THEOREMS OF BARTH-LEFSCHETZ TYPE AND MORSE THEORY ON THE
SPACE OF PATHS IN HOMOGENEOUS SPACES
By
Chaitanya Senapathi

A DISSERTATION
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
Mathematics - Doctor of Philosophy
2013

ABSTRACT
THEOREMS OF BARTH-LEFSCHETZ TYPE AND MORSE THEORY ON
THE SPACE OF PATHS IN HOMOGENEOUS SPACES
By
Chaitanya Senapathi
Homotopy connectedness theorems for complex submanifolds of homogeneous spaces
(sometimes referred to as theorems of Barth-Lefshetz type) have been established by a number of authors. Morse Theory on the space of paths lead to an elegant proof of homotopy
connectedness theorems for complex submanifolds of Hermitian symmetric spaces. In this
work we extend this proof to a larger class of compact complex manifolds namely quotients
of complex orthogonal, unitary groups and exceptional groups.

Dedicated to my loving and supporting parents and my darling sister.

iii

ACKNOWLEDGMENTS

Many thanks to my adviser, Jon G. Wolfson, for his constant belief and support in me
over the years despite all my monumental shortcomings. I would also like to thank Dr.
Ben Schmidt for the helpful discussions and encouragement. The excellent coursework and
atmosphere at the math department at MSU has been very essential for this work to take
place. I would like to also mention Prof Zhengfang Zhou, the departmental chair for his
timely interventions during some rough times.
I am very grateful to Dr Robert Moody and Apritam Roy for encouraging me to think
about maths during my formative years, their influence gave me a lot of confidence as I was
growing up. Also Prof Yogish Holla for his constant mentorship from my school days till the
end of my college education. A very special thanks to my teachers Prof Siddhartha Gadgil at
ISI(Bangalore) and Prof Vasudevan Srinivas at (TIFR) for their dedicated effort at teaching
algebraic topology and differential geometry.

iv

TABLE OF CONTENTS

Chapter 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1

Chapter 2 Morse Theory and Homotopy groups . . . . . . . . . . . . . . . .
2.1 Morse Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Homotopy Theory and Index . . . . . . . . . . . . . . . . . . . . . . . . . . .

6
6
9

Chapter 3 Homogeneous Spaces
3.1 Notation and Background . .
3.2 The canonical connection . . .
3.3 The Levi-Civita Connection .
3.4 Complex Gc /P . . . . . . . .
3.5 The map I . . . . . . . . . . .

.
.
.
.
.
.

12
12
16
23
25
28

Chapter 4 The complex hat connection . . . . . . . . . . . . . . . . . . . . . .
4.1 The second variation formula . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 Properties of the complex-hat connection . . . . . . . . . . . . . . . . . . . .

34
35
38

Chapter 5 Index calculations
5.1 The variations T . . . . .
5.2 The variations S . . . . .
5.3 Reconciling Sk and T . . .

.
.
.
.

40
43
44
47

Chapter 6 Lie algebra calculations . . . . . . . . . . . . . . . . . . . . . . . . .

53

BIBLIOGRAPHY

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Chapter 1
Introduction
In the 1920s Lefschetz [Le] stated the following theorem now known as the Lefschetz
theorem on hyperplane sections. Let H ⊂ Pv be a connected complex submanifold of
complex dimension n. Let H be a hyperplane and N ∩ H be a non singular hyperplane
section. Then the relative cohomology groups satisfy:

H j (N, N ∩ H, C) = 0

j

n−1

Fifty years later Barth [B] generalized Lefschetz’ theorem: Let M, N ∈ Pv be complex
submanifolds of complex dimensions m, n respectively. If M and N meet properly, then

Hj (N, N ∩ M ) = 0 , j ≤ min(n + m − v; 2m − v + 1).

Generalizations of Barth's results to homotopy groups were first obtained by Larsen [La],
Barth-Larsen [B-L] and later by Sommese. Sommese [S1][S2] and Goldstein [G] generalized
these results to submanifolds of generalized flag manifolds, i.e manifolds of the form Gc /P
where Gc is a semi-simple complex Lie group and P a parabolic subgroup.
In 1961 T. Frankel [F] proved a connectedness theorem for complex submanifolds of a
K¨hler manifold of positive holomorphic sectional curvature: Let V be a complete K¨hler
a
a
manifold of positive holomorphic sectional curvature and of complex dimension v. Let
1

M, N ⊂ V be compact complex submanifolds of dimensions m and n, respectively. If m+n ≥
v then M and N must intersect. Later [K-W]and [S-W] expanded on this idea to prove
the Barth-Lefschetz theorems on a class of generalized flag manifolds, namely Hermitian
Symmetric Spaces and hence reproduced the results of [S1], [S2] and [G].
In this thesis we extend the theorem of [S-W] and [K-W] to a larger class of generalized
flag manifolds. In the main theorem of the work we deal with the case when Gc is simple
and later, state how to deal with the semisimple case.
Theorem 1.1. Let Gc be a simple complex Lie group and P be a parabolic subgroup. Let
V be the complex homogeneous space Gc /P with dimension v. Let M, N ⊂ V be compact
complex submanifolds dimension m and n respectively. Then there exists a number
number λ0 = m + n − (v − ) − v such that

ι∗ : πj (N, N ∩ M ) → πj (V, M )

is an isomorphism for j ≤ λ0 and a surjection for j = λ0 + 1.
(i) If Gc = SLr+1 (C) then
(ii) If Gc = SO2r (C) then

=r
= 2r − 3

(iii) If Gc = SO2r+1 (C) then
(iv) If Gc = Spr (C) then

=r

(v) If Gc = E6 ,E7 ,E8 then
(vi) If Gc = F4 the

= 11, 17 and 29 respectively

=8

(vii) If Gc = G2 the

= 2r − 2

=3
2

and a

Corollary 1.2. Suppose that V, M, and N satisfy the same hypotheses as Theorem 1.1 and
remains the same then
(a) If j ≤ 2m − v − (v − ) + 1 then πj (V, M ) = 0
(b) If j ≤ min(2m − v − (v − ) + 1, n + m − v − (v − )) then
πj (N, N ∩ M ) = 0
In part (iii) of Theorem 1.1, the case where Gc = SO2r+1 (C) and P is the parabolic
corresponding to the painted Dynkin diagram with all long roots painted, the result can be
improved. By a Theorem of [O] the corresponding homogeneous space can be written in the
form SO2r+2 (C)/P where P is a parabolic subgroup of SO2r+2 (C), in this case

can be

improved to 2r − 1.
Also the case Gc = Spr (C) and for a special type of parabolic subgroup P , Gc /P is
biholomorphic to CP 2r−1 ([O]) so the number

can be improved to 2r − 1. The parabolic

P is a maximal parabolic containing a copy of Spr−1 (C).
The results obtained from Theorem 1.1 also follow from the work of [S1] ,[S2] and [G].
In [S1] and [S2] Sommese shows that the number

can be replaced by the co-ampleness of

the respective homogeneous space, and in [G] the co-ampleness of the respective spaces are
calculated. In the case where Gc = Spr (C), F4 or G2 and P is a parabolic such that the
corresponding painted Dynkin diagram contains a long root, the approach of [S1] ,[S2] and
[G] leads to stronger results. All other cases treated in this work, the results are same as
those obtained in [S1], [S2] and [G]. To prove these connectedness Theorems [S1] and [S2]
apply Morse Theory locally on the ambient space but in this work we apply Morse theory
on the space of paths following the work of [F],[S-W],[K-W],[N-W],[C],[FM] and [W].

3

The basic idea of [S-W] and [K-W] is to demonstrate that the index of the critical points
in the space of paths joining two submanifolds has the appropriate index for a chosen Morse
function. The Morse function that they choose on the space of paths is the energy function
with respect to the K¨hler metric. To compute a lower bound of the index at the critical
a
points, variational vector fields are constructed along these geodesics and used in the second
variation formula.
In this work we generalize the idea of [S-W] and [K-W]. Their argument cannot be generalized to non-symmetric homogeneous spaces as [M1] has shown that other homogeneous
spaces that are not covers of products of Hermitian Symmetric Spaces don't posses K¨hler
a
metrics with non-negative curvature. But complex Gc /P , being a quotient of a compact Lie
group, does posses a metric induced by the standard bi-invariant metric of the compact Lie
group. This metric, which is commonly referred to as the 'normal metric' is what we use
in this paper. This metric has non-negative curvature and is K¨hler only in the case of a
a
Hermitian Symmetric Space. Using this metric allows us to naturally generalize the work of
[S-W] and [K-W].
Using the canonical connection we construct a new connection referred here as the
complex-hat connection. We use this connection to form variational vector fields along
geodesics. This connection is invariant and is compatible with the complex structure. The
connection is also amenable towards the root structure, as a result most of the computations follow naturally. We also use certain types of linear combinations of these variational
vector fields and show the existence of a quaternionic structure on the linear combinations.
To demonstrate a lower bound on the index, we take an average using this quaternionic
structure.
An outline of this thesis is as follows. In Chapter 2 we review Morse theory on the space
4

of paths and its relation to homotopy theory. In Chapter 3 we review the basic properties
of reductive homogeneous spaces. In Chapter 4 we construct the complex-hat connection,
describe its properties, and describe its relation with the second variation formula. In Chapter 5 we establish a lower bound on the index of geodesics in terms on an invariant of a Lie
algebra. In the final Chapter we compute this invariant, thereby proving Theorem 1.1.

5

Chapter 2
Morse Theory and Homotopy groups
In this chapter we follow [S-W] and talk about Morse Theory on the space of paths, and
relate the index of geodesics with the vanishing of relative homotopy groups.

