Orthogonal group
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Modular groups

Infinite dimensional Lie group

In mathematics, the orthogonal group of dimension n, denoted O(n), is the group of distancepreserving transformations of a Euclidean space of dimension n, which preserve a fixed point, where the group operation is given by composing transformations. Equivalently, it is the group of n×n orthogonal matrices of a given dimension, where the group operation is given by matrix multiplication, and an orthogonal matrix is a real matrix whose inverse equals its transpose.
The determinant of an orthogonal matrix being either 1 or −1, an important subgroup of O(n) is the special orthogonal group, denoted SO(n), of the orthogonal matrices of determinant 1. This group is also called the rotation group, because, in dimensions 2 and 3, its elements are the usual rotations around a point (in dimension 2) or a line (in dimension 3). In low dimension, these groups have been widely studied, see SO(2), SO(3) and SO(4).
The term "orthogonal group" may also refer to a generalization of the above case: the group of invertible linear operators which preserve a nondegenerate symmetric bilinear form or quadratic form^{1} on a vector space over a field. In particular, when the bilinear form is the scalar product on the vector space F^{ n} of dimension n over a field F, with quadratic form the sum of squares, then the corresponding orthogonal group, denoted O(n, F ), is the set of n × n orthogonal matrices with entries from F, with the group operation of matrix multiplication. This is a subgroup of the general linear group GL(n, F ) given by
where Q^{T} is the transpose of Q and I is the identity matrix.
This article mainly discusses the orthogonal groups of quadratic forms that may be expressed over some bases as the dot product; over the reals, they are the positive definite quadratic forms. Over the reals, for any nondegenerate quadratic form, there is a basis, on which the matrix of the form is a diagonal matrix such that the diagonal entries are either 1 or −1. Thus the orthogonal group depends only on the numbers of 1 and of −1, and is denoted O(p, q), where p is the number of ones and q the number of −1. For details, see indefinite orthogonal group.
The derived subgroup Ω(n, F ) of O(n, F) is an often studied object because, when F is a finite field, Ω(n, F ) is often^{clarification needed} a central extension of a finite simple group.
Both O(n, F ) and SO(n, F ) are algebraic groups, because the condition that a matrix be orthogonal, i.e. have its own transpose as inverse, can be expressed as a set of polynomial equations in the entries of the matrix. The Cartan–Dieudonné theorem describes the structure of the orthogonal group for a nonsingular form.
Contents
 1 Name
 2 Even and odd dimension
 3 Over the real number field
 4 Over the complex number field
 5 Over finite fields
 6 The Dickson invariant
 7 Orthogonal groups of characteristic 2
 8 The spinor norm
 9 Galois cohomology and orthogonal groups
 10 Lie algebra
 11 Related groups
 12 Principal homogeneous space: Stiefel manifold
 13 See also
 14 Notes
 15 References
 16 External links
Name
The determinant of any orthogonal matrix is either 1 or −1. The orthogonal nbyn matrices with determinant 1 form a normal subgroup of O(n, F ) known as the special orthogonal group SO(n, F ), consisting of all proper rotations. (More precisely, SO(n, F ) is the kernel of the Dickson invariant, discussed below.). By analogy with GL–SL (general linear group, special linear group), the orthogonal group is sometimes called the general orthogonal group and denoted GO, though this term is also sometimes used for indefinite orthogonal groups O(p, q). The term rotation group can be used to describe either the special or general orthogonal group.
Even and odd dimension
The structure of the orthogonal group differs in certain aspects between even and odd dimensions – for example, over ordered fields (such as R) the −I element is orientationpreserving in even dimensions, but orientationreversing in odd dimensions. When this distinction wishes to be emphasized, the groups are generally denoted O(2k) and O(2k + 1), reserving n for the dimension of the space (n = 2k or n = 2k + 1). The letters p or r are also used, indicating the rank of the corresponding Lie algebra; in odd dimension the corresponding Lie algebra is , while in even dimension the Lie algebra is .
Over the real number field
Over the field R of real numbers, the orthogonal group O(n, R) and the special orthogonal group SO(n, R) are often simply denoted by O(n) and SO(n) if no confusion is possible. They form real compact Lie groups of dimension n(n − 1)/2. O(n, R) has two connected components, with SO(n, R) being the identity component, i.e., the connected component containing the identity matrix.
