Generalized flag variety

In mathematics, a generalized flag variety (or simply flag variety) is a homogeneous space whose points are flags in a finite-dimensional vector space V over a field F. When F is the real or complex numbers, a generalized flag variety is a smooth or complex manifold, called a real or complex flag manifold. Flag varieties are naturally projective varieties.

Flag varieties can be defined in various degrees of generality. A prototype is the variety of complete flags in a vector space V over a field F, which is a flag variety for the special linear group over F. Other flag varieties arise by considering partial flags, or by restriction from the special linear group to subgroups such as the symplectic group. For partial flags, one needs to specify the sequence of dimensions of the flags under consideration. For subgroups of the linear group, additional conditions must be imposed on the flags.

The most general concept of a generalized flag variety is a conjugacy class of parabolic subgroups of a semisimple algebraic or Lie group G: G acts transitively on such a conjugacy class by conjugation, and the stabilizer of a parabolic P is P itself, so that the generalized flag variety is isomorphic to G/P. It may also be realised as the orbit of a highest weight space in a projectivized representation of G. In the algebraic setting, generalized flag varieties are precisely the homogeneous spaces for G which are complete as algebraic varieties. In the smooth setting, generalized flag manifolds are compact, and are homogeneous Riemannian manifolds under any maximal compact subgroup of G.

Flag manifolds can be symmetric spaces. Over the complex numbers, the corresponding flag manifolds are the Hermitian symmetric spaces. Over the real numbers, an R-space is a synonym for a real flag manifold and the corresponding symmetric spaces are called symmetric R-spaces.

Flags in a vector space

Main article: flag (linear algebra)

A flag in a finite dimensional vector space V over a field F is an increasing sequence of subspaces, where "increasing" means each is a proper subspace of the next (see filtration):

$\{0\}=V_{0}\subset V_{1}\subset V_{2}\subset \cdots \subset V_{k}=V.$

If we write the dim V_i = d_i then we have

$0=d_{0}<d_{1}<d_{2}<\cdots <d_{k}=n,$

where n is the dimension of V. Hence, we must have k ≤ n. A flag is called a complete flag if d_i = i, otherwise it is called a partial flag. The signature of the flag is the sequence (d₁, … d_k).

A partial flag can be obtained from a complete flag by deleting some of the subspaces. Conversely, any partial flag can be completed (in many different ways) by inserting suitable subspaces.

Prototype: the complete flag variety

According to basic results of linear algebra, any two complete flags in an n-dimensional vector space V over a field F are no different from each other from a geometric point of view. That is to say, the general linear group acts transitively on the set of all complete flags.

Fix an ordered basis for V, identifying it with Fⁿ, whose special linear group is the group GL(n,F) of n × n matrices. The standard flag associated with this basis is the one where the i th subspace is spanned by the first i vectors of the basis. Relative to this basis, the stabilizer of the standard flag is the group of nonsingular upper triangular matrices, which we denote by B_n. The complete flag variety can therefore be written as a homogeneous space GL(n,F) / B_n, which shows in particular that it has dimension n(n−1)/2 over F.

Note that the multiples of the identity act trivially on all flags, and so one can restrict attention to the special linear group SL(n,F) of matrices with determinant one, which is a semisimple algebraic group; the set of upper triangular matrices of determinant one is a Borel subgroup.

If the field F is the real or complex numbers we can introduce an inner product on V such that the chosen basis is orthonormal. Any complete flag then splits into a direct sum of one dimensional subspaces by taking orthogonal complements. It follows that the complete flag manifold over the complex numbers is the homogeneous space

$U(n)/T^{n}$

where U(n) is the unitary group and Tⁿ is the n-torus of diagonal unitary matrices. There is a similar description over the real numbers with U(n) replaced by the orthogonal group O(n), and Tⁿ by the diagonal orthogonal matrices (which have diagonal entries ±1).

Partial flag varieties

The partial flag variety

$F(d_{1},d_{2},\ldots d_{k},\mathbb {F} )$

is the space of all flags of signature (d₁, d₂, … d_k) in a vector space V of dimension n = d_k over F. The complete flag variety is the special case that d_i = i for all i. When k=2, this is a Grassmannian of d₁-dimensional subspaces of V.

This is a homogeneous space for the general linear group G of V over F. To be explicit, take V = Fⁿ so that G = GL(n,F). The stabilizer of a flag of nested subspaces V_i of dimension d_i can be taken to be the group of nonsingular block upper triangular matrices, where the dimensions of the blocks are n_i := d_i − d_i−1 (with d₀ = 0).

