Indefinite orthogonal group

In mathematics, the indefinite orthogonal group, O(p,q) is the Lie group of all linear transformations of an n-dimensional real vector space that leave invariant a nondegenerate, symmetric bilinear form of signature (p,q), where n = p + q. The dimension of the group is n(n − 1)/2.

The indefinite special orthogonal group, SO(p,q) is the subgroup of O(p,q) consisting of all elements with determinant 1. Unlike in the definite case, SO(p,q) is not connected – it has 2 components – and there are two additional finite index subgroups, namely the connected SO⁺(p,q) and O⁺(p,q), which has 2 components – see the topology section for definition and discussion.

The signature of the form determines the group up to isomorphism; interchanging p with q amounts to replacing the metric by its negative, and so gives the same group. If either p or q equals zero, then the group is isomorphic to the ordinary orthogonal group O(n). We assume in what follows that both p and q are positive.

The group O(p,q) is defined for vector spaces over the reals. For complex spaces, all groups O(p,q; C) are isomorphic to the usual orthogonal group O(p + q; C), since the transform $z_j \mapsto iz_j$ changes the signature of a form.

In even dimension, the middle group O(n,n) is known as the split orthogonal group, and is of particular interest. In odd dimension, split form is the almost-middle group O(n,n + 1).

Examples

Squeeze mappings, here r = 3/2, are the basic hyperbolic symmetries.

The basic example is the squeeze mappings, which is the group SO⁺(1,1) of (the identity component of) linear transforms preserving the unit hyperbola. Concretely, these are the matrices $\left[\begin{smallmatrix} \cosh(\alpha) & \sinh(\alpha) \\ \sinh(\alpha) & \cosh(\alpha) \end{smallmatrix}\right],$ and can be interpreted as hyperbolic rotations, just as the group SO(2) can be interpreted as circular rotations.

In physics, the Lorentz group O(1,3) is of central importance, being the setting for electromagnetism and special relativity.

Matrix definition

One can define O(p,q) as a group of matrices, just as for the classical orthogonal group O(n). The standard inner product on R^p,q is given in coordinates by the diagonal matrix:

\eta = \mathrm{diag}(\underbrace{1,\ldots,1}_{p},\underbrace{-1,\ldots,-1}_{q}).\,

As a quadratic form, $Q(x_1,\dots,x_n) = x_1^2 + \cdots + x_p^2 - x_{p+1}^2 - \cdots - x_{p+q}^2.$

The group O(p,q) is then the group of an n×n matrices M (where n = p+q) such that $Q(Mv)=Q(v)$ ; as a bilinear form,

M^T\eta M = \eta.\,

Here M^T denotes the transpose of the matrix M. One can easily verify that the set of all such matrices forms a group. The inverse of M is given by

M^{-1} = \eta^{-1}M^T\eta.\,

One obtains an isomorphic group (indeed, a conjugate subgroup of GL(V)) by replacing η with any symmetric matrix with p positive eigenvalues and q negative ones (such a matrix is necessarily nonsingular); equivalently, any quadratic form with signature (p,q). Diagonalizing this matrix gives a conjugation of this group with the standard group O(p,q).

Topology

Assuming both p and q are nonzero, neither of the groups O(p,q) or SO(p,q) are connected, having four and two components respectively. π₀(O(p,q)) ≅ C₂ × C₂ is the Klein four-group, with each factor being whether an element preserves or reverses the respective orientations on the p and q dimensional subspaces on which the form is definite; note that reversing orientation on only one of these subspaces reverses orientation on the whole space. The special orthogonal group has components π₀(SO(p,q)) = {(1,1),(−1,−1)} which either preserves both orientations or reverses both orientations, in either case preserving the overall orientation.

The identity component of O(p,q) is often denoted SO⁺(p,q) and can be identified with the set of elements in SO(p,q) which preserves both orientations. This notation is related to the notation O⁺(1,3) for the orthochronous Lorentz group, where the + refers to preserving the orientation on the first (temporal) dimension.

The group O(p,q) is also not compact, but contains the compact subgroups O(p) and O(q) acting on the subspaces on which the form is definite. In fact, O(p) × O(q) is a maximal compact subgroup of O(p,q), while S(O(p) × O(q)) is a maximal compact subgroup of SO(p,q). Likewise, SO(p) × SO(q) is a maximal compact subgroup of SO⁺(p, q). Thus up to homotopy, the spaces are products of (special) orthogonal groups, from which algebro-topological invariants can be computed.

In particular, the fundamental group of SO⁺(p,q) is the product of the fundamental groups of the components, π₁(SO⁺(p,q)) = π₁(SO(p)) × π₁(SO(q)), and is given by:

π₁(SO⁺(p,q))	p = 1	p = 2	p ≥ 3
q = 1	{1}	Z	Z₂
q = 2	Z	Z × Z	Z × Z₂
q ≥ 3	Z₂	Z₂ × Z	Z₂ × Z₂

Split orthogonal group

In even dimension, the middle group O(n,n) is known as the split orthogonal group, and is of particular interest. It is the split Lie group corresponding to the complex Lie algebra so_2n (the Lie group of the split real form of the Lie algebra); more precisely, the identity component is the split Lie group, as non-identity components cannot be reconstructed from the Lie algebra. In this sense it is opposite to the definite orthogonal group O(n) := O(n,0) = O(0,n), which is the compact real form of the complex Lie algebra.

The case (1,1) corresponds to the split-complex numbers.

In terms of being a group of Lie type – i.e., construction of an algebraic group from a Lie algebra – split orthogonal groups are Chevalley groups, while the non-split orthogonal groups require a slightly more complicated construction, and are Steinberg groups.

Split orthogonal groups are used to construct the generalized flag variety over non-algebraically closed fields.

In odd dimension, the split form is the almost-middle group O(n, n + 1).

References