Symplectic group

The name "symplectic" was introduced by Hermann Weyl as a replacement for older terminology such as line complex group. It was intended as a Greek-based analogue of the word "complex".

The notation Sp(2n, F) usually denotes the symplectic group of a 2n-dimensional symplectic vector space over a field F. A related but different family is the compact symplectic group, denoted Sp(n) or ${\displaystyle \mathrm {USp} (n)}$, which is the compact real form of the complex symplectic group.

Many authors use slightly different notations, often differing by factors of 2. In Cartan's classification, the Lie algebra of Sp(2n, C) has type C_n.

The symplectic group is a classical group defined as the set of linear transformations of a 2n-dimensional vector space over the field F which preserve a non-degenerate skew-symmetric bilinear form. Such a vector space is called a symplectic vector space, and the symplectic group of an abstract symplectic vector space V is denoted Sp(V). Upon fixing a basis for V, the symplectic group becomes the group of 2n × 2n symplectic matrices, with entries in F, under the operation of matrix multiplication. This group is denoted either Sp(2n, F) or Sp(n, F). If the bilinear form is represented by the nonsingular skew-symmetric matrix Ω, then

${\displaystyle \operatorname {Sp} (2n,F)=\{M\in M_{2n\times 2n}(F):M^{\mathrm {T} }\Omega M=\Omega \},}$

where M^T is the transpose of M. Often Ω is defined to be

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

where I_n is the identity matrix. In this case, Sp(2n, F) can be expressed as those block matrices ${\displaystyle ({\begin{smallmatrix}A&B\\C&D\end{smallmatrix}})}$, where ${\displaystyle A,B,C,D\in M_{n\times n}(F)}$, satisfying the three equations:

${\displaystyle {\begin{aligned}-C^{\mathrm {T} }A+A^{\mathrm {T} }C&=0,\\-C^{\mathrm {T} }B+A^{\mathrm {T} }D&=I_{n},\\-D^{\mathrm {T} }B+B^{\mathrm {T} }D&=0.\end{aligned}}}$

Since all symplectic matrices have determinant 1, the symplectic group is a subgroup of the special linear group SL(2n, F). When n = 1, the symplectic condition on a matrix is satisfied if and only if the determinant is one, so that Sp(2, F) = SL(2, F). For n > 1, there are additional conditions, i.e. Sp(2n, F) is then a proper subgroup of SL(2n, F).

Typically, the field F is the field of real numbers R or complex numbers C. In these cases Sp(2n, F) is a real or complex Lie group of real or complex dimension n(2n + 1), respectively. These groups are connected but non-compact.

The center of Sp(2n, F) consists of the matrices I_2n and −I_2n as long as the characteristic of the field is not 2.^[1] Since the center of Sp(2n, F) is discrete and its quotient modulo the center is a simple group, Sp(2n, F) is considered a simple Lie group.

The real rank of the corresponding Lie algebra, and hence of the Lie group Sp(2n, F), is n.

The Lie algebra of Sp(2n, F) is the set

${\displaystyle {\mathfrak {sp}}(2n,F)=\{X\in M_{2n\times 2n}(F):\Omega X+X^{\mathrm {T} }\Omega =0\},}$

equipped with the commutator as its Lie bracket.^[2] For the standard skew-symmetric bilinear form ${\displaystyle \Omega =({\begin{smallmatrix}0&I\\-I&0\end{smallmatrix}})}$, this Lie algebra is the set of all block matrices ${\displaystyle ({\begin{smallmatrix}A&B\\C&D\end{smallmatrix}})}$ subject to the conditions

${\displaystyle {\begin{aligned}A&=-D^{\mathrm {T} },\\B&=B^{\mathrm {T} },\\C&=C^{\mathrm {T} }.\end{aligned}}}$

Sp(2n, C)

The symplectic group over the field of complex numbers is a non-compact, simply connected, simple Lie group. The definition of this group includes no conjugates (contrary to what one might naively expect) but instead it is exactly the same as the definition bar the field change.^[3]

Sp(2n, R)

Sp(n, C) is the complexification of the real group Sp(2n, R). Sp(2n, R) is a real, non-compact, connected, simple Lie group.^[4] It has a fundamental group isomorphic to the group of integers under addition. As the real form of a simple Lie group its Lie algebra is a splittable Lie algebra.

Some further properties of Sp(2n, R):

• The exponential map from the Lie algebra sp(2n, R) to the group Sp(2n, R) is not surjective. However, any element of the group can be represented as the product of two exponentials.^[5] In other words,

${\displaystyle \forall S\in \operatorname {Sp} (2n,\mathbf {R} )\,\,\exists X,Y\in {\mathfrak {sp}}(2n,\mathbf {R} )\,\,S=e^{X}e^{Y}.}$

• For all S in Sp(2n, R):

${\displaystyle S=OZO'\quad {\text{such that}}\quad O,O'\in \operatorname {Sp} (2n,\mathbf {R} )\cap \operatorname {SO} (2n)\cong U(n)\quad {\text{and}}\quad Z={\begin{pmatrix}D&0\\0&D^{-1}\end{pmatrix}}.}$

The matrix D is positive-definite and diagonal. The set of such Zs forms a non-compact subgroup of Sp(2n, R) whereas U(n) forms a compact subgroup. This decomposition is known as 'Euler' or 'Bloch–Messiah' decomposition.^[6][7] Further symplectic matrix properties can be found on that Wikipedia page.