2.1

Morse Theory

Let V be a complete Riemannian manifold and let M and N be closed submanifolds with
M compact and N a closed subset of V. We let P (V ; M, N ) denote the set of continuous
paths γ : [0; 1] → V such that γ(0) ∈ M and γ(1) ∈ N and let Ω(V ; M, N ) denote the set of
piecewise smooth paths. To learn about the topology of the path space P (V, M, N ) it suffices
to look at the space Ω(V ; M, N ), as the natural inclusion i∗ : Ω(V ; M, N ) → P (V ; M, N ) is
a homotopy equivalence [M]. To study the topology of Ω(V ; M, N ) we use a Morse function
following [M]. We will denote Ω(V ; M, N ) by simply Ω.
In [M], Milnor approximates the path space by finite-dimensional manifolds and employs
techniques from finite-dimensional Morse Theory. However the problem that Milnor deals
with is Ω(V ; p, q), but the proofs given in [M] apply to the general case with only minor
changes that can easily be made. Accordingly, in this section, we will describe the general
set-up, state the results we will need and give the appropriate references to [M].
The set Ω(V ; M, N ) can be topologized as follows. Let ρ denote the Riemannian distance
function on V . Let γ1 , γ2 ∈ Ω(V ; M, N ). Define the distance d(γ1 , γ2 ) by :
6

1

d(γ1 , γ2 ) = max ρ(γ1 (t), γ2 (t)) +
0≤t≤1

0

(|γ˙1 (t)| − |γ˙2 (t)|)2 dt

The energy of a path, given by
1

E(γ) =

|γ(t))|2 dt,
˙

0

defines a continuous map from Ω(V ; M, N ) → R.
Define the 'tangent space' of Ω at γ, Tγ Ω, to be the vector space of piecewise smooth
vector fields W along γ such that W (0) ∈ Tγ(0) Ω and W (1) ∈ Tγ(1) Ω. A standard computation in [C-E] shows that the first variation of E in the direction of W ∈ Tγ Ω is given by
:
1
E∗ (W ) = W, γ |1 −
˙ 0
2

1

W (t), ∆t γ −
˙
t

W,
0

Dγ
˙
dt
dt

where ∆t γ = γ(t+ ) − γ(t− ), is the discontinuity of γ at t. It follows that γ is a critical point
˙
˙
˙
˙
of E if :
(a) γ is a smooth geodesic.
(b) γ is normal to M and N at γ(0) and γ(1) , respectively.
Let W1 , W2 ∈ Tγ Ω. If γ is a critical point of E then the second variation of E along γ, is
given by:

1
E∗∗ (W1 , W2 ) =
2

W2 (t), ∆t
t

1
DW1
D 2 W1
−
W2 ,
+ R(γ, W1 )γ
˙
˙
dt
dt2
0

(2.1)

Let Ωc denote the closed subset E −1 ([0, c]) ⊂ Ω and let Ω∗ denote the open subset
c
E −1 ([0, c)). We construct a finite dimensional approximation of Ωc . Choose some subdivi7

sion 0 = t0 < t1 < ...... < tk = 1 of [0, 1]. Let Ω(t0 , .....tk ) be the subspace of Ω consisting
of paths γ : [0, 1] → V such that :
(a) γ(0) ∈ M and γ(1) ∈ N
(b) γ|[t

i−1 ,ti

] is a geodesic for each i = 1, .....k

Define the subspace Ω∗ (t0 , .....tk ) = Ω∗ ∩ Ω(t0 , ....., tk )
c
c
Theorem 2.1. Let V be a complete Riemannian manifold and let M and N be sub-manifolds
with M compact and N a closed subset of V . Let c be a fixed positive number such that
Ωc = φ. Then for all sufficiently fine subdivisions 0 = t0 < t1 < ......tk = 1 of [0, 1] the set
Ω∗ (t0 , .....tk ) can be given the structure of a smooth finite dimensional manifold.
c
Proof. [M] Sect 16.
Denote the manifold Ω∗ (t0 , ....tk ), of broken geodesics by B. Let E' : B → R denote the
restriction to B of the energy function.
Theorem 2.2. E : B → R is a smooth map. For each a < c the set BA = (EB )−1 ([0, a]) is
compact and is a deformation retract of the set Ωa . The critical points of E are precisely the
same as the critical points of E in Ω∗ , that is the smooth geodesics from M to N , intersecting
c
M and N orthogonally, and with energy less than c. The index of the hessian of E at each
such critical point γ is equal to the index of E∗∗ at γ.
Proof. [M] Sect. 14 and Sect 16.

Now suppose that every nontrivial critical point γ of E on Ω has index λ > λ0 ≥ 0 .
We remark that this implies that N ∩ M = ∅, otherwise there exists a nontrivial minimizing
8

geodesic from M to N and the index of such a geodesic must be zero. It follows that if every
nontrivial critical point γ on Ω has index λ > λ0 ≥ 0 then the space Ω0 of minimal (i.e
trivial ) geodesics can be identified with the subspace N ∩ M ⊂ Ω.
Proposition 2.3. Suppose N intersects M transversely and that every non-trivial critical
point of E on Ω has index λ > λ0 ≥ 0. Then the relative homotopy groups πj (Ω, Ω0 ) are
zero for 0 ≤ j ≤ λ0
The essential element in the proof of the proposition is the following simple lemma whose
proof can be found in [M], about functions on finite-dimensional manifolds. Let X be a
smooth manifold and f : X → R be a smooth real-valued function with minimum value 0
such that each Xc = f −1 ([0, c]) is compact.
Lemma 2.4. If the set X0 of minimal points has a neighborhood U with retraction r : U →
X0 and if every critical point in X\X0 has index > λ0 then

πj (X, X0 ) = 0 for 0 ≤ j ≤ λ0
For the proof of Proposition 2.3 we refer back to [S-W].

2.2

Homotopy Theory and Index

In this section we will apply Morse Theory to the path spaces Ω(V ; M, N ), in the spirit of
[S-W]. Here we cite Theorem 1.5 of [S-W] which is an improvement of Proposition 2.3. The
assumption of transversality is dropped.
Theorem 2.5. Let V be a complete complex manifold. Let M, N ⊂ V be closed complex
submanifolds and suppose that M is compact and N is a closed subset of V . If every nontrivial
9

critical point of E on Ω has index λ > λ0 ≥ 0, then the relative homotopy groups πj (Ω, Ω0 ) =
0 for 0 ≤ j ≤ λ0 .
As we can identify Ω0 with M ∩ N we have πj (Ω, M ∩ N ) = 0 for 0 < j < λ0 . This
observation, along with the long exact sequence of the pair (Ω, N ∩ M ) implies that the
homomorphism induced by the inclusion:

ι∗ : πj (N ∩ M ) → πj (Ω)

(2.2)

is an isomorphism when j < λ0 and is a surjection j = λ0 .
Now consider the fibration

e

Ω(V ; M, x) → Ω(V ; M, N ) − N
→

where e is the evaluation map e : γ → γ(1) and x ∈ N . The long exact homotopy sequence
of the fibration is:

e

∗
...πj+1 (N ) → πj (Ω(V ; M, x)) → πj (Ω) −→ πj (N ) → πj−1 (Ω(V ; M, x))...
−

(2.3)

It is well known that the homotopy groups of the fiber Ω(V ; M, x) satisfy.

πj (Ω(V ; M, x))

πj+1 (V, M )

(2.4)

for all j and hence the sequence (2.3) becomes

e

∗
...πj+1 (N ) → πj+1 (V, M ) → πj (Ω) −→ πj (N ) → πj (V, M )...
−

10

(2.5)

Theorem 2.6. Let V be a complete complex manifold. Let M, N ⊂ V be complex submanifolds and suppose that M is compact and N is a closed subset of V . If every nontrivial
critical point of E on Ω has index λ > λ0 ≥ 0, then the homomorphism induced by the
inclusion.
ι∗ : πj (N, N ∩ M ) → πj (V, M )
is an isomorphism for j ≤ λ0 and a surjection for j = λ0 + 1.
Proof. Consider the following diagram :
...πj+1 (N )

πj+1 (V, M )

πj (Ω)

πj (N )

πj (V, M )...

...πj+1 (N )

πj+1 (N, N ∩ M )

πj (N ∩ M )

πj (N )

πj (N, N ∩ M )..

The first horizontal line is just equation 2.5, the second is the long exact sequence for the
pair (N, N ∩ M ) and the vertical arrows are the inclusion maps. For j < λ0 the middle map
is an isomorphism (2.2) and hence by the five lemma ι∗ : πj (N, N ∩ M ) → πj (V, M ) is an
isomorphism. For j = λ (2.2) is onto so the corresponding map is onto via the five lemma.

11

Chapter 3
Homogeneous Spaces

In this Chapter we introduce and set up notation to objects associated to compact complex homogeneous spaces, including the canonical connection and the Levi-Civita connection.
The canonical connection associated to a reductive homogeneous space is well known. The
construction of this connection can be found in [K]. Here we describe an easier construction
of this connection. We also talk about the Levi-Civita connection of the normal metric
as well as briefly describe compact homogeneous spaces of the form Gc /P , where Gc is a
complex semi-simple Lie group and P is a parabolic subgroup.

3.1

Notation and Background

Let G be a compact Lie group with identity element e. We identify the Lie algebra g of G,
with the tangent space Te G at e.
Each element a ∈ G defines diffeomorphisms
La : G → G by La (g) = ag
Ra : G → G by Ra (g) = ga

(left translation)
(right translation)

Ca : G → G by ca (g) = aga−1

(conjugation)

The adjoint map Ada : g → g is the differential of the map Ca : G → G at the identity.
12

Since Ca ◦ Ca = Ca a that gives us a representation of G on the vector space g. Which
1
2
1 2
we denote by the following map Ad : G → GL(g)
After taking the derivative, we get the adjoint representation ad : g → gl(g) of the lie
algebra which is given by ad(X)Y = [X, Y ] ∀ X, Y ∈ g. We denote the exponential map
by exp : g → G. So if t ∈ R, X ∈ g the map t → exp(tX) is the unique one parameter
subgroup whose tangent vector at t = 0 is X. Let ·, · be a bi-invariant metric on G. This
implies that

X, [Y, Z] e = [X, Y ], Z e for all X, Y, Z ∈ g

(3.1)

Let K ⊆ G be a closed connected subgroup of G and with Lie algebra k then M = G/K
is naturally a differentiable manifold. Let π : G → G/K be the canonical projection map.
If g ∈ G we denote its image in G/K by g or gK depending on the situation. Let a ∈ G
¯
we define the following map La : G/K → G/K by La (gK) = agK. It is obvious that
La ◦ π = π ◦ La
Definition 1. A reductive homogeneous space is a homogeneous space with a decomposition
g = k ⊕ m such that Ad

K

(m) ⊆ m.

