The Scientific World Journal

Volume 2014 (2014), Article ID 696231, 12 pages

http://dx.doi.org/10.1155/2014/696231

## On Lightlike Geometry of Para-Sasakian Manifolds

^{1}Faculty of Arts and Sciences, Department of Mathematics, Adıyaman University, 02040 Adıyaman, Turkey^{2}Faculty of Arts and Sciences, Department of Mathematics, İnönü University, 44280 Malatya, Turkey

Received 4 February 2014; Accepted 3 April 2014; Published 28 April 2014

Academic Editor: Michele Zappalorto

Copyright © 2014 Bilal Eftal Acet et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

We study lightlike hypersurfaces of para-Sasakian manifolds tangent to the characteristic vector field. In particular, we define invariant lightlike hypersurfaces and screen semi-invariant lightlike hypersurfaces, respectively, and give examples. Integrability conditions for the distributions on a screen semi-invariant lightlike hypersurface of para-Sasakian manifolds are investigated. We obtain a para-Sasakian structure on the leaves of an integrable distribution of a screen semi-invariant lightlike hypersurface.

#### 1. Introduction

It is well known that the main difference between the geometry of submanifolds in Riemannian manifolds and in semi-Riemannian manifolds is that in the latter case the induced metric tensor field by the semi-Riemannian metric on the ambient space is not necessarily nondegenerate. If the induced metric tensor field is degenerate the classical theory of Riemannian submanifolds fails since the normal bundle and the tangent bundle of the submanifold have a nonzero intersection. In particular, from the point of physics lightlike hypersurfaces are important as they are models of various types of horizons, such as Killing, dynamical and conformal horizons, studied in general relativity.

Lightlike submanifolds of semi-Riemannian manifolds were introduced by Duggal and Bejancu in [1]. Since then many authors studied lightlike hypersurfaces of semi-Riemannian manifolds and especially of indefinite Sasakian manifolds (for differential geometry of lightlike submanifolds we refer to the book [2]).

The study of paracontact geometry was initiated by Kaneyuki and Konzai in [3]. The authors defined almost paracontact structure on a pseudo-Riemannian manifold of dimension and constructed the almost paracomplex structure on . Recently, Zamkovoy [4] studied paracontact metric manifolds and some remarkable subclasses like para-Sasakian manifolds. In particular, in the recent years, many authors [5–9] have pointed out the importance of paracontact geometry and, in particular, of para-Sasakian geometry, by several papers giving the relationships with the theory of para-Kähler manifolds and its role in pseudo-Riemannian geometry and mathematical physics.

These circumstances motivated us to initiate the study of lightlike geometry of submanifolds in almost paracontact metric manifolds. As a first step, in the present paper, we study the lightlike hypersurfaces of almost paracontact metric manifolds. We introduce the invariant lightlike hypersurfaces and screen semi-invariant lightlike hypersurfaces of almost paracontact metric manifolds, respectively, and give examples. Moreover, integrability conditions for the distributions involved in the definition of a screen semi-invariant lightlike hypersurface are investigated in case of the ambient manifold being para-Sasakian manifold.

#### 2. Preliminaries

##### 2.1. Almost Paracontact Metric Manifolds

A differentiable manifold of dimension is called almost paracontact manifold with the almost paracontact structure if it admits a tensor field of type , a vector field , and a -form satisfying the following conditions [3]: where denotes the identity transformation. Moreover, the tensor field induces an almost paracomplex structure on the paracontact distribution ; that is, the eigen distributions corresponding to the eigenvalues of are both -dimensional.

If a -dimensional almost paracontact manifold with an almost paracontact structure admits a pseudo-Riemannian metric such that [4]
then we say that is an* almost paracontact metric manifold* with an* almost paracontact metric structure * and such metric is called* compatible metric*. Any compatible metric is necessarily of signature .

From (5) it can be easily seen that [4] for any . The fundamental -form of is defined by An almost paracontact metric structure becomes a paracontact metric structure if , for all vector fields , where .

For a -dimensional manifold with an almost paracontact metric structure one can also construct a local orthonormal basis which is called -*basis * [4].

An almost paracontact metric structure is a* para-Sasakian manifold* if and only if [4]
where and is a Levi-Civita connection on .