Geometric interpretation
The real orthogonal and real special orthogonal groups have the following geometric interpretations:
O(n, R) is a subgroup of the Euclidean group E(n), the group of isometries of R^{n}; it contains those that leave the origin fixed – O(n, R) = E(n) ∩ GL(n, R). It is the symmetry group of the sphere (n = 3) or (n − 1)sphere and all objects with spherical symmetry, if the origin is chosen at the center.
SO(n, R) is a subgroup of E^{+}(n), which consists of direct isometries, i.e., isometries preserving orientation; it contains those that leave the origin fixed – SO(n, R) = E^{+}(n) ∩ GL(n, R) = E(n) ∩ GL^{+}(n, R). It is the rotation group of the sphere and all objects with spherical symmetry, if the origin is chosen at the center.
{±I} is a normal subgroup and even a characteristic subgroup of O(n, R), and, if n is even, also of SO(n, R). If n is odd, O(n, R) is the internal direct product of SO(n, R) and {±I}. For every positive integer k the cyclic group C_{k} of kfold rotations is a normal subgroup of O(2, R) and SO(2, R).
Relative to suitable orthogonal bases, the isometries are of the form:
where the matrices R_{1}, … , R_{k} are 2by2 rotation matrices in orthogonal planes of rotation. As a special case, known as Euler's rotation theorem, any (nonidentity) element of SO(3, R) is rotation about a uniquely defined axis.
The orthogonal group is generated by reflections (two reflections give a rotation), as in a Coxeter group,^{note 1} and elements have length at most n (require at most n reflections to generate; this follows from the above classification, noting that a rotation is generated by 2 reflections, and is true more generally for indefinite orthogonal groups, by the Cartan–Dieudonné theorem). A longest element (element needing the most reflections) is reflection through the origin (the map v ↦ −v), though so are other maximal combinations of rotations (and a reflection, in odd dimension).
The symmetry group of a circle is O(2, R). It is isomorphic (as a real Lie group) to the circle group, also known as U(1). This isomorphism sends the complex number exp(φ i) = cos φ + i sin φ of absolute value 1 to the orthogonal matrix
The group SO(3, R), understood as the set of rotations of 3dimensional space, is of major importance in the sciences and engineering, and there are numerous charts on SO(3).
Lowdimensional topology
The lowdimensional (real) orthogonal groups are familiar spaces:
 O(1) = {±1}, a twopoint discrete space
 SO(1) = {1}
 SO(2) is S^{1}
 SO(3) is RP^{3}
 SO(4) is double covered by SU(2) × SU(2) = S^{3} × S^{3}.
Homotopy groups
In terms of algebraic topology, for n > 2 the fundamental group of SO(n, R) is cyclic of order 2, and the spin group Spin(n) is its universal cover. For n = 2 the fundamental group is infinite cyclic and the universal cover corresponds to the real line (the group Spin(2) is the unique 2fold cover).
Generally, the homotopy groups π_{k}(O) of the real orthogonal group are related to homotopy groups of spheres, and thus are in general hard to compute. However, one can compute the homotopy groups of the stable orthogonal group (aka the infinite orthogonal group), defined as the direct limit of the sequence of inclusions:
Since the inclusions are all closed, hence cofibrations, this can also be interpreted as a union. On the other hand S^{n} is a homogeneous space for O(n + 1), and one has the following fiber bundle:
which can be understood as "The orthogonal group O(n + 1) acts transitively on the unit sphere S^{n}, and the stabilizer of a point (thought of as a unit vector) is the orthogonal group of the perpendicular complement, which is an orthogonal group one dimension lower. Thus the natural inclusion O(n) → O(n + 1) is (n − 1)connected, so the homotopy groups stabilize, and π_{k}( π_{k}(O(n)) for n > k + 1: thus the homotopy groups of the stable space equal the lower homotopy groups of the unstable spaces.