Let m = k ⊥ , clearly g = k ⊕ m. The bi-invariance of the metric implies that the metric
is Adg -invariant ∀g ∈ G, in particular it is Adk invariant ∀k ∈ K, which along with the
orthogonality of m with k implies that Ad

K

(m) ⊆ m. So g = k ⊕ m makes G/K into a

reductive homogeneous space.
As π∗ m = Te G/K, this implies that in a reductive homogeneous space we can identify
¯
m with Te G/K. With this identification we can show how left translation is the same as
¯
pushing forward the adjoint map.
13

Proposition 3.1.
π∗ Adk (X) = Lk∗ X

∀X ∈ m, ∀k ∈ K

Proof. This follows from

π∗ Adk (X) = π∗ Lk∗ R −1 X
k ∗
= Lk∗ π∗ R −1 X
k ∗
= Lk∗ π∗ X
= Lk∗ X

The isotropy representation of the group K, AdG/K : K → GL(Te G/K) is defined by

AdG/K (k)(X) = Lk∗ (X) ∀X ∈ Te (G/K)
¯

Let ω ∈ Λr (T G/K) be a covariant vector valued r-tensor. Then ω is G-invariant if

La∗ ωg (X1 , X2 , .....Xr ) = ωag (Lg∗ X1 , Lg∗ X2 .......Lg∗ Xr ) ∀Xi ∈ Tg G/K for i = 1, 2....r
¯
¯
¯

Likewise ω ∈ Λr Te G/K is AdG/K invariant if
¯

(AdG/K (k)ωe (X1 , X2 , .....Xr ) = ωe (AdG/K (k)(X1 ), ......., AdG/K (k)(Xr ))
¯
¯

∀k ∈ K

We have similar definitions for real valued tensors. The next proposition follows from
Proposition 3.1

14

Proposition 3.2. If ω ∈ Λr Te G/K is AdG/K invariant then ω can be extended to a G
¯
invariant r-tensor.
Restrict ·, · e to m. Then the restricted metric is AdG/K invariant since ·, · e is AdG invariant. Thus we have a G invariant metric ·, ·

G/K

on the whole of G/K, which turns

out to be nothing but the push forward of the metric ·, · on G.
We give some important examples of invariant tensors. Let X, Y ∈ m, lets denote the m
and k component of the bracket by [X, Y ]m and [X, Y ]k respectively. As Ad(g) commutes
with the Lie bracket ∀g ∈ G and the fact that the decomposition m

k is reductive, we

have the following

AdG (k)[X, Y ]m = [AdG (k)X, AdG (k)Y ]m

(3.2)

AdG (k)[X, Y ]k = [AdG (k)X, AdG (k)Y ]k

(3.3)

From the first equality we have that [·, ·]m is AdG/K invariant (2, 1) tensor. From the
second equality and the fact that the metric ·, ·

G/K

is invariant under K, we have that

that |[X, Y ]k |2 is AdG/K (k) invariant real valued tensor.
Definition 2. [·, ·]m and |[·, ·]k |2 will represent the global tensors obtained by extending the
AdG/K invariant tensors [·, ·]m and |[·, ·]k |2 to G/K.
We now mention a very important property of the bracket tensor [·, ·]m

[X, Y ]m , Z = X, [Y, Z]m for all X, Y, Z ∈ TgK (G/K)

(3.4)

This property just follows from (3.1). We will refer to this property as 'associativity of the
15

bracket'.

3.2

The canonical connection

In this section we assume G/K is a reductive homogeneous space, with the decomposition
g = k ⊕ m. We will define and describe the basic properties of the canonical connection of
a reductive homogeneous space.
Consider G as a fiber bundle over the left-coset space G/K with structure group K. The
action of K on G is right multiplication. The group G itself acts on the fiber bundle; this
action clearly commutes with the projection map and the action of K.
We define a G-invariant connection on the principle bundle G. We set the horizontal
space at the identity to be the space m and to be Lg ∗ m at g. This defines a horizontal
distribution. To show that this distribution is a connection we have to show compatibility
with the right action:

Rk∗ Lg∗ m = Lg∗ Rk∗ m
= Lg∗ Lk∗ L−1 Rk∗ m
k∗
= L(gk)∗ AdG/K (k −1 )∗ m
= Lgk∗ m

Let θ : G × m → T (G/K) is defined by θ(g × X) = Lg∗ X ∈ Tg (G/K). Let φ : T (G/K) →
G/K denote the projection map and π' : G × m → (G/K) be the natural projection map.
Then clearly φ ◦ θ = π .

16

Define an equivalence relation '∼' on G × m by

−1

(g × X) ∼ (gk × Lk∗ X)

∀k ∈ K

Now θ clearly factors through the quotient space (G × m)/∼ so we have the quotient map

θ : (G × m)/ ∼→ T (G/K)

θ is clearly one-one, onto and fiber preserving, making θ an isomorphisms of vector bundles.
With this connection on the principle bundle G we have a notion of parallel transport
along a curve γ : [0, 1] → G/K as follows. Let u0 ∈ π −1 (γ(0)) and let ut denote the
horizontal lift of γ starting at u0 , then the parallel transport of u0 from γ(0) to γ(t) is
simply ut .
On the space (G × m)/ ∼ there is a notion of parallel transport (as in section 7 ch2 of
[K])that derives from the notion of parallel transport on the principle bundle G. Let [(u0 , X)]
represent the equivalence class containing(u0 , X). The parallel transport of [(u0 , X)] along
γ, from γ(0) to γ(t) will just be [(ut , X)]. This is independent of the choice of representative
[(u0 , X)].
As we can identify (G × m)/ ∼ with T (G/K) we can talk about the parallel translation
of X ∈ Tγ(0) G/K along γ. By this identification [(u0 , L−1∗ X)] can be identified with X, so
u0
the parallel translation of X will be Lut ∗ L−1∗ X.
u0
h
Let τ0 : φ−1 (γ0 ) → φ−1 (γh ) denotes the parallel translation along γ from γ(0) to γ(h),
0
and let τh denote the inverse map. With this notion of parallel transport on T (G/K) we

can find a linear connection which has parallel transport coinciding with this (as in sec 1 ch

17

3 of [K]), as follows.
Define
1 0
γ(0) X = lim h [τh (X(γh )) − X(γ0 )]
˙
h→0
This linear connection will also be called the canonical connection and will be denoted
by

. The above discussion can be summarized in the following proposition.

Proposition 3.3. The parallel transport of X ∈ Tγ(0) G/K with respect to the canonical
connection along a curve γ is given by left translation of some element of G. More precisely
h
τ0 (X) = Lut ∗ L−1∗ X where ut is any horizontal lift of γ
u0

Now for a few applications of this proposition. Let γX (t) be the integral curve to the
vector field X in G starting at the identity. Where X is the left-invariant vector field
generated by X.
Theorem 3.4. π(γX (t)) is a geodesic with respect to the canonical connection and all
geodesics are of this form or a translate of it.
Proof. To show that π(γX (t)) is a geodesic with respect to the canonical connection, it
suffices to show that the tangent vector field on the curve is itself parallel. From the
above Theorem, the parallel transport of X ∈ Te G/K along π(γX (t)) will be Lγ (t )∗ X ∈
¯
X 0
Tγ (t ) G/K. So the vector field Lγ (t)∗ X is a parallel vector field on π(γX (t)). But by
X 0
X
the construction of π(γX (t)) it is easy to see that this is in fact the tangent vector field on
the curve.
The second part of the Theorem follows from G-invariance of the connection.
Theorem 3.5. Every G-invariant tensor is parallel with respect to the canonical connection.

18

Proof. This follows from Proposition 3.3 and the definition of G-invariant tensor.
Definition 3. For a vector field X on G/K define fX : G → m by

fX (g) = L−1 (X(π(g)).
g

Proposition 3.6. Let X be vector field on G/K, let γ(t) be a path in G/K, let ut be a
horizontal lift of γ(t) and denote γ(0) by Y and u (0) by Y ∗ . Then

∗
Y X = Lu0 ∗ (Y (fX )).

Proof.

Lu0 ∗ (Y ∗ (fX )) = Lu0 ∗
= Lu0 ∗

1
[fX (uh ) − fX (u0 )]
h→0 h
lim

1 −1
[L
X(γh ) − L−1∗ X(γ0 )]
u0
h uh ∗
h→0
lim

1
[Lu0 ∗ L−1∗ X(γh ) − X(γ0 )]
uh
h→0 h
1 0
= lim [τh (X(γh ) − X(γ0 )]
h→0 h
= lim

=

YX

Proposition 3.7. Suppose X, Y, Z are vector fields on G/K and X ∗ , Y ∗ , Z ∗ be the respective horizontal lifts, let T (X, Y ) and R(X, Y )Z denote the torsion and curvature on G/K
Then
(a) T (X, Y )(¯) = Lg∗ (X ∗ (fY ) − Y ∗ (fX )) − [X, Y ](g)
g
(b) R(X, Y )Z = Lg∗ (X ∗ (Y ∗ (fZ )) − Y ∗ (X ∗ (fZ )) − h([X ∗ , Y ∗ ])(fZ )
19

= Lg∗ v([X ∗ , Y ∗ ](fZ ))
where h() and v() denote the horizontal and vertical components of the vector
Proof. (a) follows from the definition of torsion and the Proposition .
(b) This follows from Proposition 3.6, the definition of curvature, and the fact that [X, Y ]∗ =
h([X ∗ , Y ∗ ])

Proposition 3.8. For the canonical connection
(a) T (X, Y ) = −[X, Y ]m
(b) R(X, Y )Z = −[[X, Y ]k , Z]m
Proof. (a) It suffices to prove it at e as T (·, ·) and [·, ·]m are invariant tensors. For X, Y ∈ m
¯
let X, Y be right-invariant vector fields generated by X, Y in G. The right invariance
˜
implies that pushing forward makes sense. So π∗ X, π∗ Y are extensions of X, Y in G/K.
We will use these vector fields to show the first part.
Now

[π∗ X, π∗ Y ] = π∗ [X, Y ]
= π∗ (−[X, Y ])
= (−[X, Y ])m

so at the identity [π∗ X, π∗ Y ] = −[X, Y ]m

20

Now

f

π∗ X

g
(g) = L−1 (π∗ X(¯))
g
= L−1 (π∗ Rg∗ (X))
g
= π∗ (ad(g −1 )∗ X)

Now as we are only calculating the value of the tensors at the identity, we will just note
that the horizontal lifts of X, Y at the identity is just X, Y itself. Now

Y ∗ (e)(f

π∗ X

(g)) = Y (π∗ (ad(g −1 )∗ X))
=

d
(π∗ (ad(exp(−tY )∗ X))
dt

= π∗ [−Y, X]
= [X, Y ]m

Similarly we have X(f

π∗ Y

(g)) = X(π∗ (ad(g −1 )∗ Y ) = [Y, X]m

So combining the previous equations we get

T (X, Y ) = [Y, X]m − [X, Y ]m − [−X, Y ]m
= −[X, Y ]m

(b) Firstly let X, Y denote the left invariant vector fields generated by X, Y in G. Notice that
v([h(·), h(·)]) is a tensor on G, so v([h(X ∗ ), h(Y ∗ )]) = v([h(X), h(Y )]), where X ∗ , Y ∗
are horizontal lifts of any extensions of X, Y . As X, Y are in fact horizontal vector fields,

21

we have the following calculation.

v([h(X), h(Y )] = v([(X), (Y )])
= v([X, Y ])
= [X, Y ]k

From the above equations we get.

R(X, Y )Z = v([h(X ∗ ), h(Y ∗ )])(fZ ))
= v([h(X), h(Y )]
= [X, Y ]k (fZ )
= [X, Y ]k (π∗ (ad(g −1 )∗ Z)
=

d
(π∗ (ad(exp(−t[X, Y ]k )∗ Z))
dt

= π∗ [−[X, Y ]k , Z]
= [−[X, Y ]k , Z]m

Proposition 3.9. Let J be a G-invariant integrable complex structure on G/K. Then
a)

J = 0, where

is the canonical connection.

b) [X, Y ]m + J[JX, Y ]m + J[X, JY ]m − [JX, JY ]m = 0
Proof. a) This follows from Proposition 3.5
b) Let X, Y be two vectors at some point, we extend the vector fields in a small neighbour22

hood and compute the Nijenhuis tensor on the extension.