• As a Lie group, Sp(2n, R) has a manifold structure. The manifold for Sp(2n, R) is diffeomorphic to the Cartesian product of the unitary group U(n) with a vector space of dimension n(n+1).^[8]

Infinitesimal generators

The members of the symplectic Lie algebra sp(2n, F) are the Hamiltonian matrices.

These are matrices, ${\displaystyle Q}$ such that

${\displaystyle Q={\begin{pmatrix}A&B\\C&-A^{\mathrm {T} }\end{pmatrix}}}$

where B and C are symmetric matrices. See classical group for a derivation.

Example of symplectic matrices

For Sp(2, R), the group of 2 × 2 matrices with determinant 1, the three symplectic (0, 1)-matrices are:^[9]

${\displaystyle {\begin{pmatrix}1&0\\0&1\end{pmatrix}},\quad {\begin{pmatrix}1&0\\1&1\end{pmatrix}}\quad {\text{and}}\quad {\begin{pmatrix}1&1\\0&1\end{pmatrix}}.}$

Sp(2n, R)

It turns out that ${\displaystyle \operatorname {Sp} (2n,\mathbf {R} )}$ can have a fairly explicit description using generators. If we let ${\displaystyle \operatorname {Sym} (n)}$ denote the symmetric ${\displaystyle n\times n}$ matrices, then ${\displaystyle \operatorname {Sp} (2n,\mathbf {R} )}$ is generated by ${\displaystyle D(n)\cup N(n)\cup \{\Omega \},}$ where

${\displaystyle {\begin{aligned}D(n)&=\left\{\left.{\begin{bmatrix}A&0\\0&(A^{T})^{-1}\end{bmatrix}}\,\right|\,A\in \operatorname {GL} (n,\mathbf {R} )\right\}\\[6pt]N(n)&=\left\{\left.{\begin{bmatrix}I_{n}&B\\0&I_{n}\end{bmatrix}}\,\right|\,B\in \operatorname {Sym} (n)\right\}\end{aligned}}}$

are subgroups of ${\displaystyle \operatorname {Sp} (2n,\mathbf {R} )}$^{[10]pg 173[11]pg 2}.

Relationship with symplectic geometry

Symplectic geometry is the study of symplectic manifolds. The tangent space at any point on a symplectic manifold is a symplectic vector space.^[12] As noted earlier, structure preserving transformations of a symplectic vector space form a group and this group is Sp(2n, F), depending on the dimension of the space and the field over which it is defined.

A symplectic vector space is itself a symplectic manifold. A transformation under an action of the symplectic group is thus, in a sense, a linearised version of a symplectomorphism which is a more general structure preserving transformation on a symplectic manifold.

The compact symplectic group^[13] Sp(n) is the intersection of Sp(2n, C) with the ${\displaystyle 2n\times 2n}$ unitary group:

${\displaystyle \operatorname {Sp} (n):=\operatorname {Sp} (2n;\mathbf {C} )\cap \operatorname {U} (2n)=\operatorname {Sp} (2n;\mathbf {C} )\cap \operatorname {SU} (2n).}$

It is sometimes written as USp(2n). Alternatively, Sp(n) can be described as the subgroup of GL(n, H) (invertible quaternionic matrices) that preserves the standard hermitian form on Hⁿ:

${\displaystyle \langle x,y\rangle ={\bar {x}}_{1}y_{1}+\cdots +{\bar {x}}_{n}y_{n}.}$

That is, Sp(n) is just the quaternionic unitary group, U(n, H).^[14] Indeed, it is sometimes called the hyperunitary group. Also Sp(1) is the group of quaternions of norm 1, equivalent to SU(2) and topologically a 3-sphere S³.

Note that Sp(n) is not a symplectic group in the sense of the previous section—it does not preserve a non-degenerate skew-symmetric H-bilinear form on Hⁿ: there is no such form except the zero form. Rather, it is isomorphic to a subgroup of Sp(2n, C), and so does preserve a complex symplectic form in a vector space of twice the dimension. As explained below, the Lie algebra of Sp(n) is the compact real form of the complex symplectic Lie algebra sp(2n, C).

Sp(n) is a real Lie group with (real) dimension n(2n + 1). It is compact and simply connected.^[15]

The Lie algebra of Sp(n) is given by the quaternionic skew-Hermitian matrices, the set of n-by-n quaternionic matrices that satisfy

${\displaystyle A+A^{\dagger }=0}$

where A^† is the conjugate transpose of A (here one takes the quaternionic conjugate). The Lie bracket is given by the commutator.