From Bott periodicity we obtain Ω^{8}O ≅ O, therefore the homotopy groups of O are 8fold periodic, meaning π_{k + 8}(O) = π_{k}(O), and one needs only to compute the lower 8 homotopy groups:
Relation to KOtheory
Via the clutching construction, homotopy groups of the stable space O are identified with stable vector bundles on spheres (up to isomorphism), with a dimension shift of 1: π_{k}(O) = π_{k + 1}(BO). Setting KO = BO × Z = Ω^{−1}O × Z (to make π_{0} fit into the periodicity), one obtains:
Computation and interpretation of homotopy groups
Lowdimensional groups
The first few homotopy groups can be calculated by using the concrete descriptions of lowdimensional groups.
 π_{0}(O) = π_{0}(O(1)) = Z/2, from orientationpreserving/reversing (this class survives to O(2) and hence stably)
 π_{1}(O) = π_{1}(SO(3)) = Z/2, which is spin comes from SO(3) = RP^{3} = S^{3}/(Z/2).
 π_{2}(O) = π_{2}(SO(3)) = 0, which surjects onto π_{2}(SO(4)); this latter thus vanishes.
Lie groups
From general facts about Lie groups, π_{2}(G) always vanishes, and π_{3}(G) is free (free abelian).
Vector bundles
From the vector bundle point of view, π_{0}(KO) is vector bundles over S^{0}, which is two points. Thus over each point, the bundle is trivial, and the nontriviality of the bundle is the difference between the dimensions of the vector spaces over the two points, so π_{0}(KO) = Z is dimension.
Loop spaces
Using concrete descriptions of the loop spaces in Bott periodicity, one can interpret higher homotopy of O as lower homotopy of simple to analyze spaces. Using π_{0}, O and O/U have two components, KO = BO × Z and KSp = BSp × Z have countably many components, and the rest are connected.
Interpretation of homotopy groups
In a nutshell:^{2}
 π_{0}(KO) = Z is about dimension
 π_{1}(KO) = Z/2 is about orientation
 π_{2}(KO) = Z/2 is about spin
 π_{4}(KO) = Z is about topological quantum field theory.
Let R be any of four division rings R, C, H, O, and let L_{R} be the tautological line bundle over the projective line RP^{1}, and L_{R} its class in Ktheory. Noting that RP^{1} = S^{1}, CP^{1} = S^{2}, HP^{1} = S^{4}, OP^{1} = S^{8}, these yield vector bundles over the corresponding spheres, and
 π_{1}(KO) is generated by L_{R}
 π_{2}(KO) is generated by L_{C}
 π_{4}(KO) is generated by L_{H}
 π_{8}(KO) is generated by L_{O}
From the point of view of symplectic geometry, π_{0}(KO) ≅ π_{8}(KO) = Z can be interpreted as the Maslov index, thinking of it as the fundamental group π_{1}(U/O) of the stable Lagrangian Grassmannian as U/O ≅ Ω^{7}(KO), so π_{1}(U/O) = π_{1 + 7}(KO).
Over the complex number field
Over the field C of complex numbers, O(n, C) and SO(n, C) are complex Lie groups of dimension n(n − 1)/2 over C (it means the dimension over R is twice that). O(n, C) has two connected components, and SO(n, C) is the connected component containing the identity matrix. For n ≥ 2 these groups are noncompact.
Just as in the real case SO(n, C) is not simply connected. For n > 2 the fundamental group of SO(n, C) is cyclic of order 2 whereas the fundamental group of SO(2, C) is infinite cyclic.
Over finite fields
Orthogonal groups can also be defined over finite fields F_{q}, where q is a power of a prime p. When defined over such fields, they come in two types^{clarification needed} in even dimension: O^{+}(2n, q) and O^{−}(2n, q); and one type in odd dimension: O(2n + 1, q).
If V is the vector space on which the orthogonal group G acts, it can be written as a direct orthogonal sum as follows:
where L_{i} are hyperbolic lines and W contains no singular vectors. If W is the zero subspace, then G is of plus type. If W is onedimensional then G has odd dimension. If W has dimension 2, G is of minus type.
In the special case where n = 1, O^{ϵ}(2, q) is a dihedral group of order 2(q − ϵ).
