Lie group

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In mathematics, a Lie group, named after Norwegian mathematician Sophus Lie (IPA pronunciation: [liː], 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. Lie groups represent the best developed theory of continuous symmetry of mathematical objects and structures. This makes Lie groups tools for nearly all parts of contemporary mathematics, as well as for modern theoretical physics, especially particle physics.

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 linearized version, which Lie himself called an infinitesimal group and which has since become known as its Lie algebra.

Lie groups provide a natural framework to analyse continuous symmetries of differential equations (Picard-Vessiot theory), much in the same way as permutation groups are used in Galois theory to analyse discrete symmetries of algebraic equations.

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[edit] Early history

According the most authoritative source on the early history of Lie groups (Hawkins, p.1), Sophus Lie himself considered winter of 1873—1874 as the birth date of his theory of continuous groups. However, he goes on to prove 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 its creation (ibid). Lie developed some of his early ideas in close collaboration with Felix Klein, with whom he saw on a daily basis in Berlin from the end of October 1869 to end of February 1870, and in Paris, Gőttingen and Erlangen in subsequent two years, until October 1872 (ibid, p.2). Lie stated that all the principal results were obtained by the year 1884. However, during the 1870s he chose to publish all papers, except the very first note, exclusively in Norwegian journals, which impeded the recognition of his 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 exposing his theory of continuous groups. The result of this effort was the publication of three volumes of Theorie der Transformationsgruppen, which appeared in 1888, 1890, and 1893.

Lie's ideas did not stand in isolation from the rest of mathematics. In fact, his interest to geometry of differential equations was first motivated by work of Carl Gustav Jacobi on the theory of partial differential equations of first order and on equations of classical mechanics, much of which was published posthumously in 1860s and generated 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 Evarist Galois has done for agebraic equations, namely, classify them in terms of group theory. Additional impetus to consider continuous groups came from ideas of Bernhard Riemann on 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: idea of symmetry, as explicated by Galois through the algebraic notion of a group; geometric theory and explicit solution of differential equations of mechanics, worked out by Poisson and Jacobi; and 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 of geometry.

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 Elie 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 connected 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,[citation needed]). The theory of Lie groups was systematically reworked in modern mathematical language in a monograph of Claude Chevalley.

[edit] 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 requires to 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 (Lie himself called them "infinitesimal groups".) 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.

[edit] Example

For example, the 2×2 real invertible matrices,

\begin{bmatrix}a&b\\c&d\end{bmatrix} , \qquad ad-bc \ne 0 ,

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

\begin{bmatrix} \cos \lambda & -\sin \lambda \\ \sin \lambda & \cos \lambda \end{bmatrix} ,

and observe that the inverse for the element given by λ is that given by −λ, while the product of the elements given by λ and μ is that given by λ+μ; thus both group operations are continuous, as required.

[edit] Definitions

A (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 the 1950s 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.

[edit] Examples of Lie groups

Here are a few examples of Lie groups and their relations to other areas of mathematics and physics.

For many more examples see the table of Lie groups and list of simple Lie groups and article on matrix groups.

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.

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,√2 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.

[edit] Types of Lie groups

Lie groups are classified according to their algebraic properties (simple, semisimple, solvable, nilpotent, abelian), their connectedness (connected or simply connected) 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 extensions 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.

[edit] 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.

[edit] The Lie algebra associated to a Lie group

To every Lie group, we can associate a Lie algebra, whose underlying vector space is the tangent space of G at the identity element, which completely captures the local structure of the group. Informally we can think of elements of the Lie algebra as elements of the groups that are "infinitesimally close" to the identity, and the Lie bracket is something to do with the commutator of two such infinitesimal elements. Before giving the abstract definition we give a few examples:

  • The Lie algebra of the vector space Rn is just Rn with the Lie bracket given by
[A, B] = 0.

(In general the Lie bracket of a connected Lie group is always 0 if and only if the Lie group is abelian.)

  • The Lie algebra of the general linear group GLn(R) of invertible matrices is the vector space Mn(R) of square matrices with the Lie bracket given by
[A, B] = AB − BA
  • If G is a closed subgroup of GLn(R) then the Lie algebra of G can be thought of informally as the matrices m of Mn(R) such that 1 + εm is in G, where ε is an infinitesimal positive number with ε2 = 0 (of course no such real number ε exists...). For example, the orthogonal group On(R) consists of matrices A with AAT = 1, so the Lie algebra consists of the matrices m with (1 + εm)(1 + εm)T = 1, which is equivalent to m + mT = 0 because ε2 = 0.

