Research

# Semi-invariant warped product submanifolds of almost contact manifolds

Falleh R Al-Solamy1 and Meraj A Khan2*

Author Affiliations

1 Department of Mathematics, King Abdulaziz University, P.O. Box 80015, Jeddah 21589, Kingdom of Saudi Arabia

2 Department of Mathematics, University of Tabuk, Tabuk, Kingdom of Saudi Arabia

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Journal of Inequalities and Applications 2012, 2012:127 doi:10.1186/1029-242X-2012-127

 Received: 1 December 2011 Accepted: 7 June 2012 Published: 7 June 2012

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### Abstract

In this article, we have obtained necessary and sufficient conditions in terms of canonical structure F on a semi-invariant submanifold of an almost contact manifold under which the submanifold reduced to semi-invariant warped product submanifold. Moreover, we have proved an inequality for squared norm of second fundamental form and finally, an estimate for the second fundamental form of a semi-invariant warped product submanifold in a generalized Sasakian space form is obtained, which extend the results of Chen, Al-Luhaibi et al., and Hesigawa and Mihai in a more general setting.

2000 Mathematics Subject Classification: 53C25; 53C40; 53C42; 53D15.

##### Keywords:
warped product; semi-invariant; almost contact metric manifold; generalized Sasakian space-form

### 1 Introduction

Bishop and O'Neil [1] introduced the notion of warped product manifolds. These manifolds are generalization of Riemannian product manifolds and occur naturally, e.g., surface of revolution is a warped product manifold. With regard to physical applications of these manifolds, one may realize that space time around a massive star or a black hole can be modeled on a warped product manifolds for instance and warped product manifolds are widely used in differential geometry, Physics and as well as in different branches of Engineering. Due to wide applications of warped product submanifolds, this becomes a fascinating and interesting topic for research, and many articles are available in literature (c.f., [2-4]). CR-warped product was introduced by Chen [5]. He studied warped products CR-submanifolds in the setting of Kaehler manifolds and showed that there does not exist warped product CR-submanifolds of the form M×f MT, therefore, he considered warped product CR-submanifolds of the types MT ×f Mand established a relationship between the warping function f and the squared norm of the second fundamental form of the CR-warped product submanifolds in Kaehler manifolds [5]. In the available literature, many geometers have studied warped products in the setting of almost contact metric manifolds (c.f., [6-8]). Hesigawa and Mihai [9] obtained the inequality for squared norm of the second fundamental form in term of the warping function for contact CR-warped product in Sasakian manifolds. Recently Atceken [4] studied contact CR-warped product submanifolds in Cosymplectic space-forms and obtained an inequality for second fundamental form in terms of warping function. After reviewing the literature, we realized that there is very few studies on the warped product submanifold for almsot contact manifolds so it will be worthwhile to study the warped product submnifolds in the setting of almost contact metric manifold. Since generalized Sasakian space forms include all the classes of almost contact metric manifold, so we have obtained an inequality for squared norm of second fundamental form for semi-invariant warped product submanifolds in the setting of generalized Sasakian space form.

### 2 Preliminaries

Let be a (2n + 1)-dimensional C-differentiable manifold endowed with the almost contact metric structure (ϕ, ξ, η, g), where ϕ is a tensor field of type (1, 1), ξ is a vector field, η is a 1-form and g is a Riemannian metric on , all these tensor fields satisfying.

(2.1)

(2.2)

(2.3)

for any . Here, is the standard notation for the tangent bundle of . The two-form Φ denotes the fundamental two-form and is given by g(X, ϕY) = Φ(X, Y). The manifold is said to be contact if Φ = .

Most of the geometric properties of a Riemannian manifold depend on the curvature tensor R of a manifold. It is well known that the sectional curvatures of a manifold determine curvature tensor completely. A Riemannian manifold with constant sectional curvature c is known as real space form and its curvature tensor is given by

A Sasakian manifold with constant ϕ-sectional curvatures is a Sasakian space form and it has a specific form of its curvature tensor. Similar notion also holds for Kenmotsu and Cosymplectic space form. In order to generalize the notion in a common frame, Alegere et al. [10] introduced the notion of generalized Sasakian space form. In this connection, a generalized Sasakian space form is defined as follows.

Given an almost contact metric manifold , we say that is a generalized Sasakian space form if there exist three functions f1, f2, and f3 on such that, the curvature tensor R is given by

(2.4)

Let M be a submanifold of an almost contact metric manifold . Then we denote the induced metric on M by the same symbol g where as the induced connection on M by ∇. With these notation, Gauss and Weingarten formulae are written as

(2.5)

(2.6)

for each X, Y TM and N TM, where ∇denotes the induced connection on the normal bundle TM. h and AN are the second fundamental form and the shape operator of the immersion of M into and they are related as

(2.7)

For any X TM and N TM, we write

(2.8)

(2.9)

where TX and tN are the tangential components of ϕX and ϕN, respectively, where as FX and fN are the normal components of ϕX and ϕN, respectively.