N (X, Y ) = [X, Y ] + J[JX, Y ] + J[X, JY ] − [JX, JY ]
=

XY −

Y X + [X, Y ]m + J

+ J X JY −

JX Y − J

JY X + J[X, JY ]m −

Y JX + J[X, Y ]m

JX JY +

JY JX − J[X, JY ]m

= [X, Y ]m + J[JX, Y ]m + J[X, JY ]m − [JX, JY ]m

(3.5)

We have used the torsion of the canonical connection (Proposition 3.8)) and part (a)
of this proposition to do the calculation. Part (b) follows from this formula since the
Nijenhuis tensor is zero if and only if the complex structure is integrable.

3.3

The Levi-Civita Connection

We use the metric ·, · to decompose the Lie algebra g. This makes G/K into a reductive
homogeneous space. Whenever we refer to the canonical connection it will refer to the
connection from this decomposition hence forth. For the rest of this work we fix the metric
·, ·

G/K

on G/K and simply denote it by ·, · .

Definition 4. We define a connection

XY =

Proposition 3.10. (a)
(b) The geodesics of

by

1

X Y + 2 [X, Y ]m

is the Levi-Civita connection for the metric ·, · .

are the same as the geodesics of the canonical connection.
23

Proof. (a) Since the torsion of the canonical connection is −[X, Y ]m , it is easy to show
that the connection

has zero torsion. Compatibility with the metric follows from

compatibility of the metric with the canonical connection and the associativity of the
bracket (3.4).
(b) For a given path γ in G/K we have

γγ =
˙˙

γ γ. The result now follows.
˙˙

Proposition 3.11. The curvature of the metric is given by

1
¯
R(X, Y )Y, X = [X, Y ]m , [X, Y ]m + [X, Y ]k , [X, Y ]k
4

Proof. Using Definition 4 and the fact that [·, ·] is an invariant tensor we have.

¯
R(X, Y )Z =
=

X

YZ−

Y

XZ −

[X,Y ] Z

1
X ( Y Z + 2 [Y, Z]m ) −

1
Y ( X Z + 2 [X, Z]m ) − ( [X,Y ] Z

+ 1 [[X, Y ], Z]m )
2
1
= R(X, Y )Z + 2 ( X [Y, Z]m −
1
+ 2 ([X,

Y Z]m − [Y,

Y [X, Z]m − [[X, Y ], Z]m )

1
X Z]m ) + 4 ([X, [Y, Z]m ]m − [Y, [X, Z]m ]m )

1
= R(X, Y )Z + 2 ([ X Y, Z]m − [ Y X, Z]m − [[X, Y ], Z]m )
1
+ 4 ([X, [Y, Z]m ]m − [Y, [X, Z]m ]m )

Using the formulas for the torsion and curvature of the canonical connection (Proposition

24

3.8), we have

¯
R(X, Y )Z = R(X, Y )Z + 1 ([ X Y, − Y X − [X, Y ], Z]m ) + 1 ([X, [Y, Z]m ]m
2
4
− [Y, [X, Z]m ]m )
1
1
= R(X, Y )Z + 2 ([−[X, Y ]m , Z]m ) + 4 ([X, [Y, Z]m ]m − [Y, [X, Z]m ]m )
1
= [−[X, Y ]k , Z]m + 2 ([−[X, Y ]m , Z]m ) + 1 ([X, [Y, Z]m ]m − [Y, [X, Z]m ]m )
4

The result now follows from the associativity of the bracket property (3.4).

3.4

Complex Gc/P

Let Gc be a complex semi-simple Lie group and let gC be the corresponding Lie algebra.
C
Let h be a Cartan subalgebra. Let ∆ ⊂ h∗ be the set of roots. Let Vα = {E ∈ gC |[h, E] =

α(h)E} denote the root space corresponding to α. We also have the following decomposition
gC = h ⊕

C
α∈∆ Vα .

For a semi-simple Lie algebra gC the roots and root spaces satisfy the following properties.
C
If α is a root then so is −α. Each Vα is one-dimensional. The root space satisfies an
C
C
C
important property [Vα , Vα ] ⊂ Vα +α , the bracket is zero if α1 + α2 is not a root and
1
2
1 2
C
C
[Vα , Vα ] ⊂ h if α1 + α2 = 0.
1
2

We can choose a base Σ ⊂ ∆ such that any element of ∆ can be uniquely written as an
integer linear combination of elements of Σ, such that all the co-efficients are either positive
or negative. A choice of such a set of roots Σ, are called simple roots. Let ∆+ , ∆− be
the set of elements of ∆ that can be written as a positive / negative linear combinations

25

respectively. ∆+ and ∆− will be referred to as positive and negative roots.
We have an inner product on the Lie algebra gC namely the Killing form κ(·, ·). κ is
associative in the sense that κ([X, Y ], Z) = κ([X, [Y, Z]) for all X, Y, Z ∈ g. It also satisfies
C
C
the following properties κ(Vα , Vα ) = 0 iff α1 + α2 = 0 and κ(Vαi , h) = 0 for i = 1, 2,
1
2

αi ∈ ∆. This inner product restricted to h is non-degenerate giving us an identification of h
and h∗ .
For a given root α we will denote its dual by tα . We also denote by hR the R linear span
of tα for α ∈ ∆+ . κ(·, ·) is positive definite on hR . Let (·, ·) be the dual of κ. Define the
structure constants cα,β by [Eα , Eβ ] = cα,β Eα+β . We state a Proposition from [H,sec 25]
C
Proposition 3.12. We can choose Eα ∈ Vα such that

(a) cα,β = −cβ,α
(b) cα,β = −c−α,−β
2tα
(c) [Eα , E−α ] = (α,α)

Eα are called root vectors, such a choice of root vectors is called a Chevalley basis[H]. As a
consequence of this choice κ(Eα , E−α ) > 0.
Let Σk ⊂ Σ and let ∆k be the set of all roots which can be written down as sums of
roots of Σk . Let ∆+ = ∆k ∩ ∆+ and ∆− = ∆k ∩ ∆− . Let p = h ⊕
k
k

V C . Let
α∈∆− ∪∆+ α
k

P be the Lie subgroup corresponding to the subalgebra p. P is a parabolic subgroup and
every parabolic subgroup is of this form, for an appropriate choice of h, ∆ and Σ. [W]
The homogeneous space V = Gc /P is a compact complex homogeneous space and can
be written as a quotient G/K where G is a compact subgroup of Gc and K = Gc ∩ P . We
now describe its Lie algebra g.
26

Let Xα = Eα − E−α and let Yα = iEα + iEα . Then g decomposes as

g = ih ⊕

Vα
α∈∆+

where Vα = spanR {Xα , Yα } is the real root space associated to α.
Since K = Gc ∩ P is the Lie algebra of K is k = ih ⊕

V , the restriction of κ
α∈∆+ α
k

to g is negative definite, hence the corresponding group G is compact. Denote by ·, · the
left-invariant metric on G such that the restriction to g is −κ|g . Since κ is associative, that
implies that ·, · is a bi-invariant metric.
Let ∆m be the complement of ∆k in ∆. Let m =

V . Using the properties of
α∈∆+ α
m

the Killing form its clear that g = k ⊕ m is an orthogonal decomposition. This makes G/K
a reductive homogeneous space. So we can identify m with Te G/K.
¯
Now we define a complex structure on this tangent space J : Te G/K → Te G/K by (note
¯
¯
that α ∈ ∆+ )
m
J(Xα ) = iYα
J(Yα ) = −Xα
We note that the complex structure we just defined, implies that JEα = iEα and
JE−α = −iE−α . This can extended to the whole G/K to give us an invariant, integrable
and hermitian complex structure.

27

3.5

The map I

In this section we define a linear operator on a subspace of g, which along with J gives us
quaternionic structure on the subspace. This structure plays an important role in the work
below.
We define a bilinear form on m by RY X = [Y, X]m + J[JY, X]m
Lemma 3.13. (a) If X, Y ∈ m then [X 1,0 , Y 1,0 ]m ∈ m1,0
(b) RY X = 0 iff [X 1,0 , Y 0,1 ]m ∈ m0,1
/
(c) If X, Y ∈ m then [X 1,0 , Y 1,0 ]k = 0.
(d) If [Y, X]k = 0 and [Y, JX]k = 0 iff [X 1,0 , Y 0,1 ]k = 0
Proof. (a) The result follows by decomposing X, Y into their {1, 0} and {0, 1} components
in the integrability condition (Proposition 3.9)
(b) After a brief calculation we find that RY X = −(Z + Z) where Z = [X 1,0 , Y 0,1 ]m −
iJ[X 1,0 , Y 0,1 ]m But Z ∈ m1,0 so Z + Z = 0 if and only if Z = 0 if and only if
[X 1,0 , Y 0,1 ]m ∈ m0,1 .
(c) It suffices to show that if α, β ∈ m the α + β ∈ k. This follows trivially from the
/
construction of m and k.
(d) Using (c) we arrive at

|[X, Y ]k |2 + |[JX, Y ]k |2 = 4 [X 1,0 , Y 0,1 ]k , [X 0,1 , Y 1,0 ]k

The result now follows from this calculation.
28

Lemma 3.14. (a) If [X 1,0 , Y 0,1 ]m ∈ m0,1 then J[Y, X]m = [JY, X]m
(b) If [X 1,0 , Y 0,1 ]m ∈ m0,1 then J[Y, X]m = −[Y, JX]m + i[Y 1,0 , X 1,0 ]m + i[Y 0,1 , X 0,1 ]m
Proof. (a) The hypothesis implies RY X = 0 from Lemma 3.13. The result now follows
directly from the definition of RY X
(b)

J[Y, X]m + [Y, JX]m = J[Y 1,0 + Y 0,1 , X 1,0 + X 0,1 ]m + i[Y 1,0 + Y 0,1 , X 1,0 − X 0,1 ]m
= i[Y 1,0 , X 1,0 ]m + [Y 0,1 , X 0,1 ]m

The last line follows since [X 1,0 , Y 0,1 ]m ∈ m0,1
For α, β ∈ ∆ we define the structure constants cα,β by [Eα , Eβ ] = cα,β Eα+β whenever
α+β =0
Lemma 3.15. Let α, β, δ ∈ ∆+ such that α + β = δ. Then

cδ,−α =

−(β, β)
c
(δ, δ) α,β

cδ,−β =

(α, α)
c
(δ, δ) α,β

Proof. By the Jacobi identity we have

[[Eα , Eβ ], E−δ ] + [[Eβ , E−δ ], Eα ] + [[E−δ , Eα ], Eβ ] = 0

29

(3.6)

Using Proposition 3.12 3), (3.6) becomes
2cα,β tδ
(δ, δ)

−

2cβ,−δ tα
(α, α)

−

2c−δ,α tβ
(β, β)

=0

(3.7)

Since tα is the dual of α. α + β = δ implies that

tδ − tα − tβ = 0

(3.8)

Using the linear independence of tα , tβ and tδ , (3.7) and (3.8) gives us

2cα,β
(δ, δ)

=

2cβ,−δ
(α, α)

=

2c−δ,α
(β, β)

Using the properties of the structure constants in Proposition 3.12, the Lemma follows.

