# First-Order Twistor Lifts

- Bruno Ascenso Simões
^{1, 2}Email author

**2010**:594843

https://doi.org/10.1155/2010/594843

© Bruno Ascenso Simões. 2010

**Received: **30 December 2009

**Accepted: **30 March 2010

**Published: **7 April 2010

## Abstract

The use of twistor methods in the study of Jacobi fields has proved quite fruitful, leading to a series of results. L. Lemaire and J. C. Wood proved several properties of Jacobi fields along harmonic maps from the two-sphere to the complex projective plane and to the three- and four-dimensional spheres, by carefully relating the infinitesimal deformations of the harmonic maps to those of the holomorphic data describing them. In order to advance this programme, we prove a series of relations between infinitesimal properties of the map and those of its twistor lift. Namely, we prove that isotropy and harmonicity to first order of the map correspond to holomorphicity to first order of its lift into the twistor space, relatively to the standard almost complex structures and . This is done by obtaining first-order analogues of classical twistorial constructions.

## 1. Introduction

A bibliography can be found in [1] and for some useful summaries on this topic, see [2, 3].

*Jacobi fields*. More precisely, let be a smooth map and denote by the set of smooth sections of the pull-back bundle . If is harmonic and is a vector field along it, is said to be a

*Jacobi field*(along ) if it satisfies the linear

*Jacobi equation*, where the

*Jacobi operator*is defined by

*Hessian*of is the bilinear operator on given by

A Jacobi field is called *integrable* if it is tangent to a deformation through harmonic maps. In [4, 5], the question of whether all Jacobi fields are integrable is answered for the case where the domain is the two-sphere and the codomain the two-dimensional complex projective space or the three- and four-dimensional sphere. This was done by relating the deformations of the map associated with the Jacobi field and those of the twistor lift of the map. More precisely, given an oriented even-dimensional manifold
, we can construct its (positive) twistor space
. This manifold admits two natural almost complex structures
and
. Given a map
from a Riemann surface
, harmonicity is intimately related with the existence of a
-holomorphic lift
, whereas isotropy is related with the existence of a
-holomorphic lift
(see [6]). On the other hand, Jacobi vector fields induce families of maps which are harmonic to first-order and, in some cases, isotropic to first order. The translation of these first order properties in terms of twistor lifts plays an important role on the study of the Jacobi fields and we shall exhibit how this translation can be established in general.

This work is divided as follows: in the next two sections, we recall some well-known results concerning twistor lifts of harmonic and isotropic maps. In Section 4, we show how this constructions generalize to their parametric versions and examine more closely the construction when the codomain is a four-dimensional manifold. We leave to the last section some technical proofs.

## 2. The Setup

### 2.1. Twistor Spaces

*Hermitian structure*on is with and for all . We say that is

*positive*if there is a positive basis of of the form and

*negative*otherwise. The set of all positive Hermitian structures (resp., all negative Hermitian structures) on is denoted by (resp., ). The Lie group acts transitively on by the formula and the isotropy subgroup at is given by

When equipped with , the manifold is a complex manifold.

These are *isotropic* subspaces, in the sense that
for all
in
(or in
). Associating an Hermitian structure
on
with its
-space
gives a 1–1 correspondence between Hermitian structures and maximal isotropic subspaces. We say that a maximal isotropic subspace is *positive* if the corresponding orthogonal complex structure is positive and we denote the set of all such subspaces by
.

*(positive) twistor space*of the bundle whose fibre at is precisely ; that is,

*vertical*and

*horizontal*parts, . Namely, if denotes the canonical projection defined by , then

When equipped with , is never a complex manifold; as for , it is integrable if and only if is conformally flat ( ) or anti-self-dual ( ) (for more details, see [6, 9, 10]; a discussion on this topic can also be found in [11] and references therein).

Definition 2.1.

*-stable subbundles*; that is, and . We shall call such a decomposition a -

*stable decomposition*. Let be a smooth map. We shall say that is

*-holomorphic*if

Analogously we define
*-holomorphic maps*.

A smooth map is holomorphic if and only if it is both and -holomorphic for some, and so any, stable decomposition . Taking , the decomposition is clearly stable for both the almost complex structures and on .

Remark 2.2.

