to the the untwisted differentials of (2.7), instead of m ′ 1 -closed.Below, we will often use the simpler product notation × to denote the m 2 map as described in Sec.

2.1.
Lemma 4.5.Let Then for a symplectically flat connection of the form A = Φλ where Φ is a constant, Proof.For By direct computation, we will show having used Leibniz rule in the first line and noting that λ×(λ×∂ When k = n, we have having used the following A ∞ relation involving m 4 (with m 4 = 0 for the TTY algebra): For βk ∈ P k − with k = n and m 1 ( βk ) = 0, (4.2) implies Φ βk = (∂ − −Φλ×)∂ +A βk , or equivalently, Similar to above, it then follows We now give a proof of Theorem 4.4.
Proof of Theorem 4.4.The proof of the theorem is divided into five cases.
where to obtain the third line, we noted ξ n = ξn as forms and applied the relations in (4.14) and (4.15).

Global cohomologies and relation to the twisted cone complex
Tanaka-Tseng in [6] gave a homotopy equivalence between the cochain complex of P * (M ) and the cone complex of C * (M ) = Ω * (M ) ⊕ θ Ω * (M ).For differential forms taking values in E, a symplectically flat vector bundle over a symplectic manifold (M, ω), we will construct here a similar relation.
Primitive forms with values in E, denoted by P * (M, E), form a twisted cochain complex with differential m ′ 1 as in (2.15).Similar to (2.6), we use F * (M, E) to denote this complex, i.e.
For the cone, the differential forms with values in E For α k ∈ C k (M, E), (4.16) implies the following decomposition we have the following Throughout this subsection, all β i ∈ P i (M, E) are primitive forms with values in E. These primitive forms can be acted upon by d A .
We now define two maps.
Definition 4.6.In terms of the decompositions (4.17)-(4.18),we define the map f : In the next two lemmas, we will show that both f and g are chain maps.
Lemma 4.7.The map f is a chain map, i.e the following graph commutes for all 0 ≤ j ≤ 2n.
On the other hand, having used (2.14).On the other hand, Lemma 4.8.The map g is a chain map, i.e the following graph commutes for all 0 ≤ j ≤ 2n.
Lemma 4.9.The maps f, g and G are related as follows: Proof.That f g = id F follows immediately from the definitions of the maps in Definition 4.6.
For the second relation, we consider first the left-hand side.We have from the definitions

Example 3 . 1 .Example 3 . 3 . 1 (n− 1 )Example 3 . 4 .
When the principal bundle P is rank 1, the condition of symplectically-flat becomes F = dA = c ω for some constant c.Specifically, a circle bundle whose Euler class is c ω would be symplectically flat.Example 3.2.A projectively flat bundle has curvature F = c ω I where c is a constant and I is the identity map over the fiber.Hence, projectively flat bundles are symplectically flat.When dim M = 2, the symplectically flat condition is exactly identical to satisfying the Yang-Mills equations in the presence of a compatible metric.More generally, for dim M ≥ 2, a symplectically flat connection is always a critical point of the Yang-Mills functional with respect to a compatible metric M tr (F ∧ * F ) .Using the relation * ω = !ω n−1 , it is straightforward to see that a symplectically flat curvature satisfies the Yang-Mills equation d * A F = − * d A * (Φ ω) = − 1 (n − 1)! * (d A Φ) ω n−1 = 0 .Hence, symplectically flat connections are a special subset of Yang-Mills solutions.When the symplectic manifold has dimension dim M = 4, a symplectically flat connection satisfies the self-dual condition with respect to a compatible metric F = * F .

Proposition 3 . 5 .Lemma 3 . 6 .
Suppose there is a symplectically-flat connection on a manifold M with curvature F = Φ ω.Locally, there exists a trivialization such that Φ is represented as a constant matrix, and the covariant derivative can be written as d + Φλ for some local 1-form λ satisfying dλ = ω.The proof of the theorem is based on the following lemma (see for example, [Locally, if d + Ã is a flat covariant derivative, then Ã = g −1 dg for some matrix valued function g.Proof of Proposition 3.5.First choose local sections {s 1 , . . ., s r } forming a frame of Γ(U, E),

Corollary 4 . 3 .
When rank E = 1 and E is non-flat, then P H * (M, E) = 0.The above Theorem 4.2 is a vanishing statement for P H * (M, E) of which Φ plays a central role.