Let cα,β =

(α,α)(β,β)c2
α,β
and let [·, ·]α,β denote the projection of the bracket on to
2
(δ,δ)

the subspace Vα ⊕ Vβ . We have following lemma
Lemma 3.16. Let X ∈ Vα ⊕ Vβ and δ = α + β. For Xδ = aXδ + bJXδ

[Xδ , [Xδ , X]α,β ]α,β = −(a2 + b2 )c2 X
α,β

Proof. Using the properties of the structure constants (Proposition 3.12) we have

[Xδ , Xα ] =[Eδ − E−δ , Eα − E−α ]
=cδ,α Eα+δ + c−δ,−α E−α−δ − c−δ,α Eβ − cδ,−α Eβ
=cδ,α Xα+δ − cδ,−α Xβ
30

So finally we have [Xδ , Xα ]α,β = −cδ,−α Xβ . Similarly we also have [Xδ , Xβ ]α,β =
−cδ,−β Xα .
Using Lemma 3.15 we have

[Xδ , [Xδ , Xα ]α,β ]α,β = −c2 Xα
α,β
[Xδ , [Xδ , Xβ ]α,β ]α,β = −c2 Xβ
α,β

Using these calculations and Lemma 3.14 we also observe that

[Xδ , [Xδ , JXα ]α,β ]α,β = −c2 JXα
α,β
[JXδ , [JXδ , Xα ]α,β ]α,β = −c2 Xα
α,β
[JXδ , [JXδ , JXα ]α,β ]α,β = −c2 JXα
α,β

Using these equations and Lemma 3.14 the lemma follows.
We can define an operator Ia,b : Vα ⊕ Vβ → Vα ⊕ Vβ by

Ia,b X =

1
1
(a2 + b2 ) 2 |c

[Xδ , X]α,β
α,β |

2
We can readily see that Ia,b = −Id and Ia,b JX = −Ia,b JX which follows from Lemma

3.14 and 3.16. Let S be a set of unordered pairs {α, β} such that α + β = δ. We can
naturally extend Ia,b to be linear operator on the subspace

Vα ⊕ Vβ

S0 =
{α,β}∈S

31

Lemma 3.17. The map Ia,b : S0 → S0 defined above satisfies the following properties
2
(a) Ia,b = −Id

(b) Ia,b J = −JIa,b
(c) For X ∈ S0 we have |Ia,b X| = |X|
(d) If X ∈ S0 then [Ia,b X, X], Xδ

1

−N0 (a2 + b2 ) 2 |X|2 where N0 is a constant only

dependant on the Lie algebra g
Proof. For the sake of convenience we refer to Ia,b as I in the proof. For (a) and (b) this
follows from the preceding discussion.
(c)

1

IX, IX =
α+β=δ

(a2 + b2 )|c

2
α,β |

−1

=
α+β=δ

(a2 + b2 )|c

2
α,β |

[Xδ , X]α,β , [Xδ , X]α,β

(3.9)

X, [Xδ , [Xδ , X]α,β ]α,β

(3.10)

Xα,β , Xα,β

=

(3.11)

α+β=δ

In the first line we use the fact that the root spaces Vα are orthogonal. In the second
line we use associativity of bracket (3.4) and later we use Lemma 3.16.

32

(d) In the computation we use the associativity of the bracket and part (c) of this lemma.

[IX, X], Xδ = − IX, [Xδ , X]
=−

IX, [Xδ , X]α,β
{α,β}∈S
1

(a2 + b2 ) 2 |cα,β ||IXα,β |2

=−
{α,β}∈S

1

(a2 + b2 ) 2 |cα,β ||X|2

=−
{α,β}∈S

1

< −(a2 + b2 ) 2 N0 |Xα,β |2

33

Chapter 4
The complex hat connection
In this Chapter we assume that G/K is equipped with an invariant integrable complex
structure J such that the normal metric is hermitian. Let M and N be two complex submanifolds of dimensions m, n respectively. Let γ : [0, 1] → G/K be a critical point to the energy
functional on the space of paths joining M and N and so it is a geodesic perpendicular to
the both manifolds at the endpoints.
If X(t) is any vector field along the geodesic γ such that X(0) and X(1) are in the tangent
space of M and N respectively then X(t) is called admissible. For an admissible vector field
X(t) we recall the second variation formula

E∗∗ (X, X) =

1
X X |0 −

1
γ X,
˙

0

˙
˙
γ X − R(γ, X)X, γ) dt
˙

Observe that if X(t) is admissible then JX(t) is admissible too, the following quadratic
form can be defined.
Definition 5. The following quantity is defined as the complex energy hessian.

1
1
C
E∗∗ (X, X) = E∗∗ (X, X) + E∗∗ (JX, JX)
2
2

34

(4.1)

Definition 6. Define the complex-hat connection

YX =

by

1

Y X + 2 RY (X)

where RY (X) = [Y, X]m + J[JY, X]m .

4.1

The second variation formula

In this section we rewrite the second variation formula.
Theorem 4.1. Suppose that X(t) is admissible and parallel with respect to the complex hat
connection. Then

C
E∗∗ (X, X) = −

11
0 2

(|Rγ (X)|2 ) + |[X, γ]k |2 + |[JX, γ]k |2 dt
˙
˙
˙

Proof. Using Proposition 2.1 we have

C
E∗∗ (X, X) =

XX +

˙ 1
JX JX, γ |0 −

1
0

γ X,
˙

− R(γ, X)X, γ) − R(γ, JX)JX, γ) dt
˙
˙
˙
˙

35

γX +
˙

γ JX,
˙

γ JX
˙

(4.2)

We begin with the boundary term at t = 0.

XX +

˙
JX JX, γ t=0 =
=

XX + J

˙
JX X, γ t=0

˙
X X + J( X JX + [JX, X] − [JX, X]m ), γ t=0

= −J[JX, X]m ), γ t=0
˙

In the first line we use the formula for the Levi-Civita connection along with fact that
J commutes with

. In the next line we use the formula for the torsion of the canonical

connection (Proposition 3.8). For the last line we observe that the variations for X, JX at
t = 0 are tangent to M hence J[X, JX] ∈ T M is tangent too. As γ is perpendicular to M ,
so J([JX, X]), γ(0) = 0.
˙
We can make a similar conclusion for t = 1 and we have the following.

XX +

˙ 1
˙ 1
JX JX, γ |0 = −J[JX, X]m , γ |0

(4.3)

The covariant derivative with respect to the canonical connection vanishes for all G
invariant tensors (Theorem 3.5). Applying this to the tensors J, [·, ·]m and the metric ·, ·
we have.

−J[JX, X]m , γ |1 =
˙ 0

1 d
0 dt
1

=
0
1

=
0

J[X, JX]m , γ dt
˙

J[ γ X, JX]m + J[X, J γ X]m , γ + J[X, JX]m , γ γ dt
˙
˙
˙
˙˙
˙
˙
γ X, [J γ, JX]m + J[J γ, X]m dt
˙

In the last line we use the associativity of the bracket (eq 3.4).

36

(4.4)

Now let us simplify the second term in equation 4.2 and write it in terms of the canonical
connection and apply the formula for the curvature (Proposition 3.11)

1
0

| γ X|2 + | γ JX|2 − R(γ, X)X, γ) − R(γ, JX)JX, γ) dt
˙
˙
˙
˙
˙
˙

(4.5)

1

=

1
1
| γ X + [γ, X]m |2 + |J γ X + [γ, JX]m |2 − R(γ, X)X, γ)
˙
˙
˙
˙
˙
˙
2
2
0

− R(γ, JX)JX, γ) dt
˙
˙
1

=
0

2| γ X|2 +
˙

(4.6)
(4.7)

˙
γ X, [γ, X]m −
˙

˙
˙ 2
˙ 2
γ X, J[γ, JX]m − |[X, γ]k | − |[JX, γ]k | dt (4.8)
˙

Now combining eq 4.3 4.4 and 4.8 in equation 4.2 we have

C
E∗∗ (X, X) =

1
0

2| γ X|2 +
˙

˙
˙
˙
˙
γ X, [γ, X]m − J[γ, JX]m + [J γ, JX]m + J[J γ, X]m
˙

− [X, γ]k [X, γ]k − [JX, γ]k , [JX, γ]k dt
˙
˙
˙
˙
1

=
0

2(| γ X|2 +
˙

˙
˙
˙
˙
γ X, [γ, X]m + J[J γ, X]m ) − [X, γ]k , [X, γ]k
˙

− [JX, γ]k , [JX, γ]k dt
˙
˙
1

=

2(
0

γ X,
˙

˙ 2
˙ 2
γ X + Rγ (X) − |[X, γ]k | − |[JX, γ]k | dt
˙
˙

Note that we use the integrability condition (Proposition 3.9) in the eq 4.9. As
1
˙
2 Rγ (X) = 0, the result follows.

37

(4.9)

γX +
˙

4.2

Properties of the complex-hat connection

Proposition 4.2. (a) The complex-hat connection commutes with the complex structure J
(b) The parallel transport along a geodesic γ with respect to the complex-hat connection
preserves orthogonality between the geodesic and the transported vector.
(c) If

γ X = 0 along a quadratic γ and Rγ (X) vanishes at t = 0 then
˙
˙

γ X = 0 along γ.
˙

Proof. (a) Using part (b) of Proposition 3.9 it is easy to show that RY (JX) = JRY (JX).
The result now follows from this observation and part (a) of Proposition 3.9.
(b) Let X(t) be a vector field along a geodesic curve γ which is parallel with respect to the
˙
complex-hat connection i.e

1
˙
γ X = − 2 Rγ (X) then
˙

d
X(t), γ(t) =
˙
dt

˙
γ X, γ + X,
˙

γγ
˙˙

˙
˙
˙
= − 1 ([γ, X]m , γ + −J[J γ, X]m ), γ + 0
2 ˙
= 1 ( X, [γ, γ]m − X, [J γ, J γ]m )
˙ ˙
˙ ˙
2
=0

In this computation we have used the associativity of the bracket (3.4) and t
(c) We can use a canonical connection parallel frame. All invariant tensors are parallel with
respect to the canonical connection (Theorem 3.5) and so is γ(t) (Proposition b). Hence
˙
with respect to this frame Rγ (X) is a linear transformation with respect to X with
˙
constant co-efficients, so parallel transport with respect to the complex-hat connection
1
˙
can be thought of as the solution to the linear ODE X(t) = − 2 R(X). From linear ODE

˙
Theory it is clear that X(t) = 0 iff R(X(0)) = 0 .
38

Theorem 4.3. The complex energy hessian of a vector field X(t) which is admissible and
parallel with respect to the complex-hat connection is negative iff [X 1,0 (0), γ 0,1 (0)] ∈ m0,1 .
˙
/
Proof. The sufficient condition follows from Theorem 4.1 and Lemma 3.13. For the necessary
condition, now suppose that [X 1,0 (0), γ 0,1 (0)] ∈ m0,1 . That implies that Rγ (X(0)) = 0 and
˙
˙
|[X(0), γ(0)]k |2 + |[JX(0), γ(0)]k |2 = 0 by Lemma 3.13. Rγ (X(0)) = 0 implies that X(t)
˙
˙
˙
is parallel with respect to the canonical connection by part 3 ) of Proposition 4.2. But that
implies that the value |[X(t), γ]k (t)|2 +|[JX(t), γ]k (t)|2 remains zero along the geodesic and
˙
˙
that implies that the complex energy hessian is zero.