The covariant derivative of the tensors T, F, t, and f are defined as

(2.10)

(2.11)

(2.12)

(2.13)

On the other hand, the covariant derivative of the second fundamental form h is defined as

(2.14)

for any X, Y, Z TM. Let and R be the curvature tensors of the connections and ∇ on and M, respectively. Then the equations of Gauss and Coddazi are given by

(2.15)

(2.16)

A submanifold M of is said to be semi-invariant submanifold if there exist on M, a differentiable invariant distribution D such that its orthogonal complementary distribution Dis anti-invariant, i.e., ϕDx TxM and for each x M. For a semi-invariant submanifold of an almost contact metric manifold , we have

(2.17)

The structure vector field ξ is tangential to submanifold M, otherwise the submanifold is simply anti-invariant.

(2.18)

where μ denotes the orthogonal complemantry distribution of ϕDand is an invariant normal subbundle of TM under ϕ.

The orthogonal projection on TM of a semi-invariant submanifold M of an almost contact metric manifold are denoted by P1 and P2, i.e., for any X TM we have

(2.19)

where P1X D, P2X Dand η(X)ξ ∈ 〈ξ〉. It follows immediately that

(2.20)

Moreover, for any X, Y TM if we denote by and , the tangential and normal parts of , then we have

(2.21)

and on using Equations (2.5)-(2.11), we obtain

(2.22)

(2.23)

Definition 2.1 A semi-invariant submanifold M of an almost contact metric manifold is semi-invariant product if the distributions are involutive and their leaves are totally geodesic in M.

Definition 2.2 Let (B, gB) and (F, gF) be two Riemannian manifolds with Riemannian metric gB and gF, respectively, and f be a positive differentiable function on B. The warped product of B and F is the Riemannian manifold (B × F, g), where

For a warped product manifold N1 ×f N2, we denote by D1 and D2 the distributions defined by the vectors tangent to the leaves and fibers, respectively. In other words, D1 is obtained by the tangent vectors of N1 via the horizontal lift and D2 is obtained by the tangent vectors of N2 via vertical lift. In case of semi-invariant warped product submanifolds D1 and D2 are replaced by D and D, respectively.

The warped product manifold (B × F, g) is denoted by B ×f F. If X is the tangent vector field to M = B ×f F at (p, q) then

Bishop and O'Neill [1] proved the following

Theorem 2.1 Let M = B×f F be warped product manifolds. If X, Y TB and V, W TF then

(i) ∇XY TB

(ii) ,

(iii) .

f is the gradient of f and is defined as

(2.24)

for all X TM.

Corollary 2.1 On a warped product manifold M = N1 ×f N2, the following statements hold

(i) N1 is totally geodesic in M

(ii) N2 is totally umbilical in M.

Moreover,

(2.25)

and

(2.26)

for any X D1 and Z, W D2, where nor(∇ZW) denotes the component of ∇ZW in D1 and ∇f denotes the gradient of f.

### 3 Semi-invariant warped product submanifolds

Chen [11] obtained various conditions under which a CR-submanifolds reduces to a CR-product. In particular, he proved that a CR-submanifold of a Kaehler manifold is a CR-product if and only if . Since, warped products are the generalization of Riemannian products by taking this point Khan et al. [8] proved a characterization of CR-warped product of a Kaehler manifold in terms of P and F after that Al-Luhaibi et al. [7] find charactraziation of CR-warped of nearly Kaehler manifolds in terms of P and F. In this section, we have obtained necessary and sufficient condition in terms of F, for which a semi-invariant submanifolds is a semi-invariant warped product submanifold in the setting of almost contact metric manifolds.

Throughout, this section, we denote NT and Nthe invariant and anti-invariant submanifolds, respectively, of an almost contact metric manifold . Warped product semi-invariant and semi-invariant warped product submanifolds of an almost contact metric manifold are represented by N×f NT and NT ×f Nand we take NT tangential to ξ.

In terms of canonical structure F, we have the following charectrization.

Theorem 3.1 A semi-invariant submanifold M with involutive distributions D ⊕ 〈ξ〉 and Dof an almost contact metric manifold with . Then M is a semi-invariant warped product submanifold of if and only if

(3.1)

for each U, V TM and μ a Cfunction on M satisfying = 0 for each Z D.

