# nLab n-plectic geometry

### Context

#### Symplectic geometry

symplectic geometry

higher symplectic geometry

# Contents

## Idea

$n$-plectic geometry is a generalization of symplectic geometry to higher category theory.

It is closely related to multisymplectic geometry and n-symplectic manifolds.

## Definition

###### Definition

For $n \in \mathbb{N}$, an n-plectic vector space is a vector space $V$ (over the real numbers) equipped with an $(n+1)$-linear skew function

$\omega : \wedge^{n+1} V \to \mathbb{R}$

such that regarded as a function

$V \to \wedge^n V^*$

is has trivial kernel.

Let $X$ be a smooth manifold, $\omega \in \Omega^{n+1}(X)$ a differential form.

###### Definition

We say $(X,\omega)$ is a $n$-plectic manifold if

• $\omega$ is closed: $d \omega = 0$;

• for all $x \in X$ the map

$\hat \omega : T_x X \to \Lambda^n T_x^n X$

given by contraction of vectors with forms

$v \mapsto \iota_v \omega$

is injective.

## Examples

1. For $X$ orientable, take $\omega$ the volume form. This is $(dim(X)-1)$-plectic.

2. $\wedge^n T^* X \to X$

3. $G$ a compact simple Lie group,

let $\nu : (x,y,z) \mapsto \langle x, [y,z]\rangle$ be the canonical Lie algebra 3-cocycle and extend it left-invariantly to a 3-form $\omega_\nu$ on $G$. Then $(G,\omega_\nu)$ is 2-plectic.

## Poisson $L_\infty$-algebras

To an ordinary symplectic manifold is associated its Poisson algebra. Underlying this is a Lie algebra, whose Lie bracket is the Poisson bracket.

We discuss here how to an $n$-plectic manifold for $n \gt 1$ there is correspondingly assoociated not a Lie algebra, but a Lie n-algebra: the Poisson bracket Lie n-algebra. It is natural to call this a Poisson Lie $n$-algebra (see for instance at Poisson Lie 2-algebra).

(Not to be confused with the Lie algebra of a Poisson Lie group, which is a Lie group that itself is equipped with a compatible Poisson manifold structure. It is a bit unfortunate that there is no better established term for the Lie algebra underlying a Poisson algebra apart from “Poisson bracket”.)

Consider the ordinary case in $n=1$ for how a Poisson algebra is obtained from a symplectic manifold $(X, \omega)$.

Here

$\hat \omega : T_x X \to T^*_x X$

is an isomorphism.

Given $f \in C^\infty(X)$, $\exists ! \nu_f \in \Gamma(T X)$ such that $d f = - \omega(v_f, -)$

Define $\{f,g\} := \omega(v_f, v_g)$. Then $(C^\infty(X,), \{-,-\})$ is a Poisson algebra.

We can generalize this to $n$-plectic geometry.

Let $(X,\omega)$ be $n$-plectic for $n \gt 1$.

Observe that then $\hat \omega : T_x X \to \wedge^n T_x X$ is no longer an isomorphism in general.

Definition

Say

$\alpha \in \Omega^{n-1}(X)$

is Hamiltonian precisely if

$\exists v_\alpha \in \Gamma(T X)$

such that

$d \alpha = - \omega(v_\alpha, -) \,.$

This makes $v_\alpha$ uniquely defined.

Denote the collection of Hamiltonian forms by $\Omega^{n-1}_{Hamilt}(X)$.

Define a bracket

$\{-,-\} : \Omega^{n-1}_{Hamilt}(X)^{\otimes_2} \to \Omega^{n-1}_{Hamilt}(X)$

by

$\{\alpha, \beta\} = - \omega(v_\alpha, v_\beta, -, \cdots, -) \,.$

This satisfies

1. k

$d \{\alpha, \beta\} = - \omega([v_\alpha, v_\beta], -, \cdots, -) \,.$
2. $\{-,-\}$ is skew-symmetric;

3. $\{\alpha_1, \{\alpha_2, \alpha_3\}\}$ + cyclic permutations
$d \omega(v_{\alpha_1}, v_{\alpha_2}, v_{\alpha_3}, -, \cdots)$.

So the Jacobi dientity fails up to an exact term. This will yield the structure of an L-infinity algebra.