We have the following formulas for the order of O(n, q), when the characteristic is greater than two:
If −1 is a square in F_{q}
If −1 is a nonsquare in F_{q}
The Dickson invariant
For orthogonal groups, the Dickson invariant is a homomorphism from the orthogonal group to the quotient group Z/2Z (integers modulo 2), taking the value 0 in case the element is the product of an even number of reflections, and the value of 1 otherwise.^{3}
Algebraically, the Dickson invariant can be defined as D(f) = rank(I − f) modulo 2, where I is the identity (Taylor 1992, Theorem 11.43). Over fields that are not of characteristic 2 it is equivalent to the determinant: the determinant is −1 to the power of the Dickson invariant. Over fields of characteristic 2, the determinant is always 1, so the Dickson invariant gives more information than the determinant.
The special orthogonal group is the kernel of the Dickson invariant^{3} and usually has index 2 in O(n, F ).^{4} When the characteristic of F is not 2, the Dickson Invariant is 0 whenever the determinant is 1. Thus when the characteristic is not 2, SO(n, F ) is commonly defined to be the elements of O(n, F ) with determinant 1. Each element in O(n, F ) has determinant ±1. Thus in characteristic 2, the determinant is always 1.
The Dickson invariant can also be defined for Clifford groups and Pin groups in a similar way (in all dimensions).
Orthogonal groups of characteristic 2
Over fields of characteristic 2 orthogonal groups often exhibit special behaviors, some of which are listed in this section. (Formerly these groups were known as the hypoabelian groups but this term is no longer used.)
 Any orthogonal group over any field is generated by reflections, except for a unique example where the vector space is 4dimensional over the field with 2 elements and the Witt index is 2.^{5} Note that a reflection in characteristic two has a slightly different definition. In characteristic two, the reflection orthogonal to a vector u takes a vector v to v + B(v, u)/Q(u) · u where B is the bilinear form and Q is the quadratic form associated to the orthogonal geometry. Compare this to the Householder reflection of odd characteristic or characteristic zero, which takes v to v − 2·B(v, u)/Q(u) · u.
 The center of the orthogonal group usually has order 1 in characteristic 2, rather than 2, since I = −I.
 In odd dimensions 2n + 1 in characteristic 2, orthogonal groups over perfect fields are the same as symplectic groups in dimension 2n. In fact the symmetric form is alternating in characteristic 2, and as the dimension is odd it must have a kernel of dimension 1, and the quotient by this kernel is a symplectic space of dimension 2n, acted upon by the orthogonal group.
 In even dimensions in characteristic 2 the orthogonal group is a subgroup of the symplectic group, because the symmetric bilinear form of the quadratic form is also an alternating form.
The spinor norm
The spinor norm is a homomorphism from an orthogonal group over a field F to the quotient group F */F *^{2} (the multiplicative group of the field F up to square elements), that takes reflection in a vector of norm n to the image of n in F */F *^{2}.^{6}
For the usual orthogonal group over the reals it is trivial, but it is often nontrivial over other fields, or for the orthogonal group of a quadratic form over the reals that is not positive definite.
Galois cohomology and orthogonal groups
In the theory of Galois cohomology of algebraic groups, some further points of view are introduced. They have explanatory value, in particular in relation with the theory of quadratic forms; but were for the most part post hoc, as far as the discovery of the phenomena is concerned. The first point is that quadratic forms over a field can be identified as a Galois H^{1}, or twisted forms (torsors) of an orthogonal group. As an algebraic group, an orthogonal group is in general neither connected nor simplyconnected; the latter point brings in the spin phenomena, while the former is related to the discriminant.
The 'spin' name of the spinor norm can be explained by a connection to the spin group (more accurately a pin group). This may now be explained quickly by Galois cohomology (which however postdates the introduction of the term by more direct use of Clifford algebras). The spin covering of the orthogonal group provides a short exact sequence of algebraic groups.
Here μ_{2} is the algebraic group of square roots of 1; over a field of characteristic not 2 it is roughly the same as a twoelement group with trivial Galois action. The connecting homomorphism from H^{0}(O_{V}), which is simply the group O_{V}(F ) of Fvalued points, to H^{1}(μ_{2}) is essentially the spinor norm, because H^{1}(μ_{2}) is isomorphic to the multiplicative group of the field modulo squares.
There is also the connecting homomorphism from H^{1} of the orthogonal group, to the H^{2} of the kernel of the spin covering. The cohomology is nonabelian, so that this is as far as we can go, at least with the conventional definitions.