The concrete definition given above is easy to work with, but has some minor problems: to use it we first need to represent a Lie group as a group of matrices, but not all Lie groups can be represented in this way, and it is not obvious that the Lie algebra does not depend on which representation we use. To get round these problems we give the general definition of the Lie algebra of any Lie group (in 4 steps):

  1. Vector fields on any smooth manifold M can be thought of as derivations X of the ring of smooth functions on the manifold, and therefore form a Lie algebra under the Lie bracket [X, Y] = XY − YX, because the Lie bracket of any two derivations is a derivation.
  2. If G is any group acting smoothly on the manifold M, then it acts on the vector fields, and the vector space of vector fields fixed by the group is closed under the Lie bracket and therefore also forms a Lie algebra.
  3. We apply this construction to the case when the manifold M is the underlying space of a Lie group G, with G acting on G = M by left translations. This shows that the space of left invariant vector fields on a Lie group is a Lie algebra under the Lie bracket of vector fields.
  4. Any tangent vector at the identity of a Lie group can be extended to a left invariant vector field by left translating the tangent vector to other points of the manifold. This identifies the tangent space Te at the identity with the space of left invariant vector fields, and therefore makes the tangent space into a Lie algebra, called the Lie algebra of G, usually denoted by a lower case g or a Fraktur \mathfrak{g}.

This Lie algebra \mathfrak{g} is finite-dimensional and it has the same dimension as the manifold G. The Lie algebra of G determines G up to "local isomorphism", where two Lie groups are called locally isomorphic if they look the same near the identity element. Problems about Lie groups are often solved by first solving the corresponding problem for the Lie algebras, and the result for groups then usually follows easily. For example, simple Lie groups are usually classified by first classifying the corresponding Lie algebras.

We could also define a Lie algebra structure on Te using right invariant vector fields instead of left invariant vector fields. This leads to the same Lie algebra, because the inverse map on G can be used to identify left invariant vector fields with right invariant vector fields, and acts as −1 on the tangent space Te.

The Lie algebra structure on Te can also be described as follows : the commutator operation

(x, y) → xyx−1y−1

on G × G sends (e, e) to e, so its derivative yields a bilinear operation on TeG. This bilinear operation is actually the zero map, but the second derivative, under the proper identification of tangent spaces, yields an operation that satisfies the axioms of a Lie bracket, and it is equal to twice the one defined through left-invariant vector fields.

[edit] 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 \mathfrak{g} and \mathfrak{h}. The association G \mapsto\mathfrak{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 \mathfrak{g} over F there is a simply connected Lie group G with \mathfrak{g} 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.

[edit] The exponential map

The exponential map from the Lie algebra Mn(R) of the group GLn(R) to GLn(R) is defined by the usual power series:

\exp(A) = 1 + A + \frac{A^2}{2!} + \frac{A^3}{3!} + \cdots

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 \mathfrak{g} determines a linear map from R to \mathfrak{g} 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 \mathfrak{g} into the Lie group G. It provides a diffeomorphism between a neighborhood of 0 in \mathfrak{g} 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 \mathfrak{g}, 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.

[edit] 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."[1]

[edit] See also

[edit] References

  • Armand Borel, Essays in the history of Lie groups and algebraic groups, History of Mathematics 21, American Mathematical Society, 2001. ISBN 0-8218-0288-7
  • Thomas Hawkins, Emergence of the theory of Lie groups, Springer, 2000. ISBN 0-387-98963-3
  • Brian C. Hall Lie Groups, Lie Algebras, and Representations: An Elementary Introduction, Springer, 2003. ISBN 0-387-40122-9
  • J.-P. Serre. Lie Algebras and Lie Groups: 1964 Lectures given at Harvard University, LNM 1500, Springer. ISBN 3-540-55008-9
  • Anthony W. Knapp, Lie Groups Beyond an Introduction, Second Edition. Birkhäuser, 2002.
  • J.F. Adams, Lectures on Lie Groups (Chicago Lectures in Mathematics). ISBN 0-226-00527-5
  • Representation Theory : A First Course (Graduate Texts in Mathematics / Readings in Mathematics) by William Fulton, Joe Harris Publisher: Springer; 1 edition (July 30, 1999) ISBN 0-387-97495-4
  • Wulf Rossmann, Lie Groups: An Introduction Through Linear Groups (Oxford Graduate Texts in Mathematics), Oxford University Press ISBN 0-19-859683-9. The 2003 reprinting corrects some unfortunate typos.

[edit] Notes

  1. ^ Andrew Pressley and Graeme Segal, Loop Groups, Oxford Science Publications, 1986, page 26.
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