Proposition

Given an $n$-plectic manifold $(X,\omega)$ we get a Lie n-algebra structure on the complex

$C^\infty(X) \stackrel{d_{dR}}{\to} \Omega^1(X) \stackrel{d_{dR}}{\to} \to \cdots \to \Omega^{n-1}_{Hamilt}(X)$

(where the rightmost term is taken to be in degree 0).

where

• the unary bracket is $d_{dR}$;

• the $k$-ary bracket is

$[\alpha_1, \cdots, \alpha_k] = \left\{ \array{ \pm \omega(v_{\alpha_1}, \cdots, v_{\alpha_k}) & if \forall i : \alpha_i \in \Omega^{n-1}_{Hamilt}(X) \\ 0 & otherwise } \right.$

This is the Poisson bracket Lie n-algebra.

This appears as (Rogers 11, theorem 3.14).

For $n = 1$ this recovers the definition of the Lie algebra underlying a Poisson algebra.

## Prequantization

### Review of the symplectic situation

Recall for $n=1$ the mechanism of geometric quantization of a symplectic manifold.

Given a 2-form $\omega$ and the corresponding complex line bundle $P$, consider the Atiyah Lie algebroid sequence

$ad P \to T P/U(1) \to T X$

The smooth sections of $T P/U(1) \to X$ are the $U(1)$ invariant vector fields on the total space of $P$.

Using a connection $\nabla$ on $P$ we may write such a section as

$s(v) + f \partial_t$

for $v \in \Gamma(T X)$ a vector field downstairs, $s(v)$ a horizontal lift with respect to the given connection and $f \in C^\infty(X)$.

Locally on a suitable patch $U \subset X$ we have that $s(V)|_U = v|_U + \iota_v \theta_i|_U$ .

We say that $\tilde v = s(v) + f \partial_t$ preserves the splitting iff $\forall u \in \Gamma(X)$ we have

$[\tilde v, s(u)] = s([v,u]) \,.$

One finds that this is the case precisely if

$d f = - \iota_v \omega \,.$

This gives a homomorphism of Lie algebras

$C^\infty(X) \to \Gamma(T P / U(1))$
$f \mapsto s(v_f) + f \partial_t \,.$

### 2-plectic geometry and Courant algebroids

We consder now prequantization of 2-plectic manifolds.

Let $(X,\omega)$ be a 2-plectic manifold such that the de Rham cohomology class $[\omega]/2 \pi i$ is in the image of integral cohomology (Has integral periods.)

We can form a cocycle in Deligne cohomology from this, encoding a bundle gerbe with connection.

On a cover $\{U_i \to X\}$ of $X$ this is given in terms of Cech cohomology by data

• $(g_{i j k} : U_{i j k} \to U(1)) \in C^\infty(U_{i j k}, U(1))$

• $A_{i j} \in \Omega^1(U_{i j})$;

• $B_i \in \Omega^2(U_i)$

satisfying a cocycle condition.

Now recall that an exact Courant algebroid is given by the following data:

• a vector bundle $E \to X$;

• an anchor morphism $\rho : E \to T X$ to the tangent bundle;

• an inner product $\langle -,-\rangle$ on the fibers of $E$;

• a bracket $[-,-]$ on the sections of $E$.

Satisfying some conditions.

The fact that the Courant algebroid is exact means that

$0 \to T^* X \to E \to T X \to 0$

is an exact sequence.

The standard Courant algebroid is the example where

• $E = T X \oplus T^* X$;

• $\langle v_1 + \alpha_1, v_2 + \alpha_2\rangle = \alpha_2(v_1) + \alpha_1(v_2)$;

• the bracket is the skew-symmetrization of the Dorfman bracket

$(v_1 + \alpha_1, v_2 + \alpha_2) = [v_1, v_2] - \mathbb{L}_{v_1}\alpha_2 - (d \alpha_1)(v_2,-)$

Now with respect to the above Deligne cocycle, build a Courant algebroid as follows:

• on each patch $U_i$ is is the standard Courant algebroid $E_i := T U_i \oplus T^* U_i$;

• glued together on double intersections using the $d A_{i j}$

This gives an exact Courant algebroid $E \to X$ as well as a splitting $s : T X \to E$ given by the $\{B_i\}$.

