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Revista de la Unión Matemática Argentina
versión Online ISSN 16699637
Rev. Unión Mat. Argent. v.47 n.1 Bahía Blanca ene./jun. 2006
Integrability of fstructures on generalized flag manifolds
Sofía Pinzón
Financiado parcialmente por COLCIENCIAS, contract No. 1382004.
Abstract: Here we consider a generalized flag manifold and a differential structure which satisfy these structures are called fstructures. Such structure determines in the tangent bundle of some invariant distributions. Since flag manifolds are homogeneous reductive spaces, they certainly have combinatorial properties that allow us to make some easy calculations about integrability conditions for itself and the distributions that it determines on An special case corresponds to the case , the unitary group, this is the geometrical classical flag manifold and in fact tools coming from graph theory are very useful.
A tensor field of type (1,1) on a Riemannian manifold is called an structure if , and almost complex if Obviously, an almost complex structure is also an structure. Integrability of almost complex structures is equivalent to the associate Nijenhuis tensor being null. In [7] Ishihara and Yano present an analogous theorem in the case of structures. We use their results to study integrability conditions when one generalized flags manifold are consider.
Let be a semisimple Lie group. A generalized flag manifold with Lie group is a reductive homogeneous space where is a centralizer of a torus. This manifold can be expressed as a where is the compact connected form of and This manifold and its tangent space have a characterization in terms of the corresponding root system terms. We will consider along this paper a generalized flag manifold together with an invariant metric and an invariant structure meaning that commute with the adjoint action of Ww will denote by the complexification of this structure, which is diagonalizable with eigenvalues and eigenspaces In analogy with the almost complex case we will distinguish between vectors of types , or corresponding to the eigenvalues of respectively.
The integrability of the distributions that it determines in and the properties of that distributions are our central topic. We characterize in root terms and in graph theoretical terms the integrability conditions.
In particular, we get that the only integrable structure, different from the null structure, in the maximal classical flag manifold is the structure which corresponds to the integrable almost complex structure, that is, in graph theoretic terms which corresponds to the canonical tournament.
Theorem A necessary and sufficient condition for to be integrable is that Therefore is integrable if in there are not exist triples of type or In this condition is equivalent to the associated digraph avoiding the subdigraphs (2), (3), (4), (5) or (6) in figure 2, that is, the digraph associated to must be isomorphic to the null digraph or the canonical tournament.
The Theorem above will appear like Theorem 8.8 and with this result we generalized a Theorem from Burstall [4] given in the context of almost complex structures where he shows that one almost complex structure is integrable if and only if its associated tournament is isomorphic to the canonical tournament, that is, the tournament which does not have threecycles in root terms it avoids {0,3,0}triples.
In this section we shall briefly review some general concepts involving generalized flag manifold and some operators and structures which we will use in all this paper. First we need to present a survey about some operators in differential geometry, then we will calculate them specifically on generalized flag manifolds, to this topic we used, specially Props I.3.2, I.3.4, I.3.5 in [8].
2.1. Operators on a general differential manifold.
 Lie derivative. This is the resulting derivative when a tensor field or a differential form is differentiated with respect to a vector field.
1) Lie derivative on tensor fields: .
2) Lie derivative on tensors: On tensors of type (1,r) we get(1) 3) Lie derivative on forms: If is an form, then
(2)  The riemannian invariant connection. Each Riemannian manifold admits a unique metric connection with vanishing torsion, called the Riemannian connection or LeviCivita connection, and it satisfies and where is the metric on the manifold and is the torsion tensor [8].
1) The covariant derivative on tensors. Given a tensor field of type the covariant differential of is a tensor field of type defined as follows.
2) The covariant derivative on forms. If is an form, then
 Exterior derivative on forms. Exterior differentiation can be characterize as follow:
 is a degreeincreasing linear mapping, that is if is a form, is a form;
 signed derivation w.r.t. the wedge product, that is, if is a form and is a form, then ;
 For 0forms is defined by
extends to forms coefficientwise, using basis expansions, resulting in
Here means that you not consider that component. On functions the 1form is defined by Otherwise we get
(3) On 1forms we get
(4) On 2forms,
(5) where is the cyclic symmetrization operator w.r.t. the vector fields involved. See [8] Prop. I.3.11. By duality we get
A generalized flag manifold is and homogeneous space of the form where is a semisimple compact Lie group and is the centralizer of some torus in If the torus is maximal, say then is called a maximal (full) flag manifold, if is not maximal, is called a partial flag manifold. Lets us describe generalized flag manifolds associated with the Lie group in terms of the root systems associated with the corresponding semisimple Lie algebra
Assume complex and let be a Cartan subalgebra of denote by and the root system and the positive root system , respectively, (of with respect to ) and
its root decomposition, where , , is the onedimensional complex root space corresponding to . As is the generator of (the dual of we have the elements defined by Denote by the subspace of generated over by Choose now a simple root system, take and denote by the set of roots generated by Each subset splits as follows
 (6) 
Fix a Weyl basis of that is, a set of vectors which satisfies or equivalently since and with , and if .
