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In mathematics, the generalized orthogonal group, O(p, q) is the Lie group of all linear transformations of a p + q = n dimensional real vector space which leave invariant a nondegenerate, symmetric, bilinear form of signature (p, q). The dimension of the group is
- n(n − 1)/2.
The generalized special orthogonal group, SO(p, q) is the subgroup of O(p, q) consisting of all elements with determinant 1.
The signature of the metric (p positive and q negative eigenvalues) 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.
Neither of the groups O(p, q) or SO(p, q) are connected, having 4 and 2 components respectively. 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 preserve the respective orientations of the p and q dimensional subspaces on which the form is definite.
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, the maximal compact subgroup of O(p,q) is given by O(p)×O(q). Likewise, the maximal compact subgroup of SO+(p, q) is SO(p)×SO(q). It follows that the fundamental group of SO+(p, q) is given by
- <math>\pi_1(\mbox{SO}^{+}(p,q)) = \pi_1(\mbox{SO}(p))\times\pi_1(\mbox{SO}(q))\;<math>
or (for p ≥ q):
- <math>\pi_1(\mbox{SO}^{+}(p,q)) = \begin{cases}
\{1\} & p=q=1 \\
\mathbb{Z} & p=2, q=1 \\
\mathbb{Z}_2 & p \ge 3, q=1 \\
\mathbb{Z}\times\mathbb{Z} & p=q=2 \\
\mathbb{Z}\times\mathbb{Z}_2 & p \ge 3, q=2 \\
\mathbb{Z}_2\times\mathbb{Z}_2 & p,q \ge 3
\end{cases}<math>
Note that O(p, q) is defined for vector spaces over the reals. For complex spaces, all groups O(p, q; C) are isomorphic with the usual orthogonal group O(p + q; C).
See also
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