Affine group

In mathematics, the affine group or general affine group of any affine space over a field is the group of all invertible affine transformations from the space into itself.

It is a Lie group if is the real or complex field or quaternions.

Relation to general linear group

Construction from general linear group

Concretely, given a vector space , it has an underlying affine space obtained by "forgetting" the origin, with acting by translations, and the affine group of can be described concretely as the semidirect product of by , the general linear group of :
:$\operatorname(V) = V \rtimes \operatorname(V)$
The action of on is the natural one (linear transformations are automorphisms), so this defines a semidirect product.

In terms of matrices, one writes:
:$\operatorname(n,K) = K^n \rtimes \operatorname(n,K)$
where here the natural action of on is matrix multiplication of a vector.

Stabilizer of a point

Given the affine group of an affine space , the stabilizer of a point is isomorphic to the general linear group of the same dimension (so the stabilizer of a point in is isomorphic to ); formally, it is the general linear group of the vector space : recall that if one fixes a point, an affine space becomes a vector space.

All these subgroups are conjugate, where conjugation is given by translation from to (which is uniquely defined), however, no particular subgroup is a natural choice, since no point is special – this corresponds to the multiple choices of transverse subgroup, or splitting of the short exact sequence
:$1 \to V \to V \rtimes \operatorname(V) \to \operatorname(V) \to 1\,.$

In the case that the affine group was constructed by ''starting'' with a vector space, the subgroup that stabilizes the origin (of the vector space) is the original .

Matrix representation

Representing the affine group as a semidirect product of by , then by construction of the semidirect product, the elements are pairs , where is a vector in and is a linear transform in , and multiplication is given by
:$(v, M) \cdot (w, N) = (v+Mw, MN)\,.$

This can be represented as the block matrix
:$\left( \begin M & v\\ \hline 0 & 1 \end\right)$
where is an matrix over , an column vector, 0 is a row of zeros, and 1 is the identity block matrix.

Formally, is naturally isomorphic to a subgroup of , with embedded as the affine plane , namely the stabilizer of this affine plane; the above matrix formulation is the (transpose of) the realization of this, with the and ) blocks corresponding to the direct sum decomposition .

A similar representation is any matrix in which the entries in each column sum to 1. The similarity for passing from the above kind to this kind is the identity matrix with the bottom row replaced by a row of all ones.

Each of these two classes of matrices is closed under matrix multiplication.

The simplest paradigm may well be the case , that is, the upper triangular matrices representing the affine group in one dimension. It is a two-parameter non-Abelian Lie group, so with merely two generators (Lie algebra elements), and , such that , where
:$A= \left( \begin 1 & 0\\ 0 & 0 \end\right), \qquad B= \left( \begin 0 & 1\\ 0 & 0 \end\right)\,,$
so that
:$e^= \left( \begin e^a & \tfrac(e^a-1)\\ 0 & 1 \end\right)\,.$

Character table of

has order . Since

:$\begin c & d \\ 0 & 1 \end\begin a & b \\ 0 & 1 \end\begin c & d \\ 0 & 1 \end^=\begin a & (1-a)d+bc \\ 0 & 1 \end\,,$

we know has conjugacy classes, namely

:\begin
C_ &= \left\\,, \\ ptC_ &= \left\\,, \\ pt\Bigg\\,.
\end

Then we know that has irreducible representations. By above paragraph (), there exist one-dimensional representations, decided by the homomorphism

:$\rho_k:\operatorname(\mathbf_p)\to\Complex^*$

for , where

:$\rho_k\begin a & b \\ 0 & 1 \end=\exp\left(\frac\right)$

and , , is a generator of the group . Then compare with the order of , we have

:$p(p-1)=p-1+\chi_p^2\,,$

hence is the dimension of the last irreducible representation. Finally using the orthogonality of irreducible representations, we can complete the character table of :

:$\begin & & & & & & \\ \hline & & & & & & \\ & & & & & & \\ & & & & & & \\ & & & & & & \\ & & & & & & \\ & & & & & & \end$

Planar affine group over the reals

The elements of $\operatorname(2,\mathbb R)$ can take a simple form on a well-chosen affine coordinate system. More precisely, given an affine transformation of an affine plane over the reals, an affine coordinate system exists on which it has one of the following forms, where , , and are real numbers (the given conditions insure that transformations are invertible, but not for making the classes distinct; for example, the identity belongs to all the classes).
:\begin
\text&& (x, y) &\mapsto (x +a,y+b),\\ pt\text&& (x, y) &\mapsto (ax,by), &\qquad \text ab\ne 0,\\ pt\text&& (x, y) &\mapsto (ax,y+b), &\qquad \text a\ne 0,\\ pt\text&& (x, y) &\mapsto (ax+y,ay), &\qquad \text a\ne 0,\\ pt\text&& (x, y) &\mapsto (x+y,y+a)\\ pt\text&& (x, y) &\mapsto (a(x\cos t + y\sin t), a(-x\sin t+y\cos t)), &\qquad \text a\ne 0.
\end

Case 1 corresponds to translations.

Case 2 corresponds to scalings that may different in two different directions. When working with a Euclidean plane these directions need not to be perpendicular, since the coordinate axes need not to be perpendicular.

Case 3 corresponds to a scaling in one direction and a translation in another one.

Case 4 corresponds to a shear mapping combined with a dilation.

Case 5 corresponds to a shear mapping combined with a dilation.

Case 6 corresponds to similarities, when the coordinate axes are perpendicular.

The affine transformations without any fixed point belong to cases 1, 3, and 5. The transformations that do not preserve the orientation of the plane belong to cases 2 (with ) or 3 (with ).

The proof may be done by first remarking that if an affine transformation has no fixed point, then the matrix of the associated linear map has an eigenvalue equal to one, and then using the Jordan normal form theorem for real matrices.

Other affine groups

General case

Given any subgroup of the general linear group, one can produce an affine group, sometimes denoted analogously as .

More generally and abstractly, given any group and a representation of on a vector space ,
:$\rho : G \to \operatorname(V)$
one getsSince . Note that this containment is in general proper, since by "automorphisms" one means ''group'' automorphisms, i.e., they preserve the group structure on (the addition and origin), but not necessarily scalar multiplication, and these groups differ if working over . an associated affine group : one can say that the affine group obtained is "a group extension by a vector representation", and as above, one has the short exact sequence:
:$1 \to V \to V \rtimes_\rho G \to G \to 1\,.$

Special affine group

The subset of all invertible affine transformations that preserve a fixed volume form is called the ''special affine group''. This group is the affine analogue of the special linear group. In terms of the semi-direct product, the special affine group consists of all pairs with of determinant 1, that is, the affine transformations
$x \mapsto Mx + v$
where is a linear transformation of determinant 1 and is any fixed translation vector.

Projective subgroup

Presuming knowledge of projectivity and the projective group of projective geometry, the affine group can be easily specified. For example, Günter Ewald wrote:
:The set $\mathfrak$ of all projective collineations of is a group which we may call the projective group of . If we proceed from to the affine space by declaring a hyperplane to be a hyperplane at infinity, we obtain the affine group $\mathfrak$ of as the subgroup of $\mathfrak$ consisting of all elements of $\mathfrak$ that leave fixed.
::$\mathfrak \subset \mathfrak$

Poincaré group

The Poincaré group is the affine group of the Lorentz group :
:$\mathbf^\rtimes \operatorname(1,3)$

This example is very important in relativity.

See also

* Affine Coxeter group – certain discrete subgroups of the affine group on a Euclidean space that preserve a lattice
* Holomorph

Notes

References

*
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Affine geometry
Group theory
Lie groups