Projective linear group: Information from Answers.com

Classical groups
General linear group
Special linear group
Orthogonal group
Special orthogonal group
Unitary group
Special unitary group
Symplectic group
edit

Relation between the projective special linear group PSL and the projective general linear group PGL.

In mathematics, especially in the group theoretic area of algebra, the projective linear group (also known as the projective general linear group or PGL) is the induced action of the general linear group of a vector space V on the associated projective space P(V). Explicitly, the projective linear group is the quotient group

PGL(V) = GL(V)/Z(V)

where GL(V) is the general linear group of V and Z(V) is the subgroup of all nonzero scalar transformations of V; these are quotiented out because they act trivially on the projective space and are exactly the kernel, and the notation "Z" is because the scalar transformations are the center of the general linear group.

The projective special linear group, PSL, is defined analogously, as the induced action of the special linear group on the associated projective space. Explicitly:

PSL(V) = SL(V)/SZ(V)

where SL(V) is the special linear group over V and SZ(V) is the subgroup of scalar transformations with unit determinant. Here SZ is the center of SL, and is naturally identified with the group of nth roots of unity in K (where n is the dimension and K is the base field).

PGL and PSL are some of the fundamental groups of study, part of the so-called classical groups, and an element of PGL is called a projective linear transformation. If V is the n-dimensional vector space over a field F, namely $V = F n,$ the alternate notations PGL(n, F) and PSL(n, F) are also used.

Note that PGL(n, F) and PSL(n, F) are equal if and only if every element of F contains a nth root in F. As an example, note that PGL(2,C)=PSL(2,C), but PGL(2,R)>PSL(2,R);^[1] this corresponds to the real projective line being orientable, and the projective special linear group only being the orientation-preserving transformations.

PGL and PSL can also be defined over a ring, with the most important example being the modular group, PSL(2, Z).

Name

The name comes from projective geometry, where the projective group acting on homogeneous coordinates (x₀:x₁: … :x_n) is the underlying group of the geometry.^{[note 1]} Stated differently, the natural action of GL(V) on V descends to an action of PGL(V) on the projective space P(V).

The projective linear groups therefore generalise the case PGL(2,C) of Möbius transformations (sometimes called the Möbius group), which acts on the projective line.

Note that unlike the general linear group, which is generally defined axiomatically as "invertible functions preserving the linear (vector space) structure", the projective linear group is defined constructively, as a quotient of the general linear group of the associated vector space, rather than axiomatically as "invertible functions preserving the projective linear structure". This is reflected in the notation: PGL(n,F) is the group associated to GL(n,F), and is the projective linear group of $(n - 1)$ -dimensional projective space, not n-dimensional projective space.

Collineations

Main article: Collineation

A related group is the collineation group, which is defined axiomatically. A collineation is an invertible (or more generally one-to-one) map which sends collinear points to collinear points. One can define a projective space axiomatically in terms of an incidence structure (a set of points P, lines L, and an incidence relation I specifying which points lie on which lines) satisfying certain axioms – an automorphism of a projective space thus defined then being an automorphism f of the set of points and an automorphism g of the set of lines, preserving the incidence relation,^{[note 2]} which is exactly a collineation of a space to itself. Projective linear transforms are collineations (planes in a vector space correspond to lines in the associated projective space, and linear transforms map planes to planes, so projective linear transforms map lines to lines), but in general not all collineations are projective linear transforms – PGL is in general a proper subgroup of the collineation group.

Specifically, for $n = 2$ (a projective line), all points are collinear, so the collineation group is exactly the symmetric group of the points of the projective line, and except for $\mathbf{F}_2$ and $\mathbf{F}_3$ (where PGL is the full symmetric group), PGL is a proper subgroup of the full symmetric group on these points.

