Symplectic vector space

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 represent 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.

tandard symplectic space

The standard symplectic space is R2"n" with the symplectic form given by a nonsingular, skew-symmetric matrix. Typically ω is chosen to be the block matrix

:omega = egin{bmatrix} 0 & I_n \ -I_n & 0 end{bmatrix}where "I""n" is the "n" × "n" identity matrix. In terms of basis vectors

:(x_1, ldots, x_n, y_1, ldots, y_n)::omega(x_i, y_j) = -omega(y_j, x_i) = delta_{ij},:omega(x_i, x_j) = omega(y_i, y_j) = 0.,

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 R"n" 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:

:omega(x oplus eta, y oplus xi) = xi(x) - eta(y).

Now choose any basis ("v"1, …, "v""n") of "V" and consider its dual basis

:(v^*_1, ldots, v^*_n).

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",

:(x_1, ldots, x_n, y_1, ldots, y_n).

The form omega defined here can be shown to have the same properties as in the beginning of this section.

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 "e"1∗ ∧ … ∧ "e""n"∗ where the "e""i" are standard basis vectors on "V".

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

:omega^n=(-1)^{n/2} x^*_1wedgeldots wedge x^*_nwedge y^*_1wedge ldots wedge y^*_n.

By reordering, one can write

:omega^n= x^*_1wedge y^*_1wedge ldots wedge x^*_nwedge y^*_n.

Authors variously define ω"n" or (−1)"n"/2ω"n" 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).

ymplectic map

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

:f^* ho(u,v)= ho(f(u),f(v))

and thus "f" is a symplectic map if and only if

: ho(f(u),f(v))=omega(u,v)

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

ymplectic group

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

:omega(f(u),f(v)) = omega(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.

ubspaces

Let "W" be a linear subspace of "V". Define the symplectic complement of "W" to be the subspace:W^{perp} = {vin V mid omega(v,w) = 0 mbox{ for all } win W}.The symplectic complement satisfies:(W^{perp})^{perp} = Wand:dim W + dim W^perp = dim V.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 R2"n" above,
* the subspace spanned by {"x"1, "y"1} is symplectic
* the subspace spanned by {"x"1, "x"2} is isotropic
* the subspace spanned by {"x"1, "x"2, …, "x""n", "y"1} is coisotropic
* the subspace spanned by {"x"1, "x"2, …, "x""n"} is Lagrangian.

Properties

Note that the symplectic form resembles the canonical commutation relations. As a result, the additive group of a symplectic vector space has a central extension, this central extension is the Heisenberg group.

ee 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.

References

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


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