39

Chapter 5
Index calculations
In this Chapter we work with the complex homogeneous space Gc /P where Gc is a
complex simple Lie group and P a parabolic subgroup. This manifold can also be written
as G/K where G is a compact Lie group and K a closed subgroup as mentioned in Chapter
section 3.4. Equip G/K with the normal metric. Let M and N be two complex submanifolds
of dimensions m, n respectively, with the dimension of G/K equal to v. Let γ : [0, 1] → G/K
be a critical point to the energy functional on the space of paths joining M and N . Thus γ
is a geodesic perpendicular to both manifolds at the endpoints. In this Chapter we will focus
on giving a lower bound on the index of each geodesic in terms of m, n, v and an invariant
of the Lie algebra g.
After applying an isometry we can assume that γ(0) = e. So we can identify Tγ(0) G/K
¯
with m. Recall that ∆ and ∆k denote the roots associated to the root system of g and k
respectively. ∆m denote the roots complementary to ∆k . Vα ⊂ g is the real root space
associated to α ∈ ∆+ . We define an ordering on ∆+ by α < δ if and only if δ − α is a
positive root. This ordering is not partial.
Define Γ = {α ∈ ∆+ |γ(0) has a non-trivial component in Vα }. Let δ ∈ Γ be a minimal
˙
element such that it also satisfies the following, α < β and β < δ implies that α ∈ Γ, we will
/
refer to such δ as superminimal.

40

Sδ = {α ∈ ∆+ |α < δ and δ − α ∈ ∆+ }
m
m

(5.1)

Tδ = {β ∈ ∆+ |β ≥ δ} ∪ {α ∈ ∆+ |δ − α ∈ ∆k }
m
m

(5.2)

To derive an optimal lower bound on the index we impose the following conditions on
the sets Sδ and Tδ . These conditions will be shown to be true if g is a simply laced Lie
algebra i.e Lie algebras of the type A, D and E in Chapter . In the non-simply laced case
the conditions are satisfied for most δ. For the remaining choices of δ the arguments given
below can be modified.
Condition 1. If β0 , β1 ∈ Tδ \{δ} with β0 = β1 then β0 − δ = β1 − λ for λ ∈ Γ
Condition 2. If α, β ∈ Sδ then α + β ∈ Γ iff it is equal to δ.
Observe that if α < δ then α, δ − α cannot both lie in ∆+ from the construction of k, it
k
follows that |Sδ | is even. Let

1
1
= 2 |Sδ | + |Tδ | and let h = 2 |Sδ |.

The main goal of this Chapter is to prove the following theorem.
Theorem 5.1. If Conditions 1 and 2 are satisfied then the index of the geodesic γ is at least
I = m + n − (v − ) − v + 1
To prove this theorem we will construct a vector space of dimension 4I. We will use a
quaternionic structure on this space and show that there exists a subspace of dimension I
such that the hessian E∗∗ is negative definite.
Let
S0 =

Vα ,
α∈Sδ

T0 =

Vβ
β∈Tδ

41

The spaces S0 , T0 are invariant under J and are of complex dimension 2h and

−h

respectively. Let U0 = S0 ⊕ T0 and τ : Tγ(0) (G/K) → Tγ(1) (G/K) denote the parallel
translation with respect to the complex-hat connection. Let

U = {X ∈ U0 ∩ Tγ(0) M |τ (X) ∈ Tγ(1) (N )}

.
Proposition 5.2. The minimum complex dimension of U is I + h.
Proof. Since the dimension of U0 is + h the minimum dimension of U0 ∩ Tγ(0) M is + h +
m − v. It is clear that the

dimC U = dimC (τ (U0 ∩ Tγ(0) M )) ∩ Tγ(0) M

Since both Tγ(0) M , Tγ(1) N are perpendicular to γ and the complex-hat parallel transport
preserves orthogonality with γ (part (b) of Proposition 4.2) we can get an extra dimension
in the count. So the dimension of U is m + + h − v + n − v + 1.

Let γδ (0) = aXδ + bJXδ denote the the Vδ component of γ(0). Lemma 3.17 gives us a
˙
˙
linear operator Ia,b : S0 → S0 satisfying the following properties, I 2 = −Id and IJ = −JI
(from now on we omit the subscript).
Let S1 = S0 ∩U and let dimC S1 = s1 . Using the properties of I we get that the subspace
S = S1 ∩IS1 is a closed under I and J. We see that dimC S ≥ 2(s1 −h) whenever s1 > h else
S is the trivial space. Now let T be the orthogonal complement of S1 in U and t = dimC T .
We observe that as s1 + t ≥ I + h, t + 1 s ≥ I.
2
42

Let S, T and U be the space of vector fields parallel with respect to the complex-hat
connection starting from S, T and U respectively. We will use Theorem 4.3 to show that the
index of E

∗∗|T

is at least t. For S Theorem 4.3 does not yield anything, instead one must

take suitable linear combinations of vector fields in S. The index of E∗∗ restricted to these
s
linear combinations is 2 .

5.1

The variations T

C
Proposition 5.3. If Z(t) ∈ T then E∗∗ (Z, Z) < 0

Proof. By Theorem 4.3 it suffices to show that [Z(0)1,0 , γ 0,1 (0)] ∈ m0,1 . Since Z(0) ∈ T we
˙
/
can write Z(0) = X + Y where X ∈ S and Y ∈ T . As [X 1,0 , γ 0,1 (0)] ∈ m0,1 it suffices to
˙
prove that [Y 1,0 , γ 0,1 (0)] ∈ m0,1 .
˙
/
Suppose Y 1,0 = Σβ∈T cβ Eβ with cβ = 0 , where β0 ∈ Tδ \{δ} then condition 1 implies
0
δ
that the co-efficient of Eβ −δ is non-zero in [Y 1,0 (0), γ 0,1 (0)]. As either β0 − δ > 0 or
˙
0
|β0 − δ| ∈ ∆+ , implies that Eβ −δ ∈ m0,1 .
/
k
0
If cδ is non-zero and cβ = 0 , for all β0 ∈ Tδ \{δ} that implies that [Y 1,0 , γ 0,1 (0)]m ∈
˙
/
0
m0,1 has a non-trivial component in h.
Theorem 5.4. The index of γ when restricted to T is t.
Proof. T has a natural complex structure J on it, since

commutes with J. To show that

the index is t, it suffices to show that every t + 1 dimensional subspace of the 2t dimensional
space T has an element X such that E∗∗ (X, X) < 0. Let W be such a subspace, then
W ∩ JW is non-empty. Let X ∈ W ∩ JW , then from Proposition 5.3 we know that the
complex energy hessian E∗∗ (X, X) + E∗∗ (JX, JX) < 0. Since both X, JX ∈ W ∩ JW the
result follows.
43

5.2

The variations S

If α ∈ Sδ then α < δ and by the choice of δ that implies that either α < λ or α is not
comparable with λ for λ ∈ Γ. If X(t) ∈ S then X(0) ∈ S which implies [X 1,0 , γ 0,1 (0)]m ∈
˙
m0,1 . Hence Rγ (X(0)) = 0 so Theorem 4.2 gives us that X is parallel to the canonical
˙
connection.
t
Let τ0 : Tγ(0) G/K → Tγ(t) G/K denote parallel translation with respect to the canonical

connection. We have I : S → S where S ⊂ Tγ(0) M , using parallel translation we can also
t
t
˙
define It : τ0 (S) → τ0 (S). Since the invariant tensors J and [·, ·]m , the vector fields γ(t) and

X(t) are all parallel the properties of Lemma 3.17 carry over to It . Using this, we can now
define a natural operation I on the vector fields S.
We define
Sk = {Z| γ Z = −kIZ and Z(0) ∈ S}
˙
¯
¯
¯
¯
On Sk we can define operations I and J. For Z ∈ Sk we have (IZ)(0) = I(Z(0)), (JZ)(0) =
¯
¯
¯¯
¯¯
J(Z(0)). We observe that I 2 = −Id,J 2 = −Id and I J = −J I.
Proposition 5.5.

¯ ¯
¯ ¯
¯¯ ¯¯
E∗∗ (Z, Z) + E∗∗ (IZ, IZ) + E∗∗ (JZ, JZ) + E∗∗ (I JZ, I JZ) < 0 for sufficiently small k
(5.3)

44

Proof. Let X ∈ S such that X(0) = Z(0). It is easy to verify that

Z = cos(kt)X − sin(kt)IX

(5.4)

¯
IZ = sin(kt)X + cos(kt)IX

(5.5)

¯
JZ = cos(kt)JX + sin(kt)JX

(5.6)

¯¯
I JZ = − cos(kt)JIX + sin(kt)JX

(5.7)

We see that all these vector fields are admissible since X and IX are. We begin by analysing
E∗∗ (Z, Z)

E∗∗ (Z, Z) =

˙ 1
Z Z, γ |0 +

=

˙ 1
Z Z, γ |0 +

=

˙ 1
Z Z, γ |0 +

1
0
1

| γ Z|2 − R(γ, Z)Z, γ dt
˙
˙
˙

1
1
˙
˙
˙
| γ Z + [γ, Z]m |2 − |[γ, Z]m |2 − |[γ, Z]k |2 dt
˙
2
4
0
1
0

k 2 |IZ|2 − k IZ, [γ, Z]m − |[γ, Z]k |2 dt
˙
˙

If α, β ∈ Sδ then from condition 2 we have that α+β ∈ Γ\{δ} and since δ is superminimal,
/
we have that η < α implies η ∈ Γ. So we have α − β ∈ Γ. We can now conclude that
/
/
[Z, IZ]m , γ(0) − γδ (0) = 0. So for sufficiently small k we have
˙
˙

45

E∗∗ (Z, Z) =

˙ 1
Z Z, γ |0 +

<

˙ 1
Z Z, γ |0 +

<

1
0
1
0

k 2 |IZ|2 − k IZ, [γδ , Z]m − |[γ, Z]k |2
˙
˙
1

k 2 |IZ|2 − kN0 (a2 + b2 ) 2

|Xα,β |2 − |[γ, Z]k |2
˙
α+β=δ

˙ 1
Z Z, γ |0

(5.8)

To get the inequality we use part 3 of Lemma 3.17. We can get a similar calculation for
¯ ¯
¯ ¯
¯¯ ¯¯
E∗∗ (IZ, IZ), E∗∗ (JZ, JZ) and E∗∗ (I JZ, I JZ). We now calculate the corresponding boundary terms.

