In mathematics, the general linear group of degree n is the set of n×n invertible matrices, together with the operation of ordinary matrix multiplication. This forms a group, because the product of two invertible matrices is again invertible, and the inverse of an invertible matrix is invertible. The name is because the columns of an invertible matrix are linearly independent, hence the vectors/points they define are in general linear position, and matrices in the general linear group take points in general linear position to points in general linear position.
To be more precise, it is necessary to specify what kind of objects may appear in the entries of the matrix. For example, the general linear group over R (the set of real numbers) is the group of n×n invertible matrices of real numbers, and is denoted by GLn(R) or GL(n, R).
More generally, the general linear group of degree n over any field F (such as the complex numbers), or a ring R (such as the ring of integers), is the set of n×n invertible matrices with entries from F (or R), again with matrix multiplication as the group operation. Typical notation is GLn(F) or GL(n, F), or simply GL(n) if the field is understood.
The group GL(n, F) and its subgroups are often called linear groups or matrix groups (the abstract group GL(V) is a linear group but not a matrix group). These groups are important in the theory of group representations, and also arise in the study of spatial symmetries and symmetries of vector spaces in general, as well as the study of polynomials. The modular group may be realised as a quotient of the special linear group SL(2, Z).
If n ≥ 2, then the group GL(n, F) is not abelian.
If V is a vector space over the field F, the general linear group of V, written GL(V) or Aut(V), is the group of all automorphisms of V, i.e. the set of all bijective linear transformations V → V, together with functional composition as group operation. If V has finite dimension n, then GL(V) and GL(n, F) are isomorphic. The isomorphism is not canonical; it depends on a choice of basis in V. Given a basis (e1, ..., en) of V and an automorphism T in GL(V), we have
In a similar way, for a commutative ring R the group GL(n, R) may be interpreted as the group of automorphisms of a free R-module M of rank n. One can also define GL(M) for any module, but in general this is not isomorphic to GL(n, R) (for any n).
Over a commutative ring R, one must be slightly more careful: a matrix over R is invertible if and only if its determinant is a unit in R, that is, if its determinant is invertible in R. Therefore GL(n, R) may be defined as the group of matrices whose determinants are units.
The general linear GL(n,R) over the field of real numbers is a real Lie group of dimension n2. To see this, note that the set of all n×n real matrices, Mn(R), forms a real vector space of dimension n2. The subset GL(n,R) consists of those matrices whose determinant is non-zero. The determinant is a polynomial map, and hence GL(n,R) is a open affine subvariety of Mn(R) (a non-empty open subset of Mn(R) in the Zariski topology), and therefore a smooth manifold of the same dimension.
As a manifold, GL(n,R) is not connected but rather has two connected components: the matrices with positive determinant and the ones with negative determinant. The identity component, denoted by GL+(n, R), consists of the real n×n matrices with positive determinant. This is also a Lie group of dimension n2; it has the same Lie algebra as GL(n,R).
The group GL(n,R) is also noncompact. "The maximal compact subgroup of GL(n, R) is the orthogonal group O(n), while "the" maximal compact subgroup of GL+(n, R) is the special orthogonal group SO(n). As for SO(n), the group GL+(n, R) is not simply connected (except when n=1), but rather has a fundamental group isomorphic to Z for n=2 or Z2 for n>2.
The general linear GL(n,C) over the field of complex numbers is a complex Lie group of complex dimension n2. As a real Lie group it has dimension 2n2. The set of all real matrices forms a real Lie subgroup.
Unlike the real case, GL(n,C) is connected. This follows, in part, since the multiplicative group of complex numbers C× is connected. The group manifold GL(n,C) is not compact; rather its maximal compact subgroup is the unitary group U(n). As for U(n), the group manifold GL(n,C) is not simply connected but has a fundamental group isomorphic to Z.
If F is a finite field with q elements, then we sometimes write GL(n, q) instead of GL(n, F). When p is prime, GL(n, p) is the outer automorphism group of the group Zpn, and also the automorphism group, because Zpn is Abelian, so the inner automorphism group is trivial.
The order of GL(n, q) is:
This can be shown by counting the possible columns of the matrix: the first column can be anything but the zero vector; the second column can be anything but the multiples of the first column; and in general, the kth column can be any vector not in the linear span of the first k-1 columns.
For example, GL(3,2) has order (8-1)(8-2)(8-4)=168. It is the automorphism group of the Fano plane and of the group Z23.
More generally, one can count points of Grassmannian over F: in other words the number of subspaces of a given dimension k. This requires only finding the order of the stabilizer subgroup of one such subspace (described on that page in block matrix form), and dividing into the formula just given, by the orbit-stabilizer theorem.
These formulas are connected to the Schubert decomposition of the Grassmannian, and are q-analogs of the Betti numbers of complex Grassmannians. This was one of the clues leading to the Weil conjectures.
The special linear group, SL(n, F), is the group of all matrices with determinant 1. They are special in that they lie on a subvariety – they satisfy a polynomial equation (as the determinant is a polynomial in the entries). Matrices of this type form a group as the determinant of the product of two matrices is the product of the determinants of each matrix. SL(n, F) is a normal subgroup of GL(n, F).
The group SL(n, C) is simply connected while SL(n, R) is not. SL(n, R) has the same fundamental group as GL+(n, R), that is, Z for n=2 and Z2 for n>2.
The set of all invertible diagonal matrices forms a subgroup of GL(n, F) isomorphic to (F×)n. In fields like R and C, these correspond to rescaling the space; the so called dilations and contractions.
A scalar matrix is a diagonal matrix which is a constant times the identity matrix. The set of all nonzero scalar matrices forms a subgroup of GL(n, F) isomorphic to F× . This group is the center of GL(n, F). In particular, it is a normal, abelian subgroup.
The center of SL(n, F) is simply the set of all scalar matrices with unit determinant, and is isomorphic to the group of nth roots of unity in the field F.
The so-called classical groups are subgroups of GL(V) which preserve some sort of bilinear form on a vector space V. These include the
These groups provide important examples of Lie groups.
The projective linear group PGL(n, F) and the projective special linear group PSL(n,F) are the quotients of GL(n,F) and SL(n,F) by their centers (which consists of some multiples of the identity matrix).
One has analogous constructions for other subgroups of the general linear group: for instance, the special affine group is the subgroup defined by the semidirect product, SL(n, F) ⋉ Fn, and the Poincaré group is the affine group associated to the Lorentz group, O(1,3,F) ⋉ Fn.