For $n\geq 3,$ the collineation group is the projective semilinear group, $P Γ L$ – this is PGL, twisted by field automorphisms; formally, $P\Gamma L \cong PGL \rtimes \operatorname{Gal}(K/k),$ where k is the prime field for K; this is the fundamental theorem of projective geometry. Thus for K a prime field ( $\mathbf{F}_p$ or $\mathbf{Q}$ ), we have $P G L = P Γ L,$ but for K a field with non-trivial Galois automorphisms (such as $\mathbf{F}_{p^n}$ for $n\geq 2$ or $\mathbf{C}$ ), the projective linear group is a proper subgroup of the collineation group, which can be thought of as "transforms preserving a projective semi-linear structure". Correspondingly, the quotient group $P\Gamma L/PGL = \operatorname{Gal}(K/k)$ corresponds to "choices of linear structure", with the identity (base point) being the existing linear structure.

One may also define collineation groups for axiomatically defined projective spaces, where there is no natural notion of a projective linear transform. However, with the exception of the non-Desarguesian planes, all projective spaces are the projectivization of a linear space over a division ring though, as noted above, there are multiple choices of linear structure, namely a torsor over $\operatorname{Gal}(K/k)$ (for $n\geq 3$ ).

Properties

PGL sends collinear points to collinear points (it preserves projective lines), but it is not the full collineation group, which is instead either PΓL (for $n > 2$ ) or the full symmetric group for $n = 2$ (the projective line).
Every algebraic automorphism of a projective space is projective linear.
PGL acts faithfully on projective space: non-identity elements act non-trivially.

Concretely, the kernel of the action of GL on projective space is exactly the scalar maps, which are quotiented out in PGL.
PGL acts 2-transitively on projective space.

This is because 2 distinct points in projective space correspond to 2 vectors that do not lie on a single linear space, and hence are linearly independent, and GL acts transitively on k-element sets of linearly independent vectors.
PGL(2, K) acts 3-transitively on the projective line.

3 arbitrary points are conventionally mapped to $[0,1],[1,1],[1,0];$ in alternative notation, $0, 1, \infty.$ In fractional linear transformation notation, the function $\frac{x-a}{x-c}\cdot \frac{b-c}{b-a}$ maps $a \mapsto 0, b \mapsto 1, c \mapsto \infty.$ This is the cross-ratio $(x, b; a, c)$ – see cross-ratio: transformational approach for details.
For $n \geq 3,$ PGL(n, K) does not act 3-transitively, because it must send 3 collinear points to 3 other collinear points, not an arbitrary set. For $n = 2$ the space is the projective line, so all points are collinear and this is no restriction.
PGL(2, K) does not act 4-transitively on the projective line (except for $\operatorname{PGL}(2,\mathbf{F}_3),$ as $\mathbf{P}^1_3$ has 3+1=4 points, so 3-transitive implies 4-transitive); the invariant that is preserved is the cross ratio, and this determines where every other point is sent: specifying where 3 points are mapped determines the map. Thus in particular it is not the full collineation group of the projective line (except for $\mathbf{F}_2$ and $\mathbf{F}_3$ ).
PSL(n, K) and PGL(n, K) are algebraic groups of dimension $n 2 - 1$ – they are both open subgroups of the projective space $\mathbf{P}^{n^2-1}.$

For PGL, the $n 2$ is the dimension of GL(n, K), and the $- 1$ is from projectivization.

For PSL, $n 2 - 1$ is the dimension of SL, which is a covering space of PSL, so they have the same dimension. More casually, PSL differs from SL and from PGL by a finite group in each case, so the dimensions agree.

This is also reflected in the order of the groups over finite fields, as the degree of the order as a polynomial in q: the order of PGL(n, q) is $q^{n^2-1}$ plus lower order terms.

Fractional linear transformations

For more details on this topic, see Möbius transformation#Projective matrix representations.