˙
Z Z, γ +
=

¯ ¯ ˙
JZ JZ, γ +

˙
X X, γ +

¯ ¯ ˙
IZ IZ, γ +

˙
JX JX, γ +

1
¯¯ ¯¯ ˙
I JZ I JZ, γ |0

˙
IX IX, γ +

˙ 1
IJX IJX, γ |0

= [−JX, X]m , γ |1 + [−JIX, IX]m , γ |1
˙ 0
˙ 0
=0

(5.9)

For the last line we observe that X, [·, ·]m and J are parallel to the canonical connection,
which implies that [−JX, X]m , γ and [−JIX, IX]m , γ are independent of t.
˙
˙
Using the inequality 5.8 and eqn 5.9 the proposition is proved.

s
Theorem 5.6. For sufficiently small k the index of γ when restricted to Sk is 2 .
s
Proof. The real dimension of Sk is 2s, so its suffices to show that for any 3 2 + 1 real

dimensional subspace W , there exists Z ∈ Sk such that E∗∗ (Z, Z) < 0. By the properties
¯
¯
¯ ¯ ¯¯
of I and J we know that there exist Z ∈ W such that Z, IZ, JZ, I JZ belong to W . From
46

Proposition 5.5 the Theorem follows .

5.3

Reconciling Sk and T

In the Theorems 5.4 and 5.6 we have shown the existence of two subspaces of dimension t
s
and 2 belonging to T and Sk such that E∗∗ is negative definite. But that is not sufficient
s
to show that the index is I = t + 2 . We will tackle this problem by twisting the space T .

Let Tk = {Z|Z = cos(kt)X − sin(kt)Y with X, Y ∈ T }. From equations 5.4 to 5.7 we
observe that Sk = {Z|Z = cos(kt)W − sin(kt)IW with W ∈ S}. We now define Uk =
Sk

Tk . So if Z ∈ Uk then Z = cos(kt)(X) − sin(kt)(Y ) for some X, Y ∈ U .
¯
We can define two operations on Uk , IZ = sin(kt)(X + W ) + cos(kt)(Y + IW ) and

¯
JZ = cos(kt)J(X + W ) + sin(kt)J(Y + IW ). Observe that Uk is closed under the two
¯
¯
¯
¯
operations I and J. It can be easily verified that I 2 = −Id and J 2 = −Id and that
¯¯
¯¯
¯
¯
I J = −J I. From eqns 5.4 to 5.7, we see that the definitions of I and J coincide.
C
Recall that the complex energy hessian E∗∗ (X, X) = E∗∗ (X, X) + E∗∗ (JX, JX). For

¯ ¯
¯ ¯
¯¯ ¯¯
Z ∈ Uk we define Q(Z) = E∗∗ (Z, Z) + E∗∗ (IZ, IZ) + E∗∗ (JZ, JZ) + E∗∗ (J IZ, J IZ)
Now we are ready to prove an important proposition.
Proposition 5.7. There exists a k > 0 such that for any Z ∈ Uk , Q(Z) < 0
We first state and prove a few lemmas
1
˙
Lemma 5.8. If X ∈ T then there exists M > 0 such that 2 ||Rγ (X(t)|2 + |[X(t), γ]k |2 +
˙

|[JX(t), γ]k |2 > M |X(t)|2 for any t ∈ [0, 1] where X(t) is a vector field which is parallel with
˙
respect to the complex-hat connection with initial value X. As a consequence E∗∗ (X, X) +
1
E∗∗ (JX, JX) > M 0 |X(t)|2 dt

47

Proof. For X ∈ T we know from the proof of the above Propsition 5.4 that either 1 |Rγ (X(0)|
2 ˙
= 0 or |[X(0), γ]k |2 + |[JX(0), γ]k |2 ) = 0. If the first case is true then we know from
˙
˙
Proposition 4.2 part (c) that Rγ (X(t)) = 0 for all t. If the first case were not true then
˙
the vector field X is parallel with respect to the canonical connection and so |[X(t), γ]k |2 +
˙
|[JX(t), γ]k |2 is the same for all t because |[·, ·]k | is an invariant tensor. But at t = 0 we
˙
know that this quantity is non-zero and hence it is non-zero for all t. As the interval [0, 1] is
compact the lemma follows.
Lemma 5.9. If Z = cos(kt)X − sin(kt)Y with X, Y ∈ U then
1

C
C
Q(Z) = E∗∗ (X, X) + E∗∗ (Y, Y ) +

0

2k 2 |X|2 + 2k 2 |Y |2 + 2k [Y, X]m − [JY, JX]m , γ dt
˙

Proof. Let us begin by rewriting the second variational form as the sum of two quadratic
forms for ease of computation. Now

E∗∗ (X, X) =

˙ 1
X X, γ |0 +

=

˙ 1
X X, γ |0 +

=

˙ 1
X X, γ |0 +

1
γ X,
˙

0
1

˙
˙
γ X − R(γ, X)X, γ) dt
˙

1
1
| γ X + [γ, X]m |2 − |[X, γ]m |2 − |[X, γ]k |2 dt
˙
˙
˙
˙
2
4
0
1

0

| γ X|2 +
˙

˙
˙ 2
γ X, [γ, X]m − |[X, γ]k | dt
˙

= H(X, X) + G(X, X)

Where H(X, X) =

1
˙ 1
˙ 2
X X, γ |0 − 0 [X, γ]k | &

1
G(X, X) = 0 | γ X|2 +
˙

˙
γ X, [γ, X]m dt We observe that H(·, ·) is linear with respect to
˙

48

differentiable functions and a simple calculation gives us

¯ ¯
H(Z, Z) + H(IZ, IZ) = H(X, X) + H(Y, Y )

(5.10)

¯ ¯
So we focus on G(Z, Z) + G(IZ, IZ). We start by simplifying

¯
| γ Z|2 + | γ IZ|2 = | γ X|2 + | γ Y |2 + k 2 |X|2 + k 2 |Y |2 − 2k cos(kt) γ X
˙
˙
˙
˙
˙
¯
− sin(kt) γ Y, IZ + 2k sin(kt) γ X + cos(kt) γ Y, Z
˙
˙
˙
= | γ X|2 + | γ Y |2 + k 2 |X|2 + k 2 |Y |2 − 2k cos2 (kt) γ X, Y
˙
˙
˙
− 2k sin2 (kt) γ X, Y + 2k cos2 (kt) γ Y, X + 2k sin2 (kt) γ Y, X
˙
˙
˙
= | γ X|2 + | γ Y |2 + k 2 |X|2 + k 2 |Y |2 − 2k
˙
˙

γ X, Y + 2k
˙

γ Y, X
˙

(5.11)
and similar computation leads us to

˙
γ Z, [γ, Z]m +
˙

¯ ˙ ¯
γ IZ, [γ, IZ]m =
˙

˙
γ X, [γ, X]m +
˙

˙
˙
γ Y, [γ, Y ]m + 2k [Y, X]m , γ
˙
(5.12)

From equation 5.11 and 5.12 we arrive at

¯ ¯
G(Z, Z) + G(IZ, IZ) = k

1

k|X|2 + k|Y |2 − 2

0

+ G(X, X) + G(Y, Y )

γ X, Y + 2
˙

˙
γ Y, X + 2 [Y, X]m , γ dt
˙
(5.13)

Equations 5.10 and 5.13 give us

49

¯ ¯
E∗∗ (Z,Z) + E∗∗ (IZ, IZ)
1

=E∗∗ (X, X) + E∗∗ (Y, Y ) +

k 2 |X|2 + k 2 |Y |2 − 2k

0

γ X, Y + 2k
˙

γ Y, X
˙

+ 2k [Y, X]m , γ
˙

(5.14)

¯
¯¯
Keeping in mind that JZ = cos(kt)JX + sin(kt)JY and J IZ = cos(kt)JY − sin(kt)JX
we can use equation 5.14 to obtain

¯ ¯
¯¯ ¯¯
E∗∗ (JZ, JZ) + E∗∗ (I JZ, I JZ)
1

= E∗∗ (JX, JX) + E∗∗ (JY, JY ) +

k 2 |JX|2 + k 2 |JY |2 − 2k

0

˙
γ J(−Y ), JX + 2k [J(−Y ), JX]m , γ dt
˙
1
= E∗∗ (JX, JX) + E∗∗ (JY, JY ) +
k 2 |JX|2 + k 2 |JY |2 + 2k
0

γ JX, J(−Y )
˙

+ 2k

− 2k

γ X, Y
˙

˙
γ Y, X − 2k [JY, JX]m , γ dt
˙

¯ ¯
¯ ¯
The result follows from adding the formulas for E∗∗ (Z, Z)+E∗∗ (IZ, IZ) and E∗∗ (JZ, JZ)+
¯¯ ¯¯
E∗∗ (J IZ, J IZ)
Lemma 5.10. Let P (X(t), Y (t)) = [Y (t), X(t)]m − [JY (t), JX(t)]m , γ
˙
(a) There exists a N such that P (X(t), Y (t)) < N |X(t)||Y (t)| for X, Y ∈ Uk ∀ t ∈ [0, 1]
1

(b) P (W (t), IW (t)) < −2kN0 (a2 + b2 ) 2 )|W (t)|2
Proof. (a) Clear.
(b) Since all the objects we deal with are parallel with the canonical connection, it suffices to
50

prove it in the case t = 0. It is easy to see that [W (0), IW (0)]m − [JW (0), JIW (0)]m =
4Re[W 1,0 , IW 1,0 ]m . Now IW, W ∈ S0 implies that [W 1,0 , IW 1,0 ]m ∈ Vδ via condition
2. As a consequence [W (0), IW (0)]m − [JW (0), JIW (0)]m , γ(0) − γδ (0) . The result
˙
˙
now follows form (c) of Lemma 3.17.

Proof. of Proposition

5.7 If Z ∈ Uk there exists X, Y ∈ T and W ∈ S so that Z =

cos(kt)(X +W )−sin(kt)(Y +IW ). Note that Rγ W (0) = Rγ (IW (0)) = 0, so from eqn 4.9 of
˙
˙
C
C
C
C
Theorem 4.1 we get that E∗∗ (X, X) = E∗∗ (X +W ) and E∗∗ (Y, Y ) = E∗∗ (Y +IW, Y +IW )).

So now we can rewrite Lemma 5.9.

C
C
Q(Z) = E∗∗ (X, X) + E∗∗ (Y, Y ) +

1

2k 2 (|X + W |2 + |Y + IW |2 ) + 2k(P (X, Y )

0

+ P (X, IW ) + P (W, Y ) + P (W, IW ))dt

Using part (a) and part (b) of Lemma 5.10 we have

C
C
Q(Z) < E∗∗ (X, X) + E∗∗ (Y, Y ) +

1

4k 2 (|X|2 + |Y |2 ) + 2kN (|X||Y | + |X||W | + |W ||Y |)

0

1

+ (8k 2 − 4k(a2 + b2 ) 2 )|W |2 dt

51

1

We choose 2 N = 2(a2 + b2 ) 2 and apply the AM-GM inequality.

