Symplectic matrix
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In mathematics, a symplectic matrix is a 2n×2n matrix M (whose entries are typically either real or complex) satisfying the condition
where MT denotes the transpose of M and Ω is a fixed nonsingular, skew-symmetric matrix. Typically Ω is chosen to be the block matrix
where In is the n×n identity matrix. Note that Ω has determinant +1 and has an inverse given by Ω−1 = ΩT = −Ω.
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[edit] Properties
Every symplectic matrix is invertible with the inverse matrix given by
- M − 1 = Ω − 1MTΩ.
Furthermore, the product of two symplectic matrices is, again, a symplectic matrix. This gives the set of all symplectic matrices the structure of a group. There exists a natural manifold structure on this group which makes it into a (real or complex) Lie group called the symplectic group. The symplectic group has dimension n(2n + 1).
It follows easily from the definition that the determinant of any symplectic matrix is ±1. Actually, it turns out that the determinant is always +1. One way to see this is through the use of the Pfaffian and the identity
- Pf(MTΩM) = det(M)Pf(Ω).
Since MTΩM = Ω and 
we have that det(M) = 1.
Suppose Ω is given in the standard form and let M be a 2n×2n block matrix given by
where A, B, C, D are n×n matrices. The condition for M to be symplectic is equivalent to the conditions
- ATD − CTB = I
- ATC = CTA
- DTB = BTD.
When n = 1 these conditions reduce to the single condition det(M) = 1. Thus a 2×2 matrix is symplectic iff it has unit determinant.
[edit] Symplectic transformations
In the abstract formulation of linear algebra, matrices are replaced with linear transformations of finite-dimensional vector spaces. The abstract analog of a symplectic matrix is a symplectic transformation of a symplectic vector space. Briefly, a symplectic vector space is a 2n-dimensional vector space V equipped with a nondegenerate, skew-symmetric bilinear form ω called the symplectic form.
A symplectic transformation is then a linear transformation L : V → V which preserves ω, i.e.
- ω(Lu,Lv) = ω(u,v).
Fixing a basis for V, ω can be written as a matrix Ω and L as a matrix M. The condition that L be a symplectic transformation is precisely the condition that M be a symplectic matrix:
- MTΩM = Ω.
Under a change of basis, represented by a matrix A, we have
One can always bring Ω to either of the standard forms given in the introduction by a suitable choice of A.
[edit] The matrix Ω
Symplectic matrices are defined relative to a fixed nonsingular, skew-symmetric matrix Ω. As explained in the previous section, Ω can be thought of as the coordinate representation of a nondegenerate skew-symmetric bilinear form. It is a basic result in linear algebra that any two such matrices differ from each other by a change of basis.
The most common alternative to the standard Ω given above is the block diagonal form
This choice differs from the previous one by a permutation of basis vectors.
Sometimes the notation J is used instead of Ω for the skew-symmetric matrix. This is a particularly unfortunate choice as it leads to confusion with the notion of a complex structure, which often has the same coordinate expression as Ω but represents a very different structure. A complex structure J is the coordinate representation of a linear transformation that squares to −1, whereas Ω is the coordinate representation of a nondegenerate skew-symmetric bilinear form. One could easily choose bases in which J is not skew-symmetric or Ω does not square to −1.
Given a hermitian structure on a vector space, J and Ω are related via
where gac is the metric. That J and Ω usually have the same coordinate expression (up to an overall sign) is simply a consequence of the fact that the metric g is usually the identity matrix.
[edit] See also
- symplectic vector space
- symplectic group
- symplectic representation
- orthogonal matrix
- unitary matrix
- Hamiltonian mechanics
