Symplectic vector space

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In mathematics, a symplectic vector space is a vector space V equipped with a nondegenerate, skew-symmetric, bilinear form ω called the symplectic form.

Explicitly, a symplectic form is a bilinear form ω : V × V → R which is

• Skew-symmetric: ω(u, v) = −ω(v, u) for all u, v ∈ V,
• Nondegenerate: if ω(u, v) = 0 for all v ∈ V then u = 0.

Working in a fixed basis, ω can be represented by a matrix. The two conditions above say that this matrix must be skew-symmetric and nonsingular. This is not the same thing as a symplectic matrix, which represents a symplectic transformation of the space.

If V is finite-dimensional then its dimension must necessarily be even since every skew-symmetric matrix of odd size has determinant zero.

A nondegenerate skew-symmetric bilinear form behaves quite differently from a nondegenerate symmetric bilinear form, such as the dot product on Euclidean vector spaces. With a Euclidean inner product g, we have g(v,v) > 0 for all nonzero vectors v, whereas a symplectic form ω satisfies ω(v,v) = 0.

Contents 1 Standard symplectic space 1.1 Analogy with complex structures 2 Volume form 3 Symplectic map 4 Symplectic group 5 Subspaces 6 Heisenberg group 7 See also 8 References

[edit] Standard symplectic space

The standard symplectic space is R²ⁿ with the symplectic form given by a nonsingular, skew-symmetric matrix. Typically ω is chosen to be the block matrix

where I_n is the n × n identity matrix. In terms of basis vectors

:

A modified version of the Gram-Schmidt process shows that any finite-dimensional symplectic vector space has such a basis, often called a Darboux basis.

There is another way to interpret this standard symplectic form. Since the model space Rⁿ used above carries much canonical structure which might easily lead to misinterpretation, we will use "anonymous" vector spaces instead. Let V be a real vector space of dimension n and V^∗ its dual space. Now consider the direct sum W := V ⊕ V^∗ of these spaces equipped with the following form:

Now choose any basis (v₁, …, v_n) of V and consider its dual basis

We can interpret the basis vectors as lying in W if we write x_i = (v_i, 0) and y_i = (0, v_i^∗). Taken together, these form a complete basis of W,

The form ω defined here can be shown to have the same properties as in the beginning of this section – in other words, every symplectic structure is isomorphic to one of the form V ⊕ V^∗. The subspace V is not unique, and a choice of subspace V is called a polarization. The subspaces that give such an isomorphism are called Lagrangian subspaces or simply Lagrangians.

Explicitly, given a Lagrangian subspace (as defined below), then a choice of basis defines a dual basis for a complement, by ω(x_i,y_j) = δ_ij.

[edit] Analogy with complex structures

Just as every symplectic structure is isomorphic to one of the form V ⊕ V^∗, every complex structure on a vector space is isomorphic to one of the form V ⊕ V. Using these structures, the tangent space of an n-manifold, considered as a 2n-manifold, has an almost complex structure, and the cotangent space of an n-manifold, considered as a 2n-manifold, has a symplectic structure:

The complex analog to a Lagrangian subspace is a real subspace, a subspace whose complexification is the whole space: W = V ⊕ J'V.

[edit] Volume form

Let ω be a form on a n-dimensional real vector space V, ω ∈ Λ²(V). Then ω is non-degenerate if and only if n is even, and ω^n/2 = ω ∧ … ∧ ω is a volume form. A volume form on a n-dimensional vector space V is a non-zero multiple of the (unique) n-form e₁^∗ ∧ … ∧ e_n^∗ where the e_i are standard basis vectors on V.

For the standard basis defined in the previous section, we have

By reordering, one can write

Authors variously define ωⁿ or (−1)^n/2ωⁿ as the standard volume form. An occasional factor of n! may also appear, depending on whether the definition of the alternating product contains a factor of n! or not. The volume form defines an orientation on the symplectic vector space (V, ω).