Lie algebra
The Lie algebra corresponding to Lie groups O(n, F ) and SO(n, F ) consists of the skewsymmetric n × n matrices, with the Lie bracket [ , ] given by the commutator. One Lie algebra corresponds to both groups. It is often denoted by or , and called the orthogonal Lie algebra or special orthogonal Lie algebra. Over real numbers, these Lie algebras for different n are the compact real forms of two of the four families of semisimple Lie algebras: in odd dimension B_{k}, where n = 2k + 1, while in even dimension D_{r}, where n = 2r.
More intrinsically, given a vector space with an inner product, the special orthogonal Lie algebra is given by the bivectors on the space, which are sums of simple bivectors (2blades) v ∧ w. The correspondence is given by the map where v* is the covector dual to the vector v; in coordinates these are exactly the elementary skewsymmetric matrices.
Over real numbers, this characterization is used in interpreting the curl of a vector field (naturally a 2vector) as an infinitesimal rotation or "curl", hence the name. Generalizing the inner product with a nondegenerate form yields the indefinite orthogonal Lie algebras
The representation theory of the orthogonal Lie algebras includes both representations corresponding to linear representations of the orthogonal groups, and representations corresponding to projective representations of the orthogonal groups (linear representations of spin groups), the socalled spin representation, which are important in physics.
Related groups
The orthogonal groups and special orthogonal groups have a number of important subgroups, supergroups, quotient groups, and covering groups. These are listed below.
The inclusions O(n) ⊂ U(n) ⊂ Sp(n) = USp(2n) and USp(n) ⊂ U(n) ⊂ O(2n) are part of a sequence of 8 inclusions used in a geometric proof of the Bott periodicity theorem, and the corresponding quotient spaces are symmetric spaces of independent interest – for example, U(n)/O(n) is the Lagrangian Grassmannian.
Lie subgroups
In physics, particularly in the areas of Kaluza–Klein compactification, it is important to find out the subgroups of the orthogonal group. The main ones are:
 – preserve an axis
 – U(n) are those that preserve a compatible complex structure or a compatible symplectic structure – see 2outof3 property; SU(n) also preserves a complex orientation.
Lie supergroups
The orthogonal group O(n) is also an important subgroup of various Lie groups:
Conformal group
Being isometries, real orthogonal transforms preserve angles, and are thus conformal maps, though not all conformal linear transforms are orthogonal. In classical terms this is the difference between congruence and similarity, as exemplified by SSS (SideSideSide) congruence of triangles and AAA (AngleAngleAngle) similarity of triangles. The group of conformal linear maps of R^{n} is denoted CO(n) for the conformal orthogonal group, and consists of the product of the orthogonal group with the group of dilations. If n is odd, these two subgroups do not intersect, and they are a direct product: CO(2k + 1) = O(2k + 1) × R*, where R* = R\{0} is the real multiplicative group, while if n is even, these subgroups intersect in ±1, so this is not a direct product, but it is a direct product with the subgroup of dilation by a positive scalar: CO(2k) = O(2k) × R^{+}.
Similarly one can define CSO(n); note that this is always: CSO(n) = CO(n) ∩ GL^{+}(n) = SO(n) × R^{+}.
Discrete subgroups
As the orthogonal group is compact, discrete subgroups are equivalent to finite subgroups.^{note 2} These subgroups are known as point group and can be realized as the symmetry groups of polytopes. A very important class of examples are the finite Coxeter groups, which include the symmetry groups of regular polytopes.
Dimension 3 is particularly studied – see point groups in three dimensions, polyhedral groups, and list of spherical symmetry groups. In 2 dimensions, the finite groups are either cyclic or dihedral – see point groups in two dimensions.
Other finite subgroups include:
 Permutation matrices (the Coxeter group A_{n})
 Signed permutation matrices (the Coxeter group B_{n}); also equals the intersection of the orthogonal group with the integer matrices.^{note 3}
Covering and quotient groups
The orthogonal group is neither simply connected nor centerless, and thus has both a covering group and a quotient group, respectively:
 Two covering Pin groups, Pin_{+}(n) → O(n) and Pin_{−}(n) → O(n),
 The quotient projective orthogonal group, O(n) → PO(n).