Let now
 (7) 
is the parabolic subalgebra determined by in Then equation ((6)) becomes
 (8) 
Then the generalized flag manifold associated to corresponds to the homogeneous space , where is the normalizer of in .
Denote by a real compact form of , and by the connected subgroup associated to . Assume the real subspace generated by with , where and Let which, by construction, is the centralizer of a torus. acts in a transitively way on and we can write . If , correspondes to the maximal flag manifold otherwise it corresponds to a partial flag manifold.
The generalized flag manifold is a reductive homogeneous space. In fact let and
Then,
 that is,
and satisfies the condition to be a a reductive homogeneous space (see [8]).
Denote by the origin of We identify with This identification is given by that is, by evaluation of in like a vector field in The tangent space to in is, naturally, identify with generated by where and In the same way, the complexified tangent space of is identified with
One special case of flag manifold corresponds to the geometrical or classical flag manifold, in this case the unitary group and has to be conjugate to some subgroup with positive integers and . If , the homogeneous space can be identified with the set of "partial flags" that is, the manifold of the flags where is an dimensional subspace of The flag manifold corresponding to the case for all will be denoted by the space of "full flags" or maximal flags in Each flag consists of the sequence In particular, the vectors y is a Weyl basis for and is the subalgebra of diagonal matrices in , then is spanned by and where is the usual canonical basis in
Example 1.1: Consider
A invariant Riemannian metric in is completely determined by its values in , that is, by an inner product in invariant under the associated action of ([3]). Any inner product in , invariant under the associated action of has the form with definite with respect to the CartanKilling form and is the Hadamard product or term by term product [3]. The inner product admits a natural extension to a bilinear symmetric form on We use the same notation for this form, as well as for the correspondent complexified form invariance of amounts to the Weyl basis being a complex basis of eigenvectors for the action of , in other words in we have
 (9) 
with for
for the real space the elements of the canonical base with , are eigenvectors for the same eigenvalue We denote by the invariant metric associated with In what follows we will use as synonymous of
K. Yano [20] in 1961 introduced structure for general manifolds; here we shall be interested in invariant structures. An invariant structure in is completely determined by an endomorphism , satisfying which commutes with the adjoint action of We also denote by its complexification which is diagonalizable with eigenvalues , and denote by the corresponding eigenspaces. Then we have with The invariance of guarantees that for all with equality when is an invariant almost complex structure (see [17]). Thus is determined uniquely by the values defined by . These values satisfy therefore is defined by its values in In the sequel we allow some abuse of notation and identify the invariant structure on with In our invariant context if the structure is an invariant almost complex structure this amounts to for all
In what follows we shall simplify notation by suppressing the subscript in the context of partial flag manifolds.
Denote by and the complementary projections onto the spaces and denoted as and , respectively and defined as follow
Since is an structure we have
 (11) 
where denote the identity. In other words and are complementary projection operators in
Consider an estructure and , like below. Then:
We are interested in studying integrability conditions for the distributions and For this purpose, we need the Nijenhuis tensor which describes the torsion of It is given by
Using Weyl basis properties we get
With some simple calculations and using, again, Weyl basis properties we obtain this other identities:
Riemannian Connection. Since is a naturally reductive homogeneous space its Riemannian connection is given by
 (16) 
where is a symmetric bilinear application defined by
Then the concrete action of the Riemannian connection on the elements of the Weyl basis is given by:
Proposition 6.1. For let and elements in the Weyl base. Then
 (17) 
Kähler form.
Derivative in the connection.
7. Graph theoretic description of
On invariant structures are in 1:1 correspondence with digraphs The correspondence is given by associating with the structure a digraph whose vertices are and whose arrows are given by the following rules: For
Example 7.1. Again in let the structure

There exists a complete classification for invariant structures on (see [5]). Here we present, up to isomorphism, the invariant structures in the case this graphs are the most relevant in our present work.

We are now ready to present our characterization of integrability in graph theoretical terms (classical case) or in root terms (general case).
8. Integrability via projections
In this section we will use all the formulas given before to establish integrability conditions for and for the associated distributions and .
First, we will call the root triple with whenever is called a zerosum triple. Given an invariant structure , each root assumes a sign in .
