Affine Groups

AUTHORS:

• Volker Braun: initial version

class sage.groups.affine_gps.affine_group.AffineGroup(degree, ring)

Bases: sage.structure.unique_representation.UniqueRepresentation, sage.groups.group.Group

An affine group.

The affine group \(\mathrm{Aff}(A)\) (or general affine group) of an affine space \(A\) is the group of all invertible affine transformations from the space into itself.

If we let \(A_V\) be the affine space of a vector space \(V\) (essentially, forgetting what is the origin) then the affine group \(\mathrm{Aff}(A_V)\) is the group generated by the general linear group \(GL(V)\) together with the translations. Recall that the group of translations acting on \(A_V\) is just \(V\) itself. The general linear and translation subgroups do not quite commute, and in fact generate the semidirect product

\[\mathrm{Aff}(A_V) = GL(V) \ltimes V.\]

As such, the group elements can be represented by pairs \((A, b)\) of a matrix and a vector. This pair then represents the transformation

\[x \mapsto A x + b.\]

We can also represent affine transformations as linear transformations by considering \(\dim(V) + 1\) dimensonal space. We take the affine transformation \((A, b)\) to

\[\begin{split}\begin{pmatrix} A & b \\ 0 & 1 \end{pmatrix}\end{split}\]

and lifting \(x = (x_1, \ldots, x_n)\) to \((x_1, \ldots, x_n, 1)\). Here the \((n + 1)\)-th component is always 1, so the linear representations acts on the affine hyperplane \(x_{n+1} = 1\) as affine transformations which can be seen directly from the matrix multiplication.

INPUT:

Something that defines an affine space. For example

• An affine space itself:
  • A – affine space
• A vector space:
  • V – a vector space
• Degree and base ring:
  • degree – An integer. The degree of the affine group, that is, the dimension of the affine space the group is acting on.
  • ring – A ring or an integer. The base ring of the affine space. If an integer is given, it must be a prime power and the corresponding finite field is constructed.
  • var – (Defalut: 'a') Keyword argument to specify the finite field generator name in the case where ring is a prime power.

EXAMPLES:

sage: F = AffineGroup(3, QQ); F
Affine Group of degree 3 over Rational Field
sage: F(matrix(QQ,[[1,2,3],[4,5,6],[7,8,0]]), vector(QQ,[10,11,12]))
      [1 2 3]     [10]
x |-> [4 5 6] x + [11]
      [7 8 0]     [12]
sage: F([[1,2,3],[4,5,6],[7,8,0]], [10,11,12])
      [1 2 3]     [10]
x |-> [4 5 6] x + [11]
      [7 8 0]     [12]
sage: F([1,2,3,4,5,6,7,8,0], [10,11,12])
      [1 2 3]     [10]
x |-> [4 5 6] x + [11]
      [7 8 0]     [12]

Instead of specifying the complete matrix/vector information, you can also create special group elements:

sage: F.linear([1,2,3,4,5,6,7,8,0])
      [1 2 3]     [0]
x |-> [4 5 6] x + [0]
      [7 8 0]     [0]
sage: F.translation([1,2,3])
      [1 0 0]     [1]
x |-> [0 1 0] x + [2]
      [0 0 1]     [3]

Some additional ways to create affine groups:

sage: A = AffineSpace(2, GF(4,'a'));  A
Affine Space of dimension 2 over Finite Field in a of size 2^2
sage: G = AffineGroup(A); G
Affine Group of degree 2 over Finite Field in a of size 2^2
sage: G is AffineGroup(2,4) # shorthand
True

sage: V = ZZ^3;  V
Ambient free module of rank 3 over the principal ideal domain Integer Ring
sage: AffineGroup(V)
Affine Group of degree 3 over Integer Ring

REFERENCES:

• Wikipedia article Affine_group

Element

alias of AffineGroupElement

degree()

Return the dimension of the affine space.

OUTPUT:

An integer.

EXAMPLES:

sage: G = AffineGroup(6, GF(5))
sage: g = G.an_element()
sage: G.degree()
6
sage: G.degree() == g.A().nrows() == g.A().ncols() == g.b().degree()
True

linear(A)

Construct the general linear transformation by A.

INPUT:

• A – anything that determines a matrix

OUTPUT:

The affine group element \(x \mapsto A x\).

EXAMPLES:

sage: G = AffineGroup(3, GF(5))
sage: G.linear([1,2,3,4,5,6,7,8,0])
      [1 2 3]     [0]
x |-> [4 0 1] x + [0]
      [2 3 0]     [0]

linear_space()

Return the space of the affine transformations represented as linear transformations.

We can represent affine transformations \(Ax + b\) as linear transformations by

\[\begin{split}\begin{pmatrix} A & b \\ 0 & 1 \end{pmatrix}\end{split}\]

and lifting \(x = (x_1, \ldots, x_n)\) to \((x_1, \ldots, x_n, 1)\).

See also

• sage.groups.affine_gps.group_element.AffineGroupElement.matrix()

EXAMPLES:

sage: G = AffineGroup(3, GF(5))
sage: G.linear_space()
Full MatrixSpace of 4 by 4 dense matrices over Finite Field of size 5

matrix_space()

Return the space of matrices representing the general linear transformations.

OUTPUT:

The parent of the matrices \(A\) defining the affine group element \(Ax+b\).

EXAMPLES:

sage: G = AffineGroup(3, GF(5))
sage: G.matrix_space()
Full MatrixSpace of 3 by 3 dense matrices over Finite Field of size 5

random_element()

Return a random element of this group.

EXAMPLES:

sage: G = AffineGroup(4, GF(3))
sage: G.random_element()  # random
      [2 0 1 2]     [1]
      [2 1 1 2]     [2]
x |-> [1 0 2 2] x + [2]
      [1 1 1 1]     [2]
sage: G.random_element() in G
True

reflection(v)

Construct the Householder reflection.

A Householder reflection (transformation) is the affine transformation corresponding to an elementary reflection at the hyperplane perpendicular to \(v\).

INPUT:

• v – a vector, or something that determines a vector.

OUTPUT:

The affine group element that is just the Householder transformation (a.k.a. Householder reflection, elementary reflection) at the hyperplane perpendicular to \(v\).

EXAMPLES:

sage: G = AffineGroup(3, QQ)
sage: G.reflection([1,0,0])
      [-1  0  0]     [0]
x |-> [ 0  1  0] x + [0]
      [ 0  0  1]     [0]
sage: G.reflection([3,4,-5])
      [ 16/25 -12/25    3/5]     [0]
x |-> [-12/25   9/25    4/5] x + [0]
      [   3/5    4/5      0]     [0]

translation(b)

Construct the translation by b.

INPUT:

• b – anything that determines a vector

OUTPUT:

The affine group element \(x \mapsto x + b\).

EXAMPLES:

sage: G = AffineGroup(3, GF(5))
sage: G.translation([1,4,8])
      [1 0 0]     [1]
x |-> [0 1 0] x + [4]
      [0 0 1]     [3]

vector_space()

Return the vector space of the underlying affine space.

EXAMPLES:

sage: G = AffineGroup(3, GF(5))
sage: G.vector_space()
Vector space of dimension 3 over Finite Field of size 5