The bracket on this $E$ is given by the skew-symmetrization of

$[ [ s(v_1) \alpha_1, s(v_2) + \alpha_2 ] ] = s([v_1, v_2]) + \mathcal{L}_{v_1} \alpha_2 - (d \alpha_2)(v_2, -) - \omega(v_1, v_2, \cdots) \,.$

Here a section $e = s(v) + ...$ preserves the splitting precisely if

for all $u \in \Gamma(T X)$ we have

$[ [ e, s(u)] ]_D = s([v,u])$

exactly if $\alpha$ is Hamiltonian and $v = v_\alpha$.

Theorem

Recall that to every Courant algebroid $E$ is associated a Lie 2-algebra $L_\infty(E)$.

Then: we have an embedding of L-infinity algebras

$\phi : L_\infty(X,\omega) \to L_\infty(E)$

given by $\phi(\alpha) = s(v_\alpha) + \alpha$.

## Properties

### Central extensions under geometric quantization

higher and integrated Kostant-Souriau extensions:

(∞-group extension of ∞-group of bisections of higher Atiyah groupoid for $\mathbb{G}$-principal ∞-connection)

$(\Omega \mathbb{G})\mathbf{FlatConn}(X) \to \mathbf{QuantMorph}(X,\nabla) \to \mathbf{HamSympl}(X,\nabla)$
$n$geometrystructureunextended structureextension byquantum extension
$\infty$higher prequantum geometrycohesive ∞-groupHamiltonian symplectomorphism ∞-groupmoduli ∞-stack of $(\Omega \mathbb{G})$-flat ∞-connections on $X$quantomorphism ∞-group
1symplectic geometryLie algebraHamiltonian vector fieldsreal numbersHamiltonians under Poisson bracket
1Lie groupHamiltonian symplectomorphism groupcircle groupquantomorphism group
22-plectic geometryLie 2-algebraHamiltonian vector fieldsline Lie 2-algebraPoisson Lie 2-algebra
2Lie 2-groupHamiltonian 2-plectomorphismscircle 2-groupquantomorphism 2-group
$n$n-plectic geometryLie n-algebraHamiltonian vector fieldsline Lie n-algebraPoisson Lie n-algebra
$n$smooth n-groupHamiltonian n-plectomorphismscircle n-groupquantomorphism n-group

(extension are listed for sufficiently connected $X$)

duality between algebra and geometry in physics:

∞-Chern-Simons theory from binary and non-degenerate invariant polynomial

$n \in \mathbb{N}$symplectic Lie n-algebroidLie integrated smooth ∞-groupoid = moduli ∞-stack of fields of $(n+1)$-d sigma-modelhigher symplectic geometry$(n+1)$d sigma-modeldg-Lagrangian submanifold/ real polarization leaf= brane(n+1)-module of quantum states in codimension $(n+1)$discussed in:
0symplectic manifoldsymplectic manifoldsymplectic geometryLagrangian submanifoldordinary space of states (in geometric quantization)geometric quantization
1Poisson Lie algebroidsymplectic groupoid2-plectic geometryPoisson sigma-modelcoisotropic submanifold (of underlying Poisson manifold)brane of Poisson sigma-model2-module = category of modules over strict deformation quantiized algebra of observablesextended geometric quantization of 2d Chern-Simons theory
2Courant Lie 2-algebroidsymplectic 2-groupoid3-plectic geometryCourant sigma-modelDirac structureD-brane in type II geometry
$n$symplectic Lie n-algebroidsymplectic n-groupoid(n+1)-plectic geometry$d = n+1$ AKSZ sigma-model

## References

### General

The observation that the would-be Poisson bracket induced by a higher degree closed form extends to the Poisson bracket Lie n-algebra is due to

with first discussion of application to prequantization in

Discussion in the more general context of higher differential geometry/extended prequantum field theory is in

A higher differential geometry-generalization of contact geometry in line with multisymplectic geometry/$n$-plectic geometry is discussed in

• Luca Vitagliano, L-infinity Algebras From Multicontact Geometry (arXiv.1311.2751)

### Applications

Some more references on application, on top of those mentioned in the articles above.

A survey of some (potential) applications of 2-plectic geometry in string theory and M2-brane models is in section 2 of

and in

Revised on May 28, 2015 10:44:16 by Urs Schreiber (195.113.30.252)