Roots triples may then be classified by their sign characteristic, which is a triple where corresponds to the quantity of roots in triple who has like its eigenvalue, corresponds to the quantity of roots in triple who has like its eigenvalue and corresponds to the quantity of roots in triple who has like its eigenvalue. There are six possible sign characteristics:
By the Frobenious Theorem we know that a distribution is integrable if and only if it is involutive. Thus is integrable if and only if that is,
 (18) 
Theorem 8.1. A necessary and sufficient condition for the distribution to be integrable is that in does not admit triples of type In case of this condition is equivalent to the associated digraph avoiding the subdigraph (2) in Figure 2.
PROOF. By equation (18) is not integrable in case where This can only occur when the triple is a triple. In the classical case, this corresponds the configuration (2) in Figure 2.
Now is integrable if and only if but
Thus we have the following.
Theorem 8.2. A necessary and sufficient condition for distribution to be integrable is that in does not amit triples of type and In case of this condition is equivalent to the associated digraph avoiding the subdigraph (3),(4) and (5) in Figure 2.
PROOF. By equation (21) is not integrable in case This can only occur when the triple is a triple or a triple. In the classical case, this corresponds to configurations (3), (4) and (5) in Figure 2.
When and are integrable, the structure of the submanifolds defined by these distributions and its properties is of interest. We hope to be able to report on this structure in a future communication.
Looking for the integrability of we need another definition from [7].
Definition 8.3. Assume integrable and let be an arbitrary vector field which is tangent to an integrable manifold of It is defined Then is an almostcomplex structure on each integral manifold of is called partially integrable if both, and are integrable.
Theorem 8.4. A necessary and sufficient condition for be partially integrable is that one of the following equivalent conditions be satisfied:
Using Weyl basis properties the Theorem 8.4 is equivalent to the following.
Theorem 8.5. A necessary and sufficient condition for to be partially integrable is that in does not admit triples of type {0,3,0}, {1,2,0} and In the case of this condition is equivalent to the associated digraph avoiding the subdigraph (3), (4), (5) and (6) in Figure 2.
PROOF. By Theorem 8.4 is enough to see the conditions Doing the respective calculations we have
When and are both integrables, it is possible to choose a local coordinates system such that the operators and can be supposed to have the components of the form:
where is the dimension of the manifold, is the rank of and means the identity matrix. This coordinate system is called "adapted." Since satisfy and then in an adapted coordinate system, we can express in the following way:
But means that the components of are independent of the coordinates, thus in the next theorem we are interested on this condition in root terms.
Theorem 8.6. Suppose that and are both integrable and that an adapted coordinate system has been chosen. A necessary and sufficient condition for the local components of to be functions independent of the coordinates is that in combinatorial terms a necessary and sufficient condition is that in does not admit triples of type In case of this condition is equivalent to the associated digraph avoiding the subdigraphs (4) and (5) in Figure 2.
PROOF. By equation (14) we have the first affirmation and with some calculus we have
Thus will be different from zero in case
This can only occur when the triple is triple. In the classical case this corresponds to configurations mentioned in the theorem.
When is an almost complex structure integrability is associated with the existence of canonical coordinate systems, which allows us to consider the manifold as a complex manifold as well is known, integrability is equivalent to The following definition in the context of general differential manifolds appears in [7]. Here we present it in the case of flag manifolds.
Definition 8.7. The structure is called integrable if it satisfies the following three conditions:
 is partially integrable.
 is integrable.
 The components of are independent of the coordinates.
With Definition 8.7, we arrive at a general integrability theorem for structures.
Theorem 8.8. A necessary and sufficient condition for to be integrable is that Therefore is integrable if in does not admit triples of type and In this condition is equivalent to the associated digraph avoiding the subdigraphs (2), (3), (4), (5) and (6) in figure 2, that is, the associated digraph to must be isomorphic to the null digraph or canonical tournament.
PROOF. It is immediate by Definition 8.7 and Theorems 8.1, 8.6 and 8.5.
Theorem 8.8 is a generalization of the results obtained by Burstall [4] in the case of almost complex structures for classical maximal flag manifolds.
At this point we would like to point out an important connection between integrability and complex structures. It is well known that for a general almost complex structure on a differential manifold is parallel, namely if the manifold with that structure is Kähler. That is, parallelism means that is integrable and the manifold is Kähler.
Now a manifold with an structure will be called Kähler if
Observe that in the case of generalized flag manifolds integrability condition is stronger than Kähler condition, for example, in the invariant structures to the digraphs (4) and (5) are Kähler but not integrable.
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Sofía Pinzón
Escuela de Matemáticas,
Universidad Industrial de Santander,
A.A. 678, Bucaramanga, Colombia.
Recibido: 30 de septiembre de 2005
Aceptado: 18 de septiembre de 2006