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Lie group
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In mathematics, a Lie group (, sounds like "Lee"), is a group which is also a differentiable manifold, with the property that the group operations are compatible with the smooth structure. They are named after the nineteenth century Norwegian mathematician Sophus Lie, who laid the foundations of the theory of continuous transformation groups.
Lie groups represent the best developed theory of continuous symmetry of mathematical objects and structures, which makes them indispensable tools for many parts of contemporary mathematics, as well as for modern theoretical physics. They provide a natural framework for analysing the continuous symmetries of differential equations, much in the same way as permutation groups are used in Galois theory for analysing the discrete symmetries of algebraic equations. An extension of Galois theory to the case of continuous symmetry groups was one of Lie's principal motivations, his idée fixe.
Overview
Since Lie groups are manifolds, they can be studied using differential calculus, in contrast with the case of more general topological groups. One of the key ideas in the theory of Lie groups, due to Sophus Lie, is to replace the global object, the group, with its local or linearised version, which Lie himself called its "infinitesimal group" and which has since become known as its Lie algebra.
Lie groups play an enormous role in modern geometry, on several different levels. Felix Klein argued in his Erlangen program that one can consider various "geometries" by specifying an appropriate transformation group that leaves certain geometric properties invariant. Thus Euclidean geometry corresponds to the choice of the group E(3) of distance-preserving transformations of the Euclidean space R3, conformal geometry corresponds to enlarging the group to the conformal group, whereas in projective geometry one is interested in the properties invariant under the projective group. This idea later led to the notion of a G-structure, where G is a Lie group of "local" symmetries of a manifold. On a "global" level, whenever a Lie group acts on a geometric object, such as a Riemannian or a symplectic manifold, this action provides a measure of rigidity and yields a rich algebraic structure. The presence of continuous symmetries expressed via a Lie group action on a manifold places strong constraints on its geometry and facilitates analysis on the manifold. Linear actions of Lie groups are especially important, and are studied in representation theory.
In the 1950s, Claude Chevalley realised that many foundational results concerning Lie groups can be developed completely algebraically, giving rise to the theory of algebraic groups defined over an arbitrary field. This insight opened new possibilities in pure algebra, by providing a uniform construction for most finite simple groups, as well as in algebraic geometry. The theory of automorphic forms, an important branch of modern number theory, deals extensively with analogues of Lie groups over adele rings.
Examples of Lie groups
Lie groups occur in abundance throughout mathematics and physics.
Examples
- Euclidean space Rn with ordinary vector addition as the group operation becomes an n-dimensional noncompact abelian Lie group.
- The circle group S1 consisting of angles mod 2p under addition or complex numbers with absolute value 1 under multiplication is a one-dimensional compact connected abelian Lie group.
- The group GLn(R) of invertible matrices (under matrix multiplication) is a Lie group of dimension n2, called the general linear group. It has a closed connected subgroup SLn(R), the special linear group, consisting of matrices of determinant 1 which is also a Lie group.
- The orthogonal group On(R), consisting of all n × n orthogonal matrices with real entries is an n(n − 1)/2-dimensional Lie group. This group is disconnected, but it has a connected subgroup SOn(R) of the same dimension consisting of orthogonal matrices of determinant 1, called the special orthogonal group (for n = 3, the rotation group).
- The Euclidean group En(R) is the Lie group of all Euclidean motions, i.e. isometric affine maps, of n-dimensional Euclidean space Rn.
- The unitary group U(n) consisting of n × n unitary matrices (with complex entries) is a compact connected Lie group of dimension n2. Unitary matrices of determinant 1 form a closed connected subgroup of dimension n2 − 1 denoted SU(n), the special unitary group.
- Spin groups are double covers of the special orthogonal groups, used for studying fermions in quantum field theory (among other things).
- The symplectic group Sp2n(R) consists of all 2n × 2n matrices preserving a nondegenerate skew-symmetric bilinear form on R2n (the symplectic form). It is a connected Lie group of dimension 2n2 + n. The fundamental group of the symplectic group is Z and this fact is related to the theory of Maslov index.
