Orbit
of a point relative to a group
acting on a set
(on the left)
The set
![]() |
The set
![]() |
is a subgroup in and is called the stabilizer or stationary subgroup of the point
relative to
. The mapping
,
, induces a bijection between
and the orbit
. The orbits of any two points from
either do not intersect or coincide; in other words, the orbits define a partition of the set
. The quotient by the equivalence relation defined by this partition is called the orbit space of
by
and is denoted by
. By assigning to each point its orbit, one defines a canonical mapping
. The stabilizers of the points from one orbit are conjugate in
, or, more precisely,
. If there is only one orbit in
, then
is a homogeneous space of the group
and
is also said to act transitively on
. If
is a topological group,
is a topological space and the action is continuous, then
is usually given the topology in which a set
is open in
if and only if the set
is open in
.
==
1) Let be the group of rotations of a plane
around a fixed point
. Then the orbits are all circles with centre at
(including the point
itself).
2) Let be the group of all non-singular linear transformations of a finite-dimensional real vector space
, let
be the set of all symmetric bilinear forms on
, and let the action of
on
be defined by
![]() |
Then an orbit of on
is the set of forms which have a fixed rank and signature.
Let be a real Lie group acting smoothly on a differentiable manifold
(see Lie transformation group). For any point
, the orbit
is an immersed submanifold in
, diffeomorphic to
(the diffeomorphism is induced by the mapping
,
). This submanifold is not necessarily closed in
(i.e., not necessarily imbedded). A classical example is the "winding of a toruswinding of a torus" , i.e. an orbit of the action of the additive group
on the torus
![]() |
defined by the formula
![]() |
where is an irrational real number; the closure of its orbit coincides with
. If
is compact, then all orbits are imbedded submanifolds.
If is an algebraic group and
is an algebraic variety over an algebraically closed field
, with regular action (see Algebraic group of transformations), then any orbit
is a smooth algebraic variety, open in its closure
(in the Zariski topology), while
always contains a closed orbit of the group
(see [5]). In this case the morphism
,
, induces an isomorphism of the algebraic varieties
and
if and only if it is separable (this condition is always fulfilled if
is a field of characteristic zero, cf. Separable mapping). The orbits of maximal dimension form an open set in
.
The description of the structure of an orbit for a given action usually reduces to giving in each orbit a unique representative , the description of the stabilizer
and the description of a suitable class of functions which are constant on the orbit (invariants) and which separate various orbits; these functions enable one to describe the location of the orbits in
(orbits are intersections of their level sets). This program is usually called the problem of orbit decomposition. Many classification problems can be reduced to this problem. Thus, Example 2) is a classification problem of bilinear symmetric forms up to equivalence; the invariants in this case — the rank and signature — are "discrete" , while the stabilizer
, where
is non-degenerate, is the corresponding orthogonal group. The classical theory of the Jordan form of matrices (as well as the theory of other normal forms of matrices, cf. Normal form) can also be incorporated in this scheme: The Jordan form is a canonical representing element (defined, admittedly, up to the order of Jordan blocks) in the orbit of the general linear group
on the space of all complex
-matrices, for the conjugation action
; the coefficients of the characteristic polynomial of a matrix
are important invariants (which, however, do not separate any two orbits). The idea of considering equivalent objects as orbits of a group is actively used in various classification problems, for example, in algebraic moduli theory (see [10]).
If and
are finite, then
![]() |
where is the number of elements of the set
, and
![]() |
If is a compact Lie group acting smoothly on a connected smooth manifold
, then the orbit structure of
is locally finite, i.e. for any point
there is a neighbourhood
such that the number of conjugacy classes of different stabilizers
,
, is finite. In particular, if
is compact, then the number of different conjugacy classes of stabilizers
,
, is finite. For any subgroup
in
, each of the sets
![]() |
is the intersection of an open and a closed -invariant subset in
. Investigation of
in this case leads to the classification of actions (see [1]).