As for Möbius transformations, the group PGL(2, K) can be interpreted as fractional linear transformations with coefficients in K, a matrix $\left(\begin{smallmatrix}a & b\\c & d\end{smallmatrix}\right)$ corresponding to the rational function

$f(x) = \frac{a x + b}{c x + d}$

where multiplication of matrices agrees with composition of functions, and quotienting out by scalar matrices corresponding to multiplying the top and bottom of the fraction by a common factor. As with Möbius transformations, these functions can be interpreted as automorphisms of the projective line over K.

Finite fields

The projective special linear groups PSL(n,F_q) for a finite field F_q are often written as PSL(n,q) or L_n(q). They are finite simple groups whenever n is at least 2, with two exceptions: L₂(2), which is isomorphic to S₃, the symmetric group on 3 letters, and is solvable; and L₂(3), which is isomorphic to A₄, the alternating group on 4 letters, and is also solvable.

The special linear groups SL(n,q) are thus quasisimple: perfect central extensions of a simple group (unless $n = 2$ and $q = 2$ or 3).

History

The groups PSL(2,p) were constructed by Évariste Galois in the 1830s, and were the second family of finite simple groups, after the alternating groups.^[2]

The groups PSL(n,q) were then constructed in the classic 1870 text by Camille Jordan, Traité des substitutions et des équations algébriques.

Order

The order of PGL(n,q) is

(qⁿ − 1)(qⁿ − q)(qⁿ − q²) … (qⁿ − qⁿ⁻¹)/(q − 1) = q^{n² – 1} – O(q^{n² – 3})

which corresponds to the order of GL(n,q), divided by $(q - 1)$ for projectivization; see q-analog for discussion of such formulas. Note that the degree is $n 2 - 1,$ which agrees with the dimension as an algebraic group. The "O" is for big O notation, meaning "terms involving lower order". This also equals the order of SL(n,q); there dividing by $(q - 1)$ is due to the determinant.

The order of PSL(n,q) is the above, divided by $|\operatorname{SZ}(n,q)|$ , the number of scalar matrices with determinant 1 – or equivalently dividing by $| F * / (F *) n |$ , the number of classes of element that have no nth root, or equivalently, dividing by the number of nth roots of unity in $\mathbf{F}_q$ .^{[note 3]}

Exceptional isomorphisms

In addition to the isomorphisms

$L_2(2) \cong S_3, L_2(3) \cong A_4,$ , and $\operatorname{PGL}(2,3) \cong S_4,$

there are other exceptional isomorphisms between projective special linear groups and alternating groups (these groups are all simple, as the alternating group over 5 or more letters is simple):

$L_2(4) \cong L_2(5) \cong A_5$

$L_2(9) \cong A_6$

$L_4(2) \cong A_8.$ ^[3]

The isomorphism $L_2(9) \cong A_6$ allows one to see the exotic outer automorphism of $A 6$ in terms of field automorphism and matrix operations. The isomorphism $L_4(2) \cong A_8$ is of interest in the structure of the Mathieu group M₂₄.

The associated extensions $\operatorname{SL}(n,q) \to \operatorname{PSL}(n,q)$ are covering groups of the alternating groups (universal perfect central extensions) for $A 4, A 5$ , by uniqueness of the universal perfect central extension; for $L_2(9) \cong A_6$ , the associated extension is a perfect central extension, but not universal: there is a 3-fold covering group.

The groups over $\mathbf{F}_5$ have a number of exceptional isomorphisms:

$\operatorname{PSL}(2,5) \cong A_5 \cong I,$ the alternating group on five elements, or equivalently the icosahedral group;

$\operatorname{PGL}(2,5) \cong S_5,$ the symmetric group on five elements;

$\operatorname{SL}(2,5) \cong 2\cdot A_5 \cong 2I,$ the double cover of the alternating group A₅, or equivalently the binary icosahedral group.

They can also be used to give a construction of an exotic map S₅ → S₆, as described below. Note however that GL(2,5) is not a double cover of $S 5,$ but is rather a 4-fold cover.