From (9), it can be seen that

*Example 1. *Let be the -dimensional real number space with standard coordinate system . Defining
where , the set is an almost paracontact metric structure on .

##### 2.2. Lightlike Hypersurfaces

In this section, we recall some basic results about lightlike hypersurfaces of a semi-Riemannian manifold [1].

Let be a -dimensional semi-Riemannian manifold with index and a hypersurface of . Assume that the induced metric on the hypersurface is degenerate on . Then there exists a vector field on such that
The radical space [10] of is defined by
whose dimension is called the nullity degree of and is called* a lightlike hypersurface* of . Since is degenerate and any null vector is perpendicular to itself, is also degenerate and
For a lightlike hypersurface , implies that
We call the radical distribution and it is spanned by the null vector field .

Consider complementary vector bundle of in . This means that
where denotes the orthogonal direct sum. is called the* screen distribution* on . Since the screen distribution is nondegenerate, there exists a complementary orthogonal vector subbundle to in which is called* screen transversal subbundle*; that is,
The rank of is 2.

Theorem 2 (see [1]). *Let be a lightlike hypersurface of an almost paracontact manifold . Then there exists a unique rank one vector subbundle of , with base space , such that, for any nonzero section of on a coordinate neighborhood , there exists a unique section of on satisfying
**
for all . is called the lightlike transversal vector bundle of with respect to .*

One can consider the following decompositions: Let be the Levi-Civita connection on . Using (20) we deduce for any and . Then and are called the induced connection on and , respectively, and, as in the classical theory of Riemannian hypersurfaces, and are called the second fundamental form and the shape operator, respectively. The above equations are cited as the Gauss and Weingarten equation, respectively [1].

Locally, let , , and be as in Theorem 2. Then, for any , putting we can write is called the local second fundamental form of , because it determines on . Moreover, is degenerate and for any .

The decomposition (16) allows to define a canonical projection . For each , we may write where is a 1-form given by From (24), for all , we get which implies that the induced connection is a nonmetric connection on .

Then for any and we can write where and are linear connections on the bundles and , respectively. Further, and are called the second fundamental form and the shape operator of the screen distribution, respectively. Locally, let be a coordinate neighborhood of and , sections on , as in Theorem 2. Then, putting , for any , one has and, locally on , (30) and (31) become

The local second fundamental forms and , respectively, of and on are related to their shape operators by Furthermore, one has , . (For more details we refer to [1, 2].)

#### 3. Lightlike Hypersurfaces of Para-Sasakian Manifolds

Let be a -dimensional para-Sasakian manifold and a lightlike hypersurface of such that the structure vector field is tangent to . For local sections and of and , respectively, in view of (7) we have From (6) it is easy to see that and are lightlike vector fields and

Now, for any , we write where and

Proposition 3. *Let be a -dimensional para-Sasakian manifold and a lightlike hypersurface of such that the structure vector field is tangent to . Then one has
**
where is any local section of and is any local section of .*

*Proof. *From (10) and (18) we have
which gives (40) by virtue of (22).

*Remark 4. *From (6) we get , which implies that there is no component of in and so . Moreover, (40) implies that there may be a component of in . Thus, in view of (27), we observe that

Proposition 5. *Let be a -dimensional para-Sasakian manifold and a lightlike hypersurface of such that the structure vector field is tangent to . Then one has
**
for any .*

*Proof. *By using (38) and (39) we obtain
Hence in view of (6) we get (43). From (38) we have
Thus, by using the last equation above and (5), we complete the proof.

Corollary 6. *Let be a lightlike hypersurface of a para-Sasakian manifold such that the structure vector field is tangent to . Then, for all , one has .*

Proposition 7. *Let be a lightlike hypersurface of a -dimensional para-Sasakian manifold such that the structure vector field is tangent to . Then, for any , one has
*

*Proof. *From (1) and (38), we get (47). Next, by using (10), (24), and (38), we have
Then by equating the tangential and transversal parts in the previous equation we get (48) and (49), respectively.

#### 4. Invariant Lightlike Hypersurfaces of Almost Paracontact Metric Manifolds

We begin with the following.

*Definition 8. *Let be a -dimensional para-Sasakian manifold and a lightlike hypersurface of such that the structure vector field is tangent to . If , then will be called an invariant lightlike hypersurface of .