We can easily introduce a metric on the twistor space : let and consider the tangent space at this point, . We know that we have the identifications and . To get a metric on , transport the metric from that on ; that is, , for all , , where denotes the metric on at . For the vertical space , we can consider the restriction of the metric on the space . Finally, we declare and to be orthogonal under the metric ; that is, , for all , . With this metric, the decomposition is orthogonal and -stable ( ), ( ) are almost Hermitian manifolds and the projection map is a Riemannian submersion.

### 2.2. Conformal and Isotropic Maps

If
, then
is said to be a *regular point* (of
) and the map
is called *conformal* at
. Moreover, a map which is conformal (resp., weakly conformal) at all points
is said to be a *conformal map* (resp., a *weakly conformal map*).

*isotropic*if [12]

Since and , both terms in the above expression vanish.

Moreover, letting in (2.13), it is easy to check that an isotropic map from a Riemann surface is a (weakly) conformal map.

Let
be a smooth map from a Riemann surface
. We shall say that
is an *umbilic point* (of
) if
is a
-linearly dependent set. If
is such that all points
are umbilic, we shall say that
is *totally umbilic* (see [6]).

## 3. Nonparametric Twistorial Constructions

The following are well-known twistorial constructions [13] (see also [6]).

Theorem 3.1.

If is holomorphic, the projection map is isotropic. Conversely, if is a conformal totally umbilic immersion, there is (locally) a holomorphic map such that .

If is holomorphic, the projection map is harmonic. Conversely, if is a conformal harmonic map, there is (locally) a holomorphic map such that .

We shall sketch the proof of this result. We start by noticing that given a smooth map
obtained as the projection of
,
, without requiring further conditions *à priori* on
, nothing guarantees that
is holomorphic relatively to the induced almost Hermitian structure
on
; if it is, we shall say that the structure
is *strictly compatible* with
(or that the map
is a *strictly compatible twistor lift* of
). Such a structure
can exist if and only if
is isotropic: in other words, if and only if
is (weakly) conformal. If
preserves
but does not necessarily render
holomorphic, we shall say that
(or the map
) is *compatible* with
.

Taking any holomorphic section of and composing with , we obtain a strictly compatible twistor lift of . Since any such lift is -holomorphic, we have just proved the following result.

Proposition 3.2.

Given , is conformal if and only if is (locally) the projection of an -holomorphic map .

To proceed, we need the following result [13].

Proposition 3.3.

and therefore we conclude that is -holomorphic.

shows that and so that is holomorphic. Therefore, taking any holomorphic section of and composing with give a -holomorphic lift of .

Remark 3.4.

Notice that to guarantee the existence of the -holomorphic lift for , the important fact was that belongs to the -part of for any almost Hermitian structure strictly compatible with . This is guaranteed if is a totally umbilic map, but it is not strictly necessary. For instance, if is an isotropic map, the vectors , span an isotropic subspace. If this vectors are linearly independent, taking this space as the -space of , then is a -holomorphic lift of , although may be a map into ; on the other hand, if is totally umbilic, then we may take either as the unique strictly compatible map into or into and both these maps are -holomorphic.

## 4. First-Order Twistorial Constructions

### 4.1. Harmonicity and Isotropy to First Order

*is harmonic to first order*if

Let
be a harmonic map,
a vector field along
, and
a one-parameter variation of
. We say that
is *tangent* to
if
. The following result is a key ingredient in what follows [4]:

Proposition 4.1.

In particular, is Jacobi if and only if any tangent one-parameter variation is harmonic to first order.

We have seen in Theorem 3.1 that harmonicity was not enough to establish a relation with possible twistor lifts of a map conformality and was also a key ingredient, as maps obtained as projections of twistorial maps must be holomorphic with respect to some almost Hermitian structure along the map. On the other hand, when the domain is the -sphere, harmonicity implies (weak) conformality or even isotropy, the last case occurring if the target manifold is itself also a sphere or the complex projective space [12, 14].

As in the nonparametric case, harmonicity to first order implies conformality to first order for maps defined on the two-sphere and even isotropy when the codomain is itself a real or complex space form [15].

### 4.2. Twistorial Constructions

As we have seen, Jacobi fields induce variations that are harmonic (and, in some cases, conformal or even isotropic) to first order. On the other hand, in Section 2 we have seen that conformality, harmonicity, and isotropy of the map correspond to , and -holomorphicity of the twistor lift . As we shall see, these results do have a first-order version as follows. We start with a definition.