A further isomorphism is:

$L_2(7) \cong L_3(2)$ is the simple group of order 168, the second smallest non-abelian simple group, and is not an alternating group; see PSL(2,7).

The above exceptional isomorphisms involving the projective special linear groups are almost all of the exceptional isomorphisms between families of finite simple groups; the only other exceptional isomorphism is $\operatorname{PSU}_4(2) \cong \operatorname{PSp}_4(3),$ between a projective special orthogonal group and a projective symplectic group.^[2]

Action on projective line

Some of the above maps can be seen directly in terms of the action of PSL and PGL on the associated projective line: $\operatorname{PGL}(n,q)$ acts on the projective space $\mathbf{P}_q^{n-1},$ which has $(q n - 1) / (q - 1)$ points, and this yields a map from the projective linear group to the symmetric group on $(q n - 1) / (q - 1)$ points. For $n = 2$ , this is the projective line $\mathbf{P}_q^1,$ which has $(q 2 - 1) / (q - 1) = q + 1$ points, so there is a map $\operatorname{PGL}(2,q) \to S_{q+1}$ .

To understand these maps, it is useful to recall these facts:

The order of $\operatorname{PGL}(2,q)$ is

(q 2 - 1)(q 2 - q) / (q - 1) = q 3 - q = (q - 1) q (q + 1);

the order of $\operatorname{PSL}(2,q)$ either equals this (if the characteristic is 2), or is half this (if the characteristic is not 2).

The action of the projective linear group on the projective line is faithful and 3-transitive, so the map is one-to-one and has image a 3-transitive subgroup.

Thus the image is a 3-transitive subgroup of known order, which allows it to be identified. This yields the following maps:

$\operatorname{PSL}(2,2) = \operatorname{PGL}(2,2) \to S_3,$ of order 6, which is an isomorphism.
$\operatorname{PSL}(2,3) < \operatorname{PGL}(2,3) \to S_4,$ of orders 12 and 24, the latter of which is an isomorphism, with PSL(2,3) being the alternating group.
$\operatorname{PSL}(2,4) = \operatorname{PGL}(2,4) \to S_5,$ of order 60, yielding the alternating group $A 5 .$
$\operatorname{PSL}(2,5) < \operatorname{PGL}(2,5) \to S_6,$ of orders 60 and 120, which yields an embedding of $S 5$ (respectively, $A 5$ ) as a transitive subgroup of $S 6$ (respectively, $A 6$ ). This is an example of an exotic map S₅ → S₆, and can be used to construct the exceptional outer automorphism of S₆.^[4] Note that the isomorphism $\operatorname{PGL}(2,5) \cong S_5$ is not transparent from this presentation: there is no particularly natural set of 5 elements on which PGL(2,5) acts.

Mathieu groups

For more details on this topic, see Mathieu group.

The group PSL(3,4) can be used to construct the Mathieu group M₂₄, one of the sporadic simple groups; in this context, one refers to PSL(3,4) as M₂₁, though it is not properly a Mathieu group itself. One begins with the projective plane over the field with four elements, which is a Steiner system of type S(2,5,21) – meaning that it has 21 points, each line ("block", in Steiner terminology) has 5 points, and any 2 points determine a line – and on which PSL(3,4) acts. One calls this Steiner system W₂₁ ("W" for Witt), and then expands it to a larger Steiner system W₂₄, expanding the symmetry group along the way: to the projective general linear group PGL(3,4), then to the projective semilinear group PΓL(3,4), and finally to the Mathieu group M₂₄.