- The 3-sphere S3 forms a Lie group by identification with the set of quaternions of unit norm, called versors. The only other spheres that admit the structure of a Lie group are the 0-sphere S0 (real numbers with absolute value 1) and the circle S1 (complex numbers with absolute value 1).
- The group of upper triangular n by n matrices is a solvable Lie group of dimension n(n + 1)/2.
- The Lorentz group and the Poincare group are the groups of linear and affine isometries of the Minkowski space (interpreted as the spacetime of the special relativity). They are Lie groups of dimensions 6 and 10.
- The Heisenberg group is a connected nilpotent Lie group of dimension 3, playing a key role in quantum mechanics.
- The group U(1)×SU(2)×SU(3) is a Lie group of dimension 1+3+8=12 that is the gauge group of the Standard Model in particle physics. The dimensions of the factors correspond to the 1 photon + 3 vector bosons + 8 gluons of the standard model.
- The (3-dimensional) metaplectic group is a double cover of SL2(R) playing an important role in the theory of modular forms. It is a connected Lie group that cannot be faithfully represented by matrices of finite size, i.e. a nonlinear group.
- The exceptional Lie groups of types G2, F4, E6, E7, E8 have dimensions 14, 52, 78, 133, and 248. There is also a group E7˝ of dimension 190.
Constructions
There are several standard ways to form new Lie groups from old ones:
- The product of two Lie groups is a Lie group.
- Any closed subgroup of a Lie group is a Lie group.
- The quotient of a Lie group by a closed normal subgroup is a Lie group.
- The universal cover of a connected Lie group is a Lie group. For example, the group R is the universal cover of the circle group S1.
Related notions Some examples of groups that are not Lie groups are:
- Infinite dimensional groups, such as the additive group of an infinite dimensional real vector space. These are not Lie groups as they are not finite dimensional manifolds.
- Some totally disconnected groups, such as the Galois group of an infinite extension of fields, or the additive group of the p-adic numbers. These are not Lie groups because their underlying spaces are not real manifolds. (Some of these groups are "p-adic Lie groups".)
- The image of a connected Lie group under a homomorphism of Lie groups need not be a Lie group. The usual example of this is the image of R in the group R2/Z2 (? S1×S1) under the map x?(x,v2 x). The image is a dense subset of R2/Z2 that is not a manifold, and so is not a Lie group. This also gives an example where a subalgebra of a Lie algebra does not correspond to a Lie subgroup of the corresponding Lie group.
- The group of rational numbers under addition, topologized as a subset of the real numbers, is not a Lie group as it is not a manifold.
Early history According to the most authoritative source on the early history of Lie groups (Hawkins, p.1), Sophus Lie himself considered the winter of 1873–1874 as the birth date of his theory of continuous groups. Hawkins, however, suggests that it was "Lie's prodigious research activity during the four-year period from the fall of 1869 to the fall of 1873" that led to the theory's creation (ibid). Some of Lie's early ideas were developed in close collaboration with Felix Klein. Lie met with Klein every day from October 1869 through 1872: in Berlin from the end of October 1869 to the end of February 1870, and in Paris, Göttingen and Erlangen in the subsequent two years (ibid, p.2). Lie stated that all of the principal results were obtained by 1884. But during the 1870s all his papers (except the very first note) were published in Norwegian journals, which impeded recognition of the work throughout the rest of Europe (ibid, p.76). In 1884 a young German mathematician, Friedrich Engel, came to work with Lie on a systematic treatise to expose his theory of continuous groups. From this effort resulted the three-volume Theorie der Transformationsgruppen, published in 1888, 1890, and 1893.
Lie's ideas did not stand in isolation from the rest of mathematics. In fact, his interest in the geometry of differential equations was first motivated by the work of Carl Gustav Jacobi, on the theory of partial differential equations of first order and on the equations of classical mechanics. Much of Jacobi's work was published posthumously in the 1860s, generating enormous interest in France and Germany (Hawkins, p.43). Lie's idée fixe was to develop a theory of symmetries of differential equations that would accomplish for them what Évariste Galois had done for algebraic equations: namely, to classify them in terms of group theory. Additional impetus to consider continuous groups came from ideas of Bernhard Riemann, on the foundations of geometry, and their further development in the hands of Klein. Thus three major themes in 19th century mathematics were combined by Lie in creating his new theory: the idea of symmetry, as exemplified by Galois through the algebraic notion of a group; geometric theory and the explicit solutions of differential equations of mechanics, worked out by Poisson and Jacobi; and the new understanding of geometry that emerged in the works of Plücker, Möbius, Grassmann and others, and culminated in Riemann's revolutionary vision of the subject.