Definition 4.2.

*holomorphic to first order*if is holomorphic and

*-holomorphic to first order*if is -holomorphic and

where
is the restriction of
to
. Changing
to
gives the definition of
*-holomorphicity to first order*.

In contrast with the nonparametric case, it is not obvious that -holomorphicity to first order implies -holomorphicity to first order. As a matter of fact, from (4.6), it only follows that . However, we do have the following.

Lemma 4.3.

Let be a smooth map and let be a -stable decomposition of , orthogonal with respect to . Then, is holomorphic to first order if and only if is both and -holomorphic to first order.

Proof.

which is true since is -holomorphic to first order. Hence, is -holomorphic to first order. Changing to shows that is -holomorphic to first order. The converse also follows using analogous arguments.

Remark 4.4.

The importance of choosing the Levi-Civita connection on
is *illusory*. In particular, we can define the concept of *holomorphicity to first order* (*or*
*,*
*-holomorphicity to first order*) for maps defined between almost complex manifolds, not necessarily equipped with any metric.

Now, since holomorphicity does not depend on the chosen connection, we can deduce that holomorphicity with respect to reduces to the same condition (4.10). Thus, being holomorphic to first order does not depend on the chosen connection. For (resp., ) holomorphicity to first order, we use similar arguments, replacing for a horizontal (resp., vertical) frame.

### 4.3. The -Holomorphic Case

*compatible to first order*(with ) if

We start with a technical lemma, whose proof the reader can find in Section 5.

Lemma 4.5.

Lemma 4.6.

Let be -holomorphic to first order. Then, is -holomorphic to first order.

Proof.

showing that and concluding our proof.

Proposition 4.7.

Let be -holomorphic to first order map. Then, the projected map is conformal to first order. Conversely, let be a conformal to first order map. Then there is a (local) -holomorphic to first order map which is compatible to first order with .

Proof.

concluding the first part of the proof.

from the given conditions on and and thus concluding the proof.

### 4.4. The -Holomorphic Case

Next, we give a useful characterization for maps to be -holomorphic to first order ( or ), whose proof is in Section 5 (compare with Proposition 3.3).

Lemma 4.8.

From the preceding lemma, we can also deduce the following.

Lemma 4.9.

Proof.

But the second condition gives , whereas the first holds precisely that , as we wanted to show.

Proposition 4.10 (projections of maps -holomorphic to first order).

Let be a map -holomorphic to first order, where is any Riemann surface. Then, the projection map is isotropic to first order.

Notice that we could replace with , as real isotropy (to first order) does not depend on the fixed orientation on .

Proof.

We now turn our attention to the existence of lifts -holomorphic to first order for a given isotropic to first-order map . Recall that in the nonparametric case such lift exists (see Remark 3.4).

Theorem 4.11.

Let be an isotropic to first order with and linearly independent. Then, there is either a (local) map or a map which is -holomorphic to first order and compatible to first order with .

Before proving Theorem 4.11, we give the following lemma, which we prove in Section 5

Lemma 4.12.

Let be as in the preceding Theorem 4.11.

(i)Suppose that the -holomorphic lift of is (resp., ). Take the unique positive (resp., negative) almost Hermitian structure on compatible with . Then, taking

We are now ready to prove Theorem 4.11.

Proof of Theorem 4.11.

where we have used , , , and . Hence, and satisfy equation (4.23), concluding our proof.

### 4.5. The Holomorphic Case

We prove the following.

Theorem 4.13.

Let be a map -holomorphic to first order. Then, is harmonic to first order (and conformal to first order from Lemma 4.3 and Proposition 4.7).

We first give the following characterization of -holomorphic to first order maps.

Lemma 4.14.

Proof.

which is the condition for -holomorphicity.

Proof of Theorem 4.13.

so that , concluding the proof.

Theorem 4.15.

Let be a harmonic and conformal to first order map. Then, there is (locally) a map which is -holomorphic to first order and with .

Since harmonicity (to first order) does not depend on the orientation on , we could have replaced by in both Theorems 4.13 and 4.15

Proof.