*Example 9. *Let be the -dimensional real number space with a coordinate system . Define a frame of vector fields on given by We*ƚ*yczko [11]:
Defining
the set is an almost paracontact metric structure on with index . Consider a hypersurface of given by
It is easy to check that is a lightlike hypersurface and
Then the lightlike transversal vector bundle is spanned by
It follows that corresponding screen distribution is spanned by
We easily check that

which gives . Thus is an invariant lightlike hypersurface of .

Now we give a characterization of an invariant lightlike hypersurface.

Theorem 10. *Let be a -dimensional para-Sasakian manifold and a lightlike hypersurface of such that the structure vector field is tangent to . Then is an invariant lightlike hypersurface of if and only if
*

*Proof. *Let be an invariant lightlike hypersurface of . From (27) and (42), for any , we get ; that is, there is no component of in . Moreover, it is obvious from (6) that has no component in and so .

On the other hand, for any local section of , we can write
By using the previous equation we have , for any , which implies that has no component in . Since , then it is also seen that there is no component of in . Hence .

Conversely, let and . For any we have
which implies that has no component in . Similarly, we get
thus there is no component of in . The proof is completed.

Corollary 11. *Let be a -dimensional para-Sasakian manifold and a lightlike hypersurface of such that the structure vector field is tangent to . Then is an invariant lightlike hypersurface of if and only if
*

Theorem 12. *Let be a -dimensional almost paracontact metric manifold and an invariant lightlike hypersurface of . Then is an almost paracontact metric manifold.*

*Proof. *Let be an invariant lightlike hypersurface of . For any , from (38), we get

Using (1) and (63), we have
Also from (63), it follows that
Next, in view of (64) and (65) one can easily see that
Moreover, from (44), we have
From (64)-(67) we complete the proof.

Proposition 13. *Let be an invariant lightlike hypersurface of a para-Sasakian manifold . Then we have
**
for any .*

*Proof. *Since , using (10), we get
From (25), we have the assertion of the proposition.

Theorem 14. *An invariant lightlike hypersurface of a para-Sasakian manifold is always para-Sasakian. Moreover,
**
for any .*

*Proof. *We have
which in view of (9) gives
Equating tangential parts in (73) provides
In view of (74) and Theorem 12, we see that is a para-Sasakian manifold. Equating transversal parts in (73) yields (70). Next, using (9) and (25), we have
In the last equation, if we equate the tangential parts, we get (71).

This completes the proof.

*Remark 15. *It is well known that, if there exists a lightlike hypersurface in an indefinite Sasakian manifold, then the dimension of the indefinite Sasakian manifold must be equal or greater than . But in a paracontact metric manifold there is not such a restriction in the dimension of the ambient manifold for the existence of lightlike hypersurfaces.

Let be a -dimensional almost paracontact metric manifold and a lightlike surface of such that the structure vector field is tangent to . Since is a nonnull vector field, it belongs to the screen distribution . Thus, is a quasi-orthonormal frame of . Also with respect to the quasi-orthonormal frame of we can write where , , , , , and . Thus we have Hence, a lightlike surface of , tangent to the structure vector field , is always an invariant lightlike surface.

*Example 16. *Let be a -dimensional almost paracontact metric manifold with the structure given in Example 1. Consider a surface of given by . It is easy to check that is a lightlike surface and and are given by
respectively. It follows that corresponding screen distribution is spanned by
Then and , which imply that is an invariant lightlike surface.

*Example 17. *Let be the standard flat para-Kähler structure on ,
for , where are the Cartesian coordinates on . Consider the hypersurface in given by the equation
Let be the normal vector field of . Then .

Define a vector field , a tensor field of type , a 1-form , and a pseudo-Riemannian metric on by assuming
Then we get an almost paracontact metric structure on . Moreover, this structure is para-Sasakian [11]. Now cut by the hyperplane and obtain a lightlike surface of with
where , . It follows that the lightlike transversal bundle is spanned by
Furthermore,
Thus is an invariant surface of .

#### 5. Screen Semi-Invariant Lightlike Hypersurfaces of Almost Paracontact Metric Manifolds

If is local section of , one has ; therefore and we get a 1-dimensional distribution on .

*Definition 18. *Let be a -dimensional almost paracontact metric manifold and a lightlike hypersurface of . If
then will be called a screen semi-invariant lightlike hypersurface of .

