# Symplectic matrix

In mathematics, a symplectic matrix is a 2n×2n matrix M with real entries that satisfies the condition

${\displaystyle M^{\text{T}}\Omega M=\Omega \,,}$

(1)

where MT denotes the transpose of M and Ω is a fixed 2n×2n nonsingular, skew-symmetric matrix. This definition can be extended to 2n×2n matrices with entries in other fields, e.g. the complex numbers.

Typically Ω is chosen to be the block matrix

${\displaystyle \Omega ={\begin{bmatrix}0&I_{n}\\-I_{n}&0\\\end{bmatrix}}}$

where In is the n×n identity matrix. The matrix Ω has determinant +1 and has an inverse given by Ω−1 = ΩT = −Ω.

Every symplectic matrix has unit determinant, and the 2n×2n symplectic matrices with real entries form a subgroup of the special linear group SL(2n, R) under matrix multiplication, specifically a connected noncompact real Lie group of real dimension n(2n + 1), the symplectic group Sp(2n, R). The symplectic group can be defined as the set of linear transformations that preserve the symplectic form of a real symplectic vector space.

An example of a group of symplectic matrices is the group of three symplectic 2x2-matrices consisting in the identity matrix, the upper triagonal matrix and the lower triangular matrix, each with entries 0 and 1.

## Properties

Every symplectic matrix is invertible with the inverse matrix given by

${\displaystyle M^{-1}=\Omega ^{-1}M^{\text{T}}\Omega .}$

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.

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 for any field. One way to see this is through the use of the Pfaffian and the identity

${\displaystyle {\mbox{Pf}}(M^{\text{T}}\Omega M)=\det(M){\mbox{Pf}}(\Omega ).}$

Since ${\displaystyle M^{\text{T}}\Omega M=\Omega }$ and ${\displaystyle {\mbox{Pf}}(\Omega )\neq 0}$ we have that det(M) = 1.

When the underlying field is real or complex, elementary proof is obtained by factoring the inequality ${\displaystyle \det(M^{\text{T}}M+I)\geq 1}$.[1]

Suppose Ω is given in the standard form and let M be a 2n×2n block matrix given by

${\displaystyle M={\begin{pmatrix}A&B\\C&D\end{pmatrix}}}$

where A, B, C, D are n×n matrices. The condition for M to be symplectic is equivalent to the two following equivalent conditions[2]

${\displaystyle A^{\text{T}}C,B^{\text{T}}D}$ symmetric, and ${\displaystyle A^{\text{T}}D-C^{\text{T}}B=I}$
${\displaystyle AB^{\text{T}},CD^{\text{T}}}$ symmetric, and ${\displaystyle AD^{\text{T}}-BC^{\text{T}}=I}$

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.

With Ω in standard form, the inverse of M is given by

${\displaystyle M^{-1}=\Omega ^{-1}M^{\text{T}}\Omega ={\begin{pmatrix}D^{\text{T}}&-B^{\text{T}}\\-C^{\text{T}}&A^{\text{T}}\end{pmatrix}}.}$

The group has dimension n(2n + 1). This can be seen by noting that the group condition implies that

${\displaystyle \Omega M^{\text{T}}\Omega M=-I}$

this gives equations of the form

${\displaystyle -\delta _{ij}=\sum _{k=1}^{n}m_{k,i+n}m_{n+k,j}-m_{n+k,i+n}m_{n,j}-m_{k,i}m_{n+k,j}+m_{k,i}m_{k,j}}$

where ${\displaystyle m_{ij}}$ is the i,j-th element of M. The sum is antisymmetric with respect to indices i,j, and since the left hand side is zero when i differs from j, this leaves n(2n-1) independent equations.

## 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 : VV which preserves ω, i.e.

${\displaystyle \omega (Lu,Lv)=\omega (u,v).}$

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:

${\displaystyle M^{\text{T}}\Omega M=\Omega .}$

Under a change of basis, represented by a matrix A, we have

${\displaystyle \Omega \mapsto A^{\text{T}}\Omega A}$
${\displaystyle M\mapsto A^{-1}MA.}$

One can always bring Ω to either the standard form given in the introduction or the block diagonal form described below by a suitable choice of A.

## 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

${\displaystyle \Omega ={\begin{bmatrix}{\begin{matrix}0&1\\-1&0\end{matrix}}&&0\\&\ddots &\\0&&{\begin{matrix}0&1\\-1&0\end{matrix}}\end{bmatrix}}.}$

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

${\displaystyle \Omega _{ab}=-g_{ac}{J^{c}}_{b}}$

where ${\displaystyle g_{ac}}$ 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.

## Diagonalisation and decomposition

• For any positive definite real symplectic matrix S there exists U in U(2n,R) such that
${\displaystyle S=U^{\text{T}}DU\quad {\text{for}}\quad D=\operatorname {diag} (\lambda _{1},\ldots ,\lambda _{n},\lambda _{1}^{-1},\ldots ,\lambda _{n}^{-1}),}$
where the diagonal elements of D are the eigenvalues of S.[3]
${\displaystyle S=UR\quad {\text{for}}\quad U\in \operatorname {U} (2n,\mathbb {R} ){\text{ and }}R\in \operatorname {Sp} (2n,\mathbb {R} )\cap \operatorname {Sym} _{+}(2n,\mathbb {R} ).}$
• Any real symplectic matrix can be decomposed as a product of three matrices:
${\displaystyle S=O{\begin{pmatrix}D&0\\0&D^{-1}\end{pmatrix}}O',}$
such that O and O' are both symplectic and orthogonal and D is positive-definite and diagonal.[4] This decomposition is closely related to the singular value decomposition of a matrix and is known as an 'Euler' or 'Bloch-Messiah' decomposition.

## Complex matrices

If instead M is a 2n×2n matrix with complex entries, the definition is not standard throughout the literature. Many authors [5] adjust the definition above to

${\displaystyle M^{*}\Omega M=\Omega \,.}$

(2)

where M* denotes the conjugate transpose of M. In this case, the determinant may not be 1, but will have absolute value 1. In the 2×2 case (n=1), M will be the product of a real symplectic matrix and a complex number of absolute value 1.

Other authors [6] retain the definition (1) for complex matrices and call matrices satisfying (2) conjugate symplectic.