Although today Sophus Lie is rightfully recognized as the creator of the theory of continuous groups, a major stride in the development of their structure theory, which was to have a profound influence on subsequent development of mathematics, was made by Wilhelm Killing, who in 1888 published the first paper in a series entitled
Die Zusammensetzung der stetigen endlichen Transformationsgruppen (The composition of continuous finite transformation groups) (Hawkins, p.100). The work of Killing, later refined and generalized by Élie Cartan, led to classification of semisimple Lie algebras, Cartan's theory of symmetric spaces, and Hermann Weyl's description of representations of compact and semisimple Lie groups using highest weights.
Weyl brought the early period of the development of the theory of Lie groups to fruition, for not only did he classify irreducible representations of semisimple Lie groups and connect the theory of groups with quantum mechanics, but he also put Lie's theory itself on firmer footing by clearly enunciating the distinction between Lie's infinitesimal groups (i.e. Lie algebras) and the Lie groups proper, and began investigations of topology of Lie groups (Borel (2001), ). The theory of Lie groups was systematically reworked in modern mathematical language in a monograph by Claude Chevalley.
The concept of a Lie group, and possibilities of classification
Lie groups may be thought of as smoothly varying families of symmetries. Examples of symmetries include rotation about an axis. What must be understood is the nature of 'small' transformations, here rotations through tiny angles, that link nearby transformations. The mathematical object capturing this structure is called a Lie algebra. It can be defined because Lie groups are manifolds, so have tangent spaces at each point.
The Lie algebra of any compact Lie group (very roughly: one for which the symmetries form a bounded set) can be decomposed as a direct sum of an abelian Lie algebra and some number of simple ones. The structure of an abelian Lie algebra is mathematically uninteresting; the interest is in the simple summands. Hence the question arises: what are the simple Lie algebras of compact groups? It turns out that they mostly fall into four infinite families, the "classical Lie algebras" An, Bn, Cn and Dn, which have simple descriptions in terms of symmetries of Euclidean space. But there are also just five "exceptional Lie algebras" that do not fall into any of these families. E8 is the largest of these.
Example
For example, the 2×2 real invertible matrices,
-
form a multiplicative group, denoted by GL2(R), which is a classic example of a Lie group; its manifold is 4-dimensional. Further restricting to 2×2 rotation matrices gives a subgroup, denoted by SO2(R), which is also a Lie group; its manifold is 1-dimensional, a circle, with rotation angle as parameter. In this latter example we can write a group element as
-
and observe that the inverse for the element given by ? is that given by -?, while the product of the elements given by ? and µ is that given by ?+µ; thus both group operations are continuous, as required.
DefinitionsA (real) Lie group is a mathematical group which is also a finite-dimensional real smooth manifold, and in which the group operations of multiplication and inversion are smooth maps.
There are several closely related concepts. A complex Lie group is defined in the same way using complex manifolds rather than real ones (example: SL2(C)), and similarly one can define a p-adic Lie group over the p-adic numbers. An Infinite dimensional Lie group is defined in the same way except that one allows the underlying manifold to be infinite dimensional. Matrix groups or algebraic groups are (roughly) groups of matrices, (for example, orthogonal and symplectic groups) and these give most of the more common examples of Lie groups.
It is possible to define analogues of many Lie groups over finite fields, and these give most of the examples of finite simple groups. One could also try varying the definition by using topological or analytic manifolds instead of smooth ones, but it turns out that this gives nothing new: Gleason, Montgomery and Zippin showed in 1952 that if G is a topological manifold with continuous group operations, then there exists exactly one analytic structure on G which turns it into a Lie group (see Hilbert's fifth problem and Hilbert-Smith conjecture).