These are all 2to1 covers.
For the special orthogonal group, the corresponding groups are:
 Spin group, Spin(n) → SO(n),
 Projective special orthogonal group, SO(n) → PSO(n).
Spin is a 2to1 cover, while in even dimension, PSO(2k) is a 2to1 cover, and in odd dimension PSO(2k + 1) is a 1to1 cover, i.e., isomorphic to SO(2k + 1). These groups, Spin(n), SO(n), and PSO(n) are Lie group forms of the compact special orthogonal Lie algebra, – Spin is the simply connected form, while PSO is the centerless form, and SO is in general neither.^{note 4}
In dimension 3 and above these are the covers and quotients, while dimension 2 and below are somewhat degenerate; see specific articles for details.
Principal homogeneous space: Stiefel manifold
The principal homogeneous space for the orthogonal group O(n) is the Stiefel manifold V_{n}(R^{n}) of orthonormal bases (orthonormal nframes).
In other words, the space of orthonormal bases is like the orthogonal group, but without a choice of base point: given an orthogonal space, there is no natural choice of orthonormal basis, but once one is given one, there is a onetoone correspondence between bases and the orthogonal group. Concretely, a linear map is determined by where it sends a basis: just as an invertible map can take any basis to any other basis, an orthogonal map can take any orthogonal basis to any other orthogonal basis.
The other Stiefel manifolds V_{k}(R^{n}) for k < n of incomplete orthonormal bases (orthonormal kframes) are still homogeneous spaces for the orthogonal group, but not principal homogeneous spaces: any kframe can be taken to any other kframe by an orthogonal map, but this map is not uniquely determined.
See also
Specific transforms
Specific groups
Related groups
Lists of groups
Notes
 ^ The analogy is stronger: Weyl groups, a class of (representations of) Coxeter groups, can be considered as simple algebraic groups over the field with one element, and there are a number of analogies between algebraic groups and vector spaces on the one hand, and Weyl groups and sets on the other.
 ^ Infinite subsets of a compact space have an accumulation point and are not discrete.
 ^ O(n) ∩ GL(n, Z) equals the signed permutation matrices because an integer vector of norm 1 must have a single nonzero entry, which must be ±1 (if it has two nonzero entries or a larger entry, the norm will be larger than 1), and in an orthogonal matrix these entries must be in different coordinates, which is exactly the signed permutation matrices.
 ^ In odd dimension, SO(2k + 1) ≅ PSO(2k + 1) is centerless (but not simply connected), while in even dimension SO(2k) is neither centerless nor simply connected.
References
 ^ For base fields of characteristic not 2, it is equivalent to use bilinear forms or quadratic forms. But in characteristic 2 these notions differ.
 ^ John Baez "This Week's Finds in Mathematical Physics" week 105
 ^ ^{a} ^{b} Knus, MaxAlbert (1991), Quadratic and Hermitian forms over rings, Grundlehren der Mathematischen Wissenschaften 294, Berlin etc.: SpringerVerlag, p. 224, ISBN 3540521178, Zbl 0756.11008
 ^ (Taylor 1992, page 160)
 ^ (Grove 2002, Theorem 6.6 and 14.16)
 ^ Cassels 1978, p. 178
This article needs additional citations for verification. (May 2010) 
 Cassels, J.W.S. (1978), Rational Quadratic Forms, London Mathematical Society Monographs 13, Academic Press, ISBN 0121632601, Zbl 0395.10029
 Grove, Larry C. (2002), Classical groups and geometric algebra, Graduate Studies in Mathematics 39, Providence, R.I.: American Mathematical Society, ISBN 9780821820193, MR 1859189
 Taylor, Donald E. (1992), The Geometry of the Classical Groups, Sigma Series in Pure Mathematics 9, Berlin: Heldermann Verlag, ISBN 3885380099, MR 1189139, Zbl 0767.20001
External links
 Hazewinkel, Michiel, ed. (2001), "Orthogonal group", Encyclopedia of Mathematics, Springer, ISBN 9781556080104
 John Baez "This Week's Finds in Mathematical Physics" week 105
 John Baez on Octonions
 (Italian) ndimensional Special Orthogonal Group parametrization