*via*.) Then defines a compatible twistor lift of . Let us check that is -holomorphic to first order. That is holomorphic is immediate. From the proof of Proposition 4.7, we deduce that is -holomorphic to first order as it is compatible to first order with and the latter is conformal to first order. Hence, we are left with proving that (4.37)

holds. We shall establish this equation by showing that both sides agree when applied to any vector . For that, we consider, in turn, the three cases , , and . The first two have similar arguments so that we prove only the first.

Now, the first condition follows from (4.43) since is Koszul-Malgrange holomorphic for each . As for the second, letting denote its left-hand side, we shall prove that for all . We do this by establishing the three cases , and (since the first two cases have similar arguments, we prove only the first).

The first term on the right side of the above equation vanishes as lies in , whereas the second is zero from -holomorphicity, concluding our proof.

### 4.6. The 4-Dimensional Case

Theorem 4.16.

Let be harmonic and isotropic to first-order map and with and being linearly independent. Then, (locally) there is either a map or a map which is simultaneously and -holomorphic to first order and with . Conversely, if (or ) is and -holomorphic to first order, the projected map is harmonic and isotropic to first order.

Proof.

The converse is obvious from Proposition 4.10 and Theorem 4.13. As for the first part, in Theorem 4.11 we saw that we can lift the map to a map -holomorphic to first order. Moreover, this lift could be defined as the unique positive or negative almost complex structure compatible with . On the other hand, in Theorem 4.15 we have seen that there is a map -holomorphic to first order with and for which is compatible. From the comment after Theorem 4.15, there is also a twistor lift of into . Therefore, from the dimension of , we conclude that the lifts constructed in both cited results are the same and, therefore, simultaneously and -holomorphic to first order.

We would now like to guarantee the *uniqueness to first order* of our twistor lift. Before stating such a result, we start with a lemma, proved in Section 5.

Lemma 4.17.

Notice that in Lemma 4.12, we were given
and *defined* the twistor lift as the unique lift compatible with
. Now, we are given the twistor map
but nothing guarantees that projecting the map to
makes
compatible; that is,
may not preserve
.

Proposition 4.18.

Let be two -holomorphic to first-order maps such that and the variational vector fields induced on are the same; that is, writing , , we have . Then, at all points for which and are linearly independent, writing , , we have .

Proof.

We consider the four possible cases for ; namely, when is equal to , , or , where and are as in the preceding lemma (notice that, since , then and ).

(i)When ( uses similar arguments), we have

where we have used Lemma 4.17, as well as the fact that and .

(ii)Taking ( uses similar arguments), we have

As , we deduce ; analogously, so that .

Hence, the twistor lifts constructed in Theorem 4.16 are *unique to first order*, in the sense that the vector field
induced on
(or
) by the map
,
depends only on the initial projected map
and on the Jacobi field
along
. Moreover, taking
the
-sphere or the complex projective plane, letting
be a harmonic map, and
a Jacobi field, isotropy to first order is immediately guaranteed. Hence, the previous construction allows a (local) unified proof of the twistor correspondence between Jacobi fields and twistor vector fields that are tangent to variations on
which are simultaneously
and
-holomorphic (*infinitesimal horizontal holomorphic deformations* in [5]). We can also conclude which different properties (namely, conformality, real isotropy or harmonicity) are related with those of the twistor lift (resp.,
,
or
-holomorphicity).

## 5. Additional Proofs

Proof of Lemma 4.5.

showing that and concluding the proof.

Proof of Lemma 4.8.

finishing the first part of our proof.

as is holomorphic. Since (4.22) holds, the last condition is trivially satisfied and we can conclude that our map is -holomorphic to first order, as desired.

As is compatible with , Lemma 4.5 guarantees that is -holomorphic to first order. On the other hand, since and are linearly independent, we deduce that , and form a basis for . Hence, (4.33) will be satisfied if and only on evaluating the inner product of and with which one of these four vectors one obtains the same result. We shall only prove for the first and fourth vectors, the other two cases being similar.

Equations (5.20) and (5.22) imply that either (which is absurd as does not lie in ) or and consequently (4.34) holds, as wanted.

Proof of Lemma 4.17.

The argument to establish the second and third identities in (4.52), will now be similar to the one in Lemma 4.12(i).

## Declarations

### Acknowledgments

The author is grateful to Professor J. C. Wood for helpful comments and stimulating discussions during the preparation of this work. The author would also like to thank the referee for valuable comments.

## Authors’ Affiliations

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