Restricting to matrices of determinant one, this is a parabolic subgroup P of SL(n,F), and thus the partial flag variety is isomorphic to the homogeneous space SL(n,F)/P.

If F is the real or complex numbers, then an inner product can be used to split any flag into a direct sum, and so the partial flag variety is also isomorphic to the homogeneous space

$U(n)/U(n_{1})\times \cdots \times U(n_{k})$

in the complex case, or

$O(n)/O(n_{1})\times \cdots \times O(n_{k})$

in the real case.

Generalization to semisimple groups

The upper triangular matrices of determinant one are a Borel subgroup of SL(n,F), and hence the stabilizers of partial flags are parabolic subgroups. Furthermore, a partial flag is determined by the parabolic subgroup which stabilizes it, and partial flags belong to the same flag variety precisely when the corresponding parabolic subgroups are conjugate.

Hence, more generally, if G is a semisimple algebraic or Lie group, then a (generalized) flag variety for G is a conjugacy class of parabolic subgroups of G. It is therefore isomorphic, as a homogeneous space, to G/P where P is a parabolic subgroup of G. The correspondence between parabolic subgroups and generalized flag varieties allows each to be understood in terms of the other.

The extension of the terminology "flag variety" is reasonable, because points of G/P can still be described using flags. When G is a classical group, such as a symplectic group or orthogonal group, this is particularly transparent. If (V, ω) is a symplectic vector space then a partial flag in V is isotropic if the symplectic form vanishes on proper subspaces of V in the flag. The stabilizer of an isotropic flag is a parabolic subgroup of the symplectic group Sp(V,ω). For orthogonal groups there is a similar picture, with a couple of complications. First, if F is not algebraically closed, then isotropic subspaces may not exist: for a general theory, one needs to use the split orthogonal groups. Second, for vector spaces of even dimension 2m, isotropic subspaces of dimension m come in two flavours ("self-dual" and "anti-self-dual") and one needs to distinguish these to obtain a homogeneous space.

Highest weight orbits and homogeneous projective varieties

If G is a semisimple algebraic group (or Lie group) and V is a (finite dimensional) highest weight representation of G, then the highest weight space is a point in the projective space P(V) and its orbit under the action of G is a projective algebraic variety. This variety is a (generalized) flag variety, and furthermore, every (generalized) flag variety for G arises in this way.

Armand Borel showed that this characterizes the flag varieties of a general semisimple algebraic group G: they are precisely the complete homogeneous spaces of G, or equivalently (in this context), the projective G-varieties.

Symmetric spaces

Let G be a semisimple Lie group with maximal compact subgroup K. Then K acts transitively on any conjugacy class of parabolic subgroups, and hence the generalized flag variety G/P is a compact homogeneous Riemannian manifold K/(K∩P) with isometry group K. Furthermore, if G is a complex Lie group, G/P is a homogeneous Kähler manifold.

Turning this around, the Riemannian homogenous spaces M =' 'K/(K∩P) admit a strictly larger Lie group of transformations, namely G. Specializing to the case that M is a symmetric space, this observation yields all symmetric spaces admitting such a larger symmetry group, and these spaces have been classified by Kobayashi and Nagano.

If G is a complex Lie group, the symmetric spaces M arising in this way are the compact Hermitian symmetric spaces: K is the isometry group, and G is the biholomorphism group of M.

Over the real numbers, a real flag manifold is also called an R-space, and the R-spaces which are Riemannian symmetric spaces under K are known as symmetric R-spaces. The symmetric R-spaces which are not Hermitian symmetric are obtained by taking G to be a real form of the biholomorphism group G^c of a Hermitian symmetric space G^c/P^c such that P := P^c∩G is a parabolic subgroup of G. Examples include projective spaces (with G the group of projective transformations) and spheres (with G the group of conformal transformations).

References

• Robert J. Baston and Michael G. Eastwood, The Penrose Transform: its Interaction with Representation Theory, Oxford University Press, 1989.
• Jürgen Berndt, Lie group actions on manifolds, Lecture notes, Tokyo, 2002.
• Jürgen Berndt, Sergio Console and Carlos Olmos, Submanifolds and Holonomy, Chapman & Hall/CRC Press, 2003.
• Michel Brion, Lectures on the geometry of flag varieties, Lecture notes, Varsovie, 2003.
• James E. Humphreys, Linear Algebraic Groups, Graduate Texts in Mathematics, 21, Springer-Verlag, 1972.
• S. Kobayashi and T. Nagano, On filtered Lie algebras and geometric structures I, II, J. Math. Mech. 13 (1964), 875–907, 14 (1965) 513–521.