[edit] Symplectic map

Suppose that (V,ω) and (W,ρ) are symplectic vector spaces. Then a linear map is called a symplectic map if and only if the pullback f ^* preserves the symplectic form, that is, if f ^* ρ = ω. The pullback form is defined by

f ^* ρ(u,v) = ρ(f(u),f(v))

and thus f is a symplectic map if and only if

ρ(f(u),f(v)) = ω(u,v)

for all u and v in V. In particular, symplectic maps are volume-preserving, orientation-preserving, and are isomorphisms.

[edit] Symplectic group

If V = W, then a symplectic map is called a linear symplectic transformation of V. In particular, in this case one has that

ω(f(u),f(v)) = ω(u,v),

and so the linear transformation f preserves the symplectic form. The set of all symplectic transformations forms a group and in particular a Lie group, called the symplectic group and denoted by Sp(V) or sometimes Sp(V,ω). In matrix form symplectic transformations are given by symplectic matrices.

[edit] Subspaces

Let W be a linear subspace of V. Define the symplectic complement of W to be the subspace

The symplectic complement satisfies

and

However, unlike orthogonal complements, W^⊥ ∩ W need not be 0. We distinguish four cases:

• W is symplectic if W^⊥ ∩ W = {0}. This is true if and only if ω restricts to a nondegenerate form on W. A symplectic subspace with the restricted form is a symplectic vector space in its own right.
• W is isotropic if W ⊆ W^⊥. This is true if and only if ω restricts to 0 on W. Any one-dimensional subspace is isotropic.
• W is coisotropic if W^⊥ ⊆ W. W is coisotropic if and only if ω descends to a nondegenerate form on the quotient space W/W^⊥. Equivalently W is coisotropic if and only if W^⊥ is isotropic. Any codimension-one subspace is coisotropic.
• W is Lagrangian if W = W^⊥. A subspace is Lagrangian if and only if it is both isotropic and coisotropic. In a finite-dimensional vector space, a Lagrangian subspace is an isotropic one whose dimension is half that of V. Every isotropic subspace can be extended to a Lagrangian one.

Referring to the canonical vector space R²ⁿ above,

• the subspace spanned by {x₁, y₁} is symplectic
• the subspace spanned by {x₁, x₂} is isotropic
• the subspace spanned by {x₁, x₂, …, x_n, y₁} is coisotropic
• the subspace spanned by {x₁, x₂, …, x_n} is Lagrangian.

[edit] Heisenberg group

A Heisenberg group can be defined for any symplectic vector space, and this is the general way that Heisenberg groups arise.

A vector space can be thought of as a commutative Lie group (under addition), or equivalently as a commutative Lie algebra, meaning with trivial Lie bracket. The Heisenberg group is a central extension of such a commutative Lie group/algebra: the symplectic form defines the commutation, analogously to the canonical commutation relations (CCR), and a Darboux basis corresponds to canonical coordinates – in physics terms, to momentum operators and position operators.

Indeed, by the Stone–von Neumann theorem, every representation satisfying the CCR (every representation of the Heisenberg group) is of this form, or more properly unitarily conjugate to the standard one.

Further, the group algebra of (the dual to) a vector space is the symmetric algebra, and the group algebra of the Heisenberg group (of the dual) is the Weyl algebra: one can think of the central extension as corresponding to quantization or deformation.

Formally, the symmetric algebra of V is the group algebra of the dual, Sym(V): = K[V ^* ], and the Weyl algebra is the group algebra of the (dual) Heisenberg group W(V) = K[H(V ^* )]. Since passing to group algebras is a contravariant functor, the central extension map becomes an inclusion

[edit] See also

• A symplectic manifold is a smooth manifold with a smoothly-varying closed symplectic form on each tangent space
• Maslov index
• A symplectic representation is a group representation where each group element acts as a symplectic transformation.

[edit] References

• Ralph Abraham and Jarrold E. Marsden, Foundations of Mechanics, (1978) Benjamin-Cummings, London ISBN 0-8053-0102-X See chapter 3.