The language of category theory provides a concise definition for Lie groups: a Lie group is a group object in the category of smooth manifolds. This is important, since it allows generalization of the notion of a Lie group to Lie supergroups.
Properties Every Lie group is parallelizable, and hence an orientable manifold.
Types of Lie groups
Lie groups are classified according to their algebraic properties, their connectedness and their compactness.
- The universal cover of any connected Lie group is a simply connected Lie group, and conversely any connected Lie group is a quotient of a simply connected Lie group by a discrete normal subgroup of the center.
- Compact Lie groups are all known: they are finite central quotients of a product of copies of the circle group S1 and simple compact Lie groups (which correspond to connected Dynkin diagrams).
- Any simply connected solvable Lie group is isomorphic to a closed subgroup of the group of invertible upper triangular matrices of some rank, and any finite dimensional irreducible representation of such a group is 1 dimensional. Solvable groups are too messy to classify except in a few small dimensions.
- Any simply connected nilpotent Lie group is isomorphic to a closed subgroup of the group of invertible upper triangular matrices with 1's on the diagonal of some rank, and any finite dimensional irreducible representation of such a group is 1 dimensional. Like solvable groups, nilpotent groups are too messy to classify except in a few small dimensions.
- Simple Lie groups are sometimes defined to be those that are simple as abstract groups, and sometimes defined to be connected Lie groups with a simple Lie algebra. For example, SL2(R) is simple according to the second definition but not according to the first. They have all been classified (for either definition).
- Semisimple Lie groups are connected groups whose Lie algebra is a product of simple Lie algebras. They are central extensions of products of simple Lie groups.
- Connected abelian Lie groups are all isomorphic to products of copies of R and the circle group S1.
Structure of a Lie group
Any Lie group G can be decomposed into discrete, simple, and abelian groups in a canonical way as follows. Write
- Gcon for the connected component of the identity
- Gsol for the largest connected normal solvable subgroup
- Gnil for the largest connected normal nilpotent subgroup
so that we have a sequence of normal subgroups
- 1 ? Gnil ? Gsol ? Gcon ? G
Then
- G/Gcon is discrete
- Gcon/Gsol is a central extension of a product of simple connected Lie groups.
- Gsol/Gnil is abelian (and a product of copies of R and S1)
- Gnil/1 is nilpotent, and therefore its ascending central series has all quotients abelian.
This can be used to reduce some problems about Lie groups (such as finding their unitary representations) to the same problems for connected simple groups.
Homomorphisms and isomorphisms
If G and H are Lie groups, then a Lie-group homomorphism f : G ? H is a smooth group homomorphism. (It is equivalent to require only that f be continuous rather than smooth.) The composition of two such homomorphisms is again a homomorphism, and the class of all Lie groups, together with these morphisms, forms a category. Two Lie groups are called isomorphic if there exists a bijective homomorphism between them whose inverse is also a homomorphism. Isomorphic Lie groups are essentially the same; they only differ in the notation for their elements.
Every homomorphism f : G ? H of Lie groups induces a homomorphism between the corresponding Lie algebras and . The association G is a functor.
One version of Ado's theorem is that every finite dimensional Lie algebra is isomorphic to a matrix Lie algebra. For every finite dimensional matrix Lie algebra, there is a linear group (matrix Lie group) with this algebra as its Lie algebra. So every abstract Lie algebra is the Lie algebra of some (linear) Lie group.
The global structure of a Lie group is not determined by its Lie algebra; for example, if Z is any discrete subgroup of the center of G then G and G/Z have the same Lie algebra (see the table of Lie groups for examples).
A connected Lie group is simple, semisimple, solvable, nilpotent, or abelian if and only if its Lie algebra has the corresponding property.
If we require that the Lie group be simply connected, then the global structure is determined by its Lie algebra: for every finite dimensional Lie algebra over F there is a simply connected Lie group G with as Lie algebra, unique up to isomorphism. Moreover every homomorphism between Lie algebras lifts to a unique homomorphism between the corresponding simply connected Lie groups.