C
C
Q(Z, Z) < E∗∗ (X, X) + E∗∗ (Y, Y ) +

+

1

(4k 2 + kN )(|X|2 + |Y |2 ) + kN (

0

|X|2
2

+ 2 |W |2

|Y |2

1
+ 2 |W |2 ) + (8k 2 − 4k(a2 + b2 ) 2 )|W |2 dt
2
1

<

1
kN
(4k 2 + kN + 2 )(|X|2 + |Y |2 ) + k(8k − 2(a2 + b2 ) 2 )|W |2 dt
0

C
C
+ E∗∗ (X, X) + E∗∗ (Y, Y )

Finally applying Lemma 5.8 we conclude that.
1

Q(Z) =

1
kN
(4k 2 + 2kN + 2 − M )(|X|2 + |Y |2 ) + k(8k − 2(a2 + b2 ) 2 )|W |2 dt
0

¯
It is clear that k and can be chosen small enough so that Q(Z) is negative for all Z ∈ Uk .
Proof. of Theorem 5.1 To prove this we demonstrate the existence of an I dimensional
subspace of the variation vector fields Uk such that E∗∗ is negative definite. We recall that
the real dimension of Uk is 4I. To show that the index is I all we have to do is show that
¯
for every 3I + 1 real subspace W there exists a vector field of negative index. As J 2 = −Id
that implies that in every 3I + 1 dimensional real space we can find a 2I + 2 dimensional
¯
¯
¯¯
¯¯
space which is closed under J. Now I 2 = −Id and I J = −J I forces the existence of a 4
¯
¯
dimensional space which is closed under both J and I. So we can find a vector field Z such
¯ ¯
¯¯
that JZ IZ and J IZ belong to W . By Theorem 5.7 we know that Q(Z) < 0 for uniform k
¯ ¯
¯¯
so the hessian is negative for one the following vector fields Z, JZ, IZ and J IZ

52

Chapter 6
Lie algebra calculations
From Theorem 5.1 have shown that the index of a geodesic is I = m + n − (v − ) − v + 1
where n, m are the dimensions of the submanifolds M , N and

= 1 |Sδ | + |Tδ |, if the roots
2

satisfy certain conditions. For the simply laced Lie algebras Ar ,Dr and Er we will show that
these conditions are satisfied. For simple Lie algebra Br these conditions are not satisfied
when δ is a short root. In this case we explicitly work with the roots and get over this
handicap.
Recall that ∆ is the set of roots for a simple Lie algebra g. Here g is not the algebra G2 .
Define Wδ = {α ∈ ∆|δ − α is a root}. Let (·, ·) be the dual of the Killing form on the Lie
algebra g restricted to h∗ . Normalize (·, ·) so that the inner product corresponding to the
R
root system ∆ of g is normalized so that the length of each long root is 2.
In this chapter we refer the reader to [H] for all standard results on root systems and Lie
algebras.
Lemma 6.1. Suppose δ and α are roots such that δ is long then (α, δ) > 0 is (α, δ) = 1
(α,δ)

Proof. Since (α, δ) > 0 the value 2 (α,α) is either 1 or 2. Using table 1 from [H,sec 9.4] it is
(α,δ)

clear that 2 (α,α) is 1 if α is long and 2 if α is short. In either case it is easy to verify that
(α, δ) = 1.

53

Lemma 6.2. If δ ∈ ∆ is a long root then, for η1 , η2 ∈ Wδ , η1 + η2 is a root if and only if
it is equal to δ.
Proof. Since the Weyl group is transitive on the set of long roots it suffices to prove this in
the case where δ is the highest root vector.
Let the α-chain of roots through δ be

−pα + δ, . . . , δ, . . . qα + δ

where p, q ≥ 0

(α,δ)

Then it is well known that p − q = 2 (α,α) . Since δ is the highest root we have q = 0.
Since α ∈ Wδ we have that p > 0 making (α, δ) > 0. From Lemma 6.1 (α, δ) = 1. So if
η1 , η2 ∈ Wδ then (η1 + η2 , δ) = 2. So if η1 + η2 is a root then it has to be equal to δ.

Recall that

Sδ = {α ∈ ∆+ |α < δ and δ − α ∈ ∆+ }
m
m
Tδ = {β ∈ ∆+ |β ≥ δ} ∪ {α ∈ ∆+ |δ − α ∈ ∆k }
m
m

Proposition 6.3. If δ is a long root then condition 1 and condition 2 are satisfied.
Proof. We first recall condition 1 and 2
Condition 1. If β0 , β1 ∈ Tδ \{δ} with β0 = β1 then β0 − δ = β1 − λ for λ ∈ Γ
Condition 2. If α, β ∈ Sδ then α + β ∈ Γ iff it is equal to δ.
For condition 1, begin by assuming that β0 − δ = β1 − λ. So λ = β1 + δ − β0 is a
root. Observe that β1 and δ − β0 belong to Wδ . So by the Lemma 6.2 the only way that
54

β1 + δ − β0 is a root is if it is equal to δ, that means β1 = β0 . A contradiction and hence
the lemma readily follows. Condition 2 follows trivially.
Proposition 6.4. If the root system associated to gC is such that all roots are long, the
value

= 1 |Sδ | + |Tδ | is independent of δ and only dependent on the lie algebra gC . The
2

values of

are given below

(a) If gC = slr+1 (C) then
(b) If gC = so2r (C) then

=r
= 2r − 3

(c) If gC = E6 ,E7 ,E8 then

= 11, 17 and 29 respectively

Proof. We begin by showing that

is independent of δ and later calculate ' 'by choosing δ

appropriately.
Define Wδ = {{α, β}|α + β = δ, α, β ∈ ∆}. It is clear that |Wδ | + 1 = 1 |Sδ | + |Tδ |. Since
2
the Weyl group acts linearly |Wδ | is preserved by the Weyl group. As these Lie algebras are
simply-laced, it is well known that the Weyl group acts transitively on the roots [H] therefore
|Wδ | is independent of δ. We observe that |Wδ | = 2|Wδ | and calculate |Wδ | for a suitable
root δ in each case. We refer to [H] for a description of the respective root systems that we
will use.
Ar : The roots are given by ∆ = {ei − ej |1 ≤ i, j ≤ r + 1} where ei ∈ Rr+1 are
the standard basis. Let us choose δ = e1 − e2 . Then it is clear that Wδ = {e1 − ej |2 <
j} ∪ {ek − e2 |k ≤ r + 1}. Thus |Wδ | = 2r − 2,so

=r

Dr : The roots are given by ∆ = {±ei ± ej |i = j, 1 ≤ i, j ≤ r + 1} where ei ∈ Rr+1
are the standard basis. Choose δ = e1 − e2 . we then have that Wδ = {e1 ± ej , ek ± e2 |2 <
j, k ≤ r}. So |Wδ | = 4r − 8
55

E8 :The roots are given by

∆ = {±ei ± ej |i = j, and 1 ≤ i, j ≤ 8} ∪


1 8
2

8

ti ei |ti = ±1,

i=1

i=1



ti = 1


where ei ∈ R8 . Choose δ = e1 − e2 . A little computation gives us that

Wδ = {e1 ± ej |2 < j} ∪ {ek ± e2 |k ≤ 8} ∪

So we have |Wδ | = 56 and so


1
2

(e1 − e2 ) +

1
2

8

8

ti ei |ti = ±1,
i=3

i=3



ti = 1


= 29. A similar calculation for E6 and E7 can be made.

Proof of Theorem 1.1
Parts i,ii and v
In Lie algebras of type A, D and E all roots are long, so parts i, ii and iv follow from
Theorem 5.1, Proposition 6.3 and Propsition 6.4.
Part iii
Referring back to [H] we see that the roots of Br are given by

∆ = {±ei ± ej |1 ≤ i, j ≤ r} ∪ {±ei |1 ≤ i ≤ r}

where ei are the standard basis of Rr . The positive roots are given by

∆+ = {ei + ej |1 ≤ i, j ≤ r} ∪ {ei − ej |1 ≤ i < j ≤ r} ∪ {ei |1 ≤ i ≤ r}.

Recall that given a Γ ⊂ ∆+ we chose δ ∈ Γ to be a superminimal element i.e a minimal
element such that if α < β and β < δ then α ∈ Γ.
/
56

Suppose we can choose a superminimal element δ such that it is a long root. In this
case we can repeat the arguments of the the simply laced case, using Proposition 6.3 and
Theorem 5.1. We can make similar calculations and conclude that

= 2r − 2.

Suppose no superminimal elements are long. In this case δ = ei where i is the largest
index such that ei ∈ Γ. Observe that el + ej ∈ Γ for i < l < j and that el − ej ∈ Γ for l < j.
/
/
For if any of these statements were not true it would imply that there exists a superminimal
long root.
We now claim that if ek ∈ ∆k for i < k ≤ r then δ = ei satisfies conditions 1 and
/
2. To verify the claim we argue as follows. It is clear that Sδ is a subset of the following
{ei −el |i < l}∪{el | i < l}. Thus condition 2 can be easily verified. Let Uδ = {β ≥ δ|β ∈ ∆+ }
m
and let Vδ = {α ∈ ∆+ |δ − α ∈ ∆+ }. Then Tδ = Uδ ∪ Vδ . We can clearly see that
m
k
Uδ = {ea |a < i} ∪ {ei + ea | a < i}. Due to the assumption that ek ∈ ∆k , we have
/
Vδ = {eb | i < b, ei − eb ∈ ∆+ }. Thus
k

Tδ = Uδ ∪ Vδ = {ea |a < i} ∪ {ei + ea | a < i} ∪ {eb | i < b, ei − eb ∈ ∆+ }
k
Note that if β1 , β2 ∈ Tδ then β1 + δ − β2 is a positive root if and only if its equal to δ or of
the form es − et where s < t. To see this note that

{δ − β|β ∈ Tδ } = {−(ea − ei )|a < i} ∪ {−ea |a < i} ∪ {ei − eb | i < b, ei − eb ∈ ∆+ }.
k

We have already observed that roots of the form es − et does not belong to Γ with s < t, we
now see that condition 1 is satisfied thereby verifying the claim.
To finish the proof we assume that ek ∈ δk for k such that i < k. Let gt = exp(tXek )
where Xek is a root vector for the root ek . Since Xek ∈ K that implies that gt ∈ K. Then
57

the left translation Lgt is an non-trivial isometry as well as a biholomorphism on the space
G/K, that fixes the base point e. So it suffices to study the index of the geodesic Lgt ∗ γ.
¯
˙
For small values of t the set Γ associated to the geodesic will contain the long root ei − ek
by the Baker-Campbell-Hausdorff formula. So δ can be chosen in the form ep − eq where
p < q. We can then use the arguments above to prove the Theorem in the main case.
Parts iv,vi and vii The proofs of the remaining cases are similar and details will appear in
a forthcoming paper.

58

BIBLIOGRAPHY

59

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61