Modular group

Main article: Modular group

The groups $\operatorname{PSL}(2,\mathbf{Z}/n)$ arise in studying the modular group, $\operatorname{PSL}(2,\mathbf{Z})$ , as quotients by reducing all elements mod n; the kernels are called the principal congruence subgroups. A noteworthy subgroup of $\operatorname{PSL}(2,\mathbf{Z})$ is the symmetries of the set $\{0, 1, \infty\} \subset \mathbf{P}^1_{\mathbf C},$ which can be expressed as matrices or as fractional linear transformations as:

$x$	$1 / (1 - x)$	$(x - 1) / x$
$\begin{pmatrix} 1 & 0\\ 0 & 1 \end{pmatrix}$	$\begin{pmatrix} 0 & 1\\ -1 & 1 \end{pmatrix}$	$\begin{pmatrix} 1 & -1\\ 1 & 0 \end{pmatrix}$
$1 / x$	$1 - x$	$x / (x - 1)$
$\begin{pmatrix} 0 & 1\\ 1 & 0 \end{pmatrix}$	$\begin{pmatrix} -1 & 1\\ 0 & 1 \end{pmatrix}$	$\begin{pmatrix} 1 & 0\\ 1 & -1 \end{pmatrix}$

This maps to the symmetries of $\{0, 1, \infty\} \subset \mathbf{P}^1_n$ under reduction mod n. Notably, for $n = 2,$ this subgroup maps isomorphically to $\operatorname{PSL}(2,\mathbf{Z}/2) \cong S_3,$ ^{[note 4]} and thus provides a splitting $\operatorname{PSL}(2,\mathbf{Z}/2) \hookrightarrow \operatorname{PSL}(2,\mathbf{Z})$ for the quotient map $\operatorname{PSL}(2,\mathbf{Z}) \twoheadrightarrow \operatorname{PSL}(2,\mathbf{Z}/2).$

Topology

Over the real and complex numbers, the topology of PGL and PSL can be determined from the fiber bundles that define them:

$\operatorname{Z} \cong K^* \to \operatorname{GL} \to \operatorname{PGL}$

$\operatorname{SZ} \cong \mu_n \to \operatorname{SL} \to \operatorname{PSL}$

via the long exact sequence of a fibration.

For both the reals and complexes, SL is a covering space of PSL, with number of sheets equal to the number of nth roots in K; thus in particular all their higher homotopy groups agree. For the reals, SL is a 2-fold cover of PSL for n even, and is a 1-fold cover for n odd, i.e., an isomorphism:

$\{\pm 1\} \to \operatorname{SL}(2n,\mathbf{R}) \to \operatorname{PSL}(2n,\mathbf{R})$

$\operatorname{SL}(2n+1,\mathbf{R}) \overset{\sim}{\to} \operatorname{PSL}(2n+1,\mathbf{R})$

For the complexes, SL is an n-fold cover of PSL.

For PGL, for the reals, the fiber is $\mathbf{R}^* \simeq \{ \pm 1 \},$ so up to homotopy, $\operatorname{GL} \to \operatorname{PGL}$ is a 2-fold covering space, and all higher homotopy groups agree.

For PGL over the complexes, the fiber is $\mathbf{C}^* \simeq S^1,$ so up to homotopy, $\operatorname{GL} \to \operatorname{PGL}$ is a circle bundle. The higher homotopy groups of the circle vanish, so the homotopy groups of $\operatorname{GL}(n,\mathbf{C})$ and $\operatorname{PGL}(n,\mathbf{C})$ agree for $n \geq 3.$ In fact, $π 2$ always vanishes for Lie groups, so the homotopy groups agree for $n \geq 2.$

Covering groups

Over the real and complex numbers, the projective special linear groups are the minimal Lie group realizations for the special linear Lie algebra $\mathfrak{sl}_n\colon$ every connected Lie group whose Lie algebra is $\mathfrak{sl}_n$ is a cover of PSL(n,F). Conversely, its universal covering group is the maximal element, and the intermediary realizations form a lattice of covering groups.

For example SL₂(R) has center {±1} and fundamental group Z, and thus has universal cover $\overline{\mathrm{SL}_2(\mathbf{R})}$ and covers the centerless PSL₂(R).