Important subgroups

Some main subgroups are:

${\displaystyle \operatorname {Sp} (n)\supset \operatorname {Sp} (n-1)}$

${\displaystyle \operatorname {Sp} (n)\supset \operatorname {U} (n)}$

${\displaystyle \operatorname {Sp} (2)\supset \operatorname {O} (4)}$

Conversely it is itself a subgroup of some other groups:

${\displaystyle \operatorname {SU} (2n)\supset \operatorname {Sp} (n)}$

${\displaystyle \operatorname {F} _{4}\supset \operatorname {Sp} (4)}$

${\displaystyle \operatorname {G} _{2}\supset \operatorname {Sp} (1)}$

There are also the isomorphisms of the Lie algebras sp(2) = so(5) and sp(1) = so(3) = su(2).

The unitary symplectic group ${\displaystyle USp(n)}$ can be represented in terms of a Clifford algebra defined as a tensor product of quaternion algebras called hyperquaternion numbers. One has, ${\displaystyle \mathbb {H} ^{\otimes 2}=\mathbb {H} \otimes _{\mathbb {R} }\mathbb {H} =M_{4\times 4}(\mathbb {R} )=Cl_{3,1}\mathbb {(R)} }$. Hence, ${\displaystyle \mathbb {H} ^{\otimes 3}=M_{4\times 4}(\mathbb {H} )}$ entails the compact symplectic group ${\displaystyle USp(4)}$.^[16]

The integral symplectic group Sp(2n, Z) is the subgroup of Sp(2n, R) consisting of matrices with integer entries. Equivalently, it is the group of 2n × 2n integer matrices preserving the standard symplectic form:

${\displaystyle \operatorname {Sp} (2n,\mathbf {Z} )=\{M\in M_{2n}(\mathbf {Z} ):M^{\mathrm {T} }JM=J\},}$

where

${\displaystyle J={\begin{pmatrix}0&I_{n}\\-I_{n}&0\end{pmatrix}}.}$

For n=1, this group coincides with the modular group SL(2, Z).

The group Sp(2n, Z) acts on the Siegel upper half-space, the space of symmetric complex n × n matrices with positive-definite imaginary part, by fractional linear transformations

${\displaystyle \gamma \cdot \tau =(A\tau +B)(C\tau +D)^{-1},\qquad \gamma ={\begin{pmatrix}A&B\\C&D\end{pmatrix}}\in \operatorname {Sp} (2n,\mathbf {Z} ).}$

For this reason, Sp(2n, Z) is also called the Siegel modular group of degree n.

Its congruence subgroups, such as the principal congruence subgroup

${\displaystyle \Gamma _{n}(m)=\{\gamma \in \operatorname {Sp} (2n,\mathbf {Z} ):\gamma \equiv I_{2n}{\pmod {m}}\},}$

play a central role in the theory of Siegel modular forms. The quotient of Siegel upper half-space by Sp(2n, Z) is the moduli space of principally polarized abelian varieties of dimension n, and quotients by congruence subgroups correspond to adding level structure.

Every complex, semisimple Lie algebra has a split real form and a compact real form; the former is called a complexification of the latter two.

The Lie algebra of Sp(2n, C) is semisimple and is denoted sp(2n, C). Its split real form is sp(2n, R) and its compact real form is sp(n). These correspond to the Lie groups Sp(2n, R) and Sp(n) respectively.

The algebras, sp(p, n − p), which are the Lie algebras of Sp(p, n − p), are the indefinite signature equivalent to the compact form.

Classical mechanics

The real symplectic group Sp(2n, R) arises in Hamiltonian mechanics as the group of linear canonical transformations of phase space. In canonical coordinates

${\displaystyle (q^{1},\dots ,q^{n},p_{1},\dots ,p_{n}),}$

its elements are precisely the linear changes of variables that preserve the standard symplectic form, or equivalently the Poisson bracket.

The Lie algebra sp(2n, R) is naturally identified with quadratic Hamiltonians on phase space: if

${\displaystyle H(z)={\tfrac {1}{2}}z^{\mathrm {T} }Kz,}$

then the corresponding Hamiltonian flow is linear and defines a one-parameter subgroup of Sp(2n, R). In this sense, the symplectic group is generated by quadratic Hamiltonians.

Quantum mechanics and the metaplectic group

Main article: metaplectic group

The symplectic group acts by linear changes of coordinates in the phase space of classical mechanics. When one tries to make the same transformations act on the wavefunctions of quantum mechanics, there is a phase ambiguity, and it is necessary to pass to a double cover. The metaplectic group is a double cover of the symplectic group over R. (It has analogues over other local fields, finite fields, and adele rings.)

• Hamiltonian mechanics
• Metaplectic group
• Orthogonal group
• Paramodular group
• Projective unitary group
• Representations of classical Lie groups
• Symplectic manifold, Symplectic matrix, Symplectic vector space, Symplectic representation
• Unitary group
• Θ10