The exponential map
The exponential map from the Lie algebra Mn(R) of the general linear group GLn(R) to GLn(R) is defined by the usual power series:
for matrices A. If G is any subgroup of GLn(R), then the exponential map takes the Lie algebra of G into G, so we have an exponential map for all matrix groups.
The definition above is easy to use, but it is not defined for Lie groups that are not matrix groups, and it is not clear that the exponential map of a Lie group does not depend on its representation as a matrix group. We can solve both problems using a more abstract definition of the exponential map that works for all Lie groups, as follows.
Every vector v in determines a linear map from R to taking 1 to v, which can be thought of as a Lie algebra homomorphism. Since R is the Lie algebra of the simply connected Lie group R, this induces a Lie group homomorphism c : R ? G so that
- c(s + t) = c(s) c(t)
for all s and t. The operation on the right hand side is the group multiplication in G. The formal similarity of this formula with the one valid for the exponential function justifies the definition
- exp(v) = c(1)
This is called the exponential map, and it maps the Lie algebra into the Lie group G. It provides a diffeomorphism between a neighborhood of 0 in and a neighborhood of e in G. This exponential map is a generalization of the exponential function for real numbers (since R is the Lie algebra of the Lie group of positive real numbers with multiplication), for complex numbers (since C is the Lie algebra of the Lie group of non-zero complex numbers with multiplication) and for matrices (since Mn(R) with the regular commutator is the Lie algebra of the Lie group GLn(R) of all invertible matrices).
Because the exponential map is surjective on some neighbourhood N of e, it is common to call elements of the Lie algebra infinitesimal generators of the group G. The subgroup of G generated by N is the identity component of G.
The exponential map and the Lie algebra determine the local group structure of every connected Lie group, because of the Baker-Campbell-Hausdorff formula: there exists a neighborhood U of the zero element of , such that for u, v in U we have
- exp(u) exp(v) = exp(u + v + 1/2 [u, v] + 1/12 [[u, v], v] − 1/12 [[u, v], u] − ...)
where the omitted terms are known and involve Lie brackets of four or more elements. In case u and v commute, this formula reduces to the familiar exponential law exp(u) exp(v) = exp(u + v).
The exponential map from the Lie algebra to the Lie group is not always onto, even if the group is connected
(though it does map onto the Lie group for connected groups that are either compact or nilpotent). For example, the exponential map of SL2(R) is not surjective.
Infinite dimensional Lie groups
Lie groups are finite dimensional by definition, but there are many groups that resemble Lie groups, except for being infinite dimensional. There is very little "general theory" of such groups, but some of the examples that have been studied include:
- The group of diffeomorphisms of a manifold. Quite a lot is known about the group of diffeomorphisms of the circle. Its Lie algebra is (more or less) the Witt algebra, which has a central extension called the Virasoro algebra, used in string theory and conformal field theory. Very little is known about the diffeomorphism groups of manifolds of larger dimension. The diffeomorphism group of spacetime sometimes appears in attempts to quantize gravity.
- The group of smooth maps from a manifold to a finite dimensional group is called a gauge group, and is used in quantum field theory and Donaldson theory. If the manifold is a circle these are called loop groups, and have central extensions whose Lie algebras are (more or less) Kac-Moody algebras.
- There are infinite dimensional analogues of general linear groups, orthogonal groups, and so on. One important aspect is that these may have simpler topological properties: see for example Kuiper's theorem.
- Just as calculus in finite-dimensional real vector spaces can be extended to calculus in Banach spaces, the definition of finite-dimensional smooth manifolds can be extended to give a definition of Banach analytic manifolds. Similarly, the standard finite-dimensional definition of Lie groups can be extended to give a definition of Banach analytic Lie groups. In this case, we have a Banach analytic manifold which simultaneously has a group structure such that multiplication and inversion are analytic maps. Some of the theorems of finite-dimensional Lie groups do not carry over to the Banach analytic case, and in particular the relation between Lie groups and Lie algebras is much more subtle in the infinite dimensional case. However, it is true that "for infinite dimensional Lie groups modeled on Banach spaces there is a well-developed theory ... which is closely parallel to the theory of finite dimensional Lie groups."
See also
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