Representation theory

Main article: Projective representation

A projective representation of G can be pulled back to a linear representation of a central extension C of G.

A group homomorphism $G \to \operatorname{PGL}(V)$ from a group G to a projective linear group is called a projective representation of the group G, by analogy with a linear representation (a homomorphism $G \to \operatorname{GL}(V)$ ). These were studied by Issai Schur, who showed that projective representations of G can be classified in terms of linear representations of central extensions of G. This lead to the Schur multiplier, which is used to address this question.

Low dimensions

The projective linear group is mostly studied for $n \geq 2,$ though it can be defined for low dimensions.

For $n = 0$ (or in fact $n < 0$ ) the projective space of $K 0$ is empty, as there are no 1-dimensional subspaces of a 0-dimensional space. Thus, PGL(0,K) is the trivial group, consisting of the unique empty map from the empty set to itself. Further, the action of scalars on a 0-dimensional space is trivial, so the map $K^* \to \operatorname{GL}(0,K)$ is trivial, rather than an inclusion as it is in higher dimensions.

For $n = 1,$ the projective space of $K 1$ is a single point, as there is a single 1-dimensional subspace. Thus, PGL(1,K) is the trivial group, consisting of the unique map from a singleton set to itself. Further, the general linear group of a 1-dimensional space is exactly the scalars, so the map $K^* \overset{\sim}{\to} \operatorname{GL}(1,K)$ is an isomorphism, corresponding to $\operatorname{PGL}(1,K) := \operatorname{GL}(1,K)/(K^*) \cong 1$ being trivial.

For $n = 2,$ PGL(2,K) is non-trivial, but is unusual in that it is 3-transitive, unlike higher dimensions when it is only 2-transitive.

Examples

PSL(2,7)
Modular group, PSL(2,Z)
PSL(2,R)
Möbius group, PGL(2,C) = PSL(2,C)

Subgroups

Notes

^ This is therefore PGL(n + 1, F) for projective space of dimension n
^ "Preserving the incidence relation" means that if point p is on line l then $f (p)$ is in $g (l)$ ; formally, if $(p,l) \in I$ then $\big(f(p),g(l)\big) \in I$ .
^ These are equal because they are the kernel and cokernel of the endomorphism $F^* \overset{x^n}{\to} F^*;$ formally, $|\mu_n|\cdot |(F^*)^n|=|F^*|.$ More abstractly, the first realizes PSL as SL/SZ, while the second realizes PSL as the kernel of $\operatorname{PGL} \to F^*/(F^*)^n.$
^ This isomorphism can be seen by removing the minus signs in matrices, which yields the matrices for $\operatorname{PSL}(2,2)$

References

^ Gareth A. Jones and David Silverman. (1987) Complex functions: an algebraic and geometric viewpoint. Cambridge UP. Discussion of PSL and PGL on page 20 in google books
^ ^a ^b Wilson, Robert (October 31, 2006), "Chapter 1: Introduction", http://www.maths.qmul.ac.uk/~raw/fsgs_files/intro.ps
^ Murray, John (December 1999), "The Alternating Group $A 8$ and the General linear Group $\operatorname{GL}_4(2)$ ", Mathematical Proceedings of the Royal Irish Academy 99A (2): 123–132, http://www.jstor.org/stable/20459753
^ Carnahan, Scott (2007-10-27), "Small finite sets", Secret Blogging Seminar, http://sbseminar.wordpress.com/2007/10/27/small-finite-sets/, notes on a talk by Jean-Pierre Serre.

This article needs additional citations for verification.
Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (February 2008)

Grove, Larry C. (2002), Classical groups and geometric algebra, Graduate Studies in Mathematics, 39, Providence, R.I.: American Mathematical Society, MR 1859189, ISBN 978-0-8218-2019-3

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

Donate to Wikimedia

Projective linear group

Contents