Analogues of these results have been obtained in the geometric theory of invariants (cf. Invariants, theory of) (see [3]). Let be a reductive algebraic group acting regularly on an affine algebraic variety
(the base field
is algebraically closed and has characteristic zero). The closure of any orbit contains a unique closed orbit. There exists a partition of
into a finite union of locally closed invariant non-intersecting subsets,
, such that: a) if
and
is closed, then the stabilizer
is conjugate in
to a subgroup in
, while if
is also closed, then
is conjugate to
; b) if
,
,
, and
and
are closed, then
and
are not conjugate in
. If
is a smooth algebraic variety (for example, in the important case of a rational linear representation of
in a vector space
), then there is a non-empty open subset
in
such that
and
are conjugate in
for any
. The latter result is an assertion about a property of points in general position in
, i.e. points of a non-empty open subset; there are also a number of other assertions of this type. For example, for a rational linear representation of a semi-simple group
in a vector space
, the orbits of the points in general position are closed if and only if their stabilizers are reductive (see [7]); when
is irreducible, an explicit expression of the stabilizers of the points in general position has been found (see [8], [9]). The study of orbit closures is important in this context. So, the set of
the closure of whose orbits contains the element
of
coincides with the variety of the zeros of all non-constant invariant polynomials on
; in many cases, and especially in the applications of the theory of invariants to the theory of moduli, this variety plays a vital part (see [10]). Any two different closed orbits can be separated by invariant polynomials. The orbit
is closed if and only if the orbit of the point
relative to the normalizer of
in
is closed (see [4]). The presence of non-closed orbits is connected with properties of
; if
is unipotent (and
is affine), then any orbit is closed (see [6]). One aspect of the theory of invariants concerns the study of orbit decompositions of different concrete actions (especially linear representations). One of these — the adjoint representation of a reductive group
— has been studied in detail (see, for example, [11]). This study is connected with the theory of representations of the group
; see Orbit method.
References
[1] | R. Palais, "The classification of ![]() |
[2] | F. Harary, "Graph theory" , Addison-Wesley (1969) pp. Chapt. 9 |
[3] | D. Luna, "Slices étales" Bull. Soc. Math. France. , 33 (1973) pp. 81–105 |
[4] | D. Luna, "Adhérence d'orbite et invariants" Invent. Math. , 29 : 3 (1975) pp. 231–238 |
[5] | A. Borel, "Linear algebraic groups" , Benjamin (1969) |
[6] | R. Steinberg, "Conjugacy classes in algebraic groups" , Lect. notes in math. , 366 , Springer (1974) |
[7] | V.L. Popov, "Stability criteria for the action of a semisimple group on a factorial manifold" Math. USSR Izv. , 4 (1970) pp. 527–535 Izv. Akad. Nauk. SSSR Ser. Mat. , 34 (1970) pp. 523–531 |
[8] | A.M. Popov, "Irreducible semisimple linear Lie groups with finite stationary subgroups of general position" Funct. Anal. Appl. , 12 : 2 (1978) pp. 154–155 Funkts. Anal. i Prilozhen. , 12 : 2 (1978) pp. 91–92 |
[9] | A.G. Elashvili, "Stationary subalgebras of points of the common state for irreducible Lie groups" Funct. Anal. Appl. , 6 : 2 (1972) pp. 139–148 Funkts. Anal. i Prilozhen. , 6 : 2 (1972) pp. 65–78 |
[10] | D. Mumford, J. Fogarty, "Geometric invariant theory" , Springer (1982) |
[11] | B. Kostant, "Lie group representations on polynomial rings" Amer. J. Math. , 85 : 3 (1963) pp. 327–404 |
[12] | J.E. Humphreys, "Linear algebraic groups" , Springer (1975) |
Comments
References
[a1] | V.L. Popov, "Modern developments in invariant theory" , Proc. Internat. Congress Mathematicians (Berkeley, 1986) , Amer. Math. Soc. (1987) pp. 394–406 |
[a2] | H. Kraft, "Geometrische Methoden in der Invariantentheorie" , Vieweg (1984) |
[a3] | H. Kraft (ed.) P. Slodowy (ed.) T.A. Springer (ed.) , Algebraische Transformationsgruppen und Invariantentheorie , DMV Sem. , 13 , Birkhäuser (1989) |
Orbit. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Orbit&oldid=13971