*Example 19. *Let be a -dimensional almost paracontact metric manifold with the structure given in Example 1. Consider a hypersurface of given by
Then the tangent bundle of is spanned by
The radical distribution and the lightlike transversal bundle are given by
It follows that the screen distribution is spanned by , where
Furthermore,

Thus, is a screen semi-invariant lightlike hypersurface of .

From now on, we will write to denote a screen semi-invariant lightlike hypersurface, together with the choices of a fixed nonzero section of , a fixed screen distribution , , and as in Theorem 2.

Since is a screen semi-invariant lightlike hypersurface, then we have and which imply that is orthogonal to by virtue of (19). Also, from (5), we obtain Therefore, is a nondegenerate vector subbundle of of rank 2.

In the following, being and nondegenerate, we can define the unique nondegenerate distribution such that [2] Then and is invariant under ; that is, . Moreover, from (16), (20), and (94) we write

For an almost paracontact metric manifold , we construct a useful local orthonormal basis. Let be a coordinate neighborhood on and any unit vector field on orthogonal to . Then is a vector field orthogonal to both and , and . Choose a unit vector field orthogonal to , , and . Then is also orthogonal to , , and and . Proceeding in this way we obtain a set of local orthonormal vector fields . Now construct the unit vector field orthogonal to , , and . Then is also orthogonal to , , and and . By a similar way set a unit vector field orthogonal to , , and . It is easy to see that is also orthogonal to , , and . Hence, from a quasi-orthonormal basis of , we obtain a local orthonormal basis where , called -basis. Thus we have the following.

Proposition 20. *Let be a -dimensional almost paracontact metric manifold and a screen semi-invariant lightlike hypersurface of . Then there always exists a -basis on generated from a quasi-orthonormal basis , of .*

Now, we consider the distributions , on . Then is a -invariant distribution and we have Thus, every can be expressed by where and are the projections of into and , respectively. Hence, we may write , for any . Let us consider the local lightlike vector fields and . From (1), (38), and (39), we obtain By comparing the tangential and transversal parts in (100) we get respectively. Next, from (3) one can easily see that Since , by using (38), we also have Furthermore, from (4), we have Finally, we get which gives

Thus we have the following.

Proposition 21. *Let be a screen semi-invariant lightlike hypersurface of an almost paracontact metric manifold . Then possesses a para -structure; that is,
*

Theorem 22. *Let be a screen semi-invariant lightlike hypersurface of a para-Sasakian manifold . Then one has
**
for all .*

Proposition 23. *Let be a para-Sasakian manifold and a screen semi-invariant lightlike hypersurface of . Then M is totally geodesic if and only if, for any and for ,
*

*Proof. *Let assume that is totally geodesic; that is, for any , . Then, for using in (109) we have
Similarly, using (109) we have
which gives
by virtue of and .

Conversely, suppose that the conditions (111) and (112) hold and we will prove that vanishes. If , using the decomposition (98), there exists such that
and for any we obtain
For , using (109) and (111), we find
which implies that .

Also, for any from (109), using (112), we get
which implies . This completes the proof.

Proposition 24. *Let be a para-Sasakian manifold and a screen semi-invariant lightlike hypersurface of . Then one has, for any ,*(i)*if the vector field ** is parallel, then*(ii)*if the vector field ** is parallel, then**where and .*

#### 6. Integrability of Distributions on a Screen Semi-Invariant Lightlike Hypersurface of a Para-Sasakian Manifold

##### 6.1. The Distribution

Firstly, we consider the distribution , defined in (94). Using (95) and putting , for any , , and we have where is a linear connection on the bundle , is an bilinear, is an linear operator on , respectively, and is a linear connection on .

Lemma 25. *Let be a screen semi-invariant lightlike hypersurface of a para-Sasakian manifold and a coordinate neighborhood as fixed in Theorem 2. Then, for any , one has
*

*Proof. *Calculation is straightforward by using (9).

Let be a coordinate neighborhood as fixed in Theorem 2. Then according to decomposition given by (95) we set for any . So (122) can be written locally as We will express , , and in terms of and . Firstly, we compute Then, by using (125), (24) and (25), we get in view of being -invariant and being a metric connection.

Next we have And, using (124), (24), and (31) in the previous equation, we obtain