Proof. Let M be a semi-invariant warped product submanifold NT ×f N, then, by Equation (2.11), we have

for any X, Y D and W D. As NT is totally geodesic in M, we get

(3.2)

On other hand, for any X D, Z, W Dby Equation (2.23)

By the assumption that , the above equation gives

(3.3)

As ξ is tangential to NT, for any X D and Z, W D, by Equations (2.11) and (2.25) we have

(3.4)

Similarly, for any Z, W, W' ∈ D, by Equation (2.11) we have

(3.5)

Moreover, for any X D, W Dand ξ ∈ 〈ξ〉, by Equations (2.23) and (2.25), it is easy to see that

(3.6)

(3.7)

Since ξ is tangential to NT, then from Equations (2.11) and (2.25), we can prove the following

(3.8)

(3.9)

(3.10)

For any U, V TM with the help of Equation (2.19), we have

In view of Equations (3.2)-(3.10), the above equation reduced to Equation (3.1).

Conversely, suppose that M be a semi-invariant submanifold, satisfying Equation (3.1). Then for any X, Y D ⊕ 〈ξ〉 by Equation (3.1), we have

therefore, by Equation (2.11) the above equation yields g(∇XY, W) = 0, this mean leaves of D ⊕ 〈ξ〉 are totally geodesic in M.

Now, for any Z, W D, by Equation (3.1), we get

or

Let Nbe a leaf of D. If ∇' denotes the induced connection on Nand h' be the second fundamental form of the immersion of Nof M, then by Gauss formula

or

or

this shows that Nis totally umbilical in M with mean curvature vector ∇μ. Moreover, as = 0 for all W Dand the mean curvature is parallel on N, this shows that Nis extrinsic sphere. Hence, by virtue of result of [12] which says that if the tangent bundle of a Riemannian manifold M splits into an orthogonal sum TM = E0 E1 of nontrivial vector subbundles such that E1 is spherical and its orthogonal complement E0 is auto parallel, then the, manifold M is locally isometric to a warped product M0 ×f M1, we can say M is a locally semi-invariant product submanifold NT ×f N, where warping function f = eμ.

Let M = NT ×f Nbe a semi-invariant warped product submanifold of an almost contact metric manifold .

In view of decomposition (2.18), we may write

(3.11)

for each U, V TM, where and hμ(U, V) ∈ μ.

If {e1, e2,..., en} be a local orthonormal frame of vector fields on M then we define

and for differentiable function f on M, the Laplacian Δf of f is defined as

(3.12)

Now we have the following proposition

Proposition 3.1 Let M be a semi-invariant warped product submanofold NT ×f Nof an almost contact metric manifold of . Then

(i)

(ii)

(iii) , for any X TNT and Z, W TN.

Proof. By Gauss formula

using the decomposition (2.21) and Equation (2.25), we get

Comparing tangential parts in above equation

taking inner product with W Don both side, we get

or equivalently

(3.13)

or

which proves the part (i) of proposition.

Now, on comparing the normal parts

or

(3.14)

taking inner product with ϕW and using Equation (3.17), we get

Taking inner product with ϕh(X, Z) in Equation (3.14), we find

which is the part (iii) of proposition.

For semi-invariant warped product submaniolds of an almost conatct metric manifold, we have the following theorem

Theorem 3.2 Let M = NT ×f Nbe a semi-invariant warped product submanifold of an almost contact manifold with , then

(i) The squared norm of the second fundamental form satisfies

(3.15)

where ∇ ln f is the gradient of ln f and q is the dimension of anti-invariant distribution.

(ii) If the equality sign in (3.15) holds identically, then NT is totally geodesic submanifolds of , Nis a totally umbilical submanifold of , M is minimal and h(D D, ξ) = 0.

Proof. Let {X0 = ξ, X1, X2,..., Xp, Xp+1 = ϕX1,..., X2p = ϕXp} be a local orthonormal frame of vector field on NT and {Z1, Z2,..., Zq} be a local orthonormal frame of vector field on N. Then by definition of squared norm of mean curvature vector

(3.16)

Thus,

On using part (i) of Proposition (3.1) with assumption , then the above inequality takes the form

Using , the above inequality can be represented as

which proves the part (i) of the Theorem.

Finally, if equality holds identically then from Equation (3.16), h(D, D) = 0, h(D, D) = 0, h(D, D) ⊆ ϕD, and h(D D, ξ) = 0. These observations proves the part (ii) of theorem.

### 4 Semi-invariant warped product submanifolds of a generalized Sasakian space-form

Hesigawa and Mihai [9] obtained the inequality for squared norm of second fundamental form for contact CR-warped product submanifolds in the setting of Sasakian space form. In the available literature, similar estimates are proved for squared norm of second fundamental form in contact manifolds (c.f., [3,4]). Since generalized Sasakian space form include the class of all almost contact metric manifold, so in this section we will obtain an estimate for the squared norm of second fundamental form for semi-invariant warped product submani-folds in the setting of generalized Sasakian space form.

Theorem 4.1 Let M = NT ×f Nbe a semi-invariant warped product submanifold of a generalized Sasakian space form with . Then we have

Proof. For X D and Z D, by formula (2.4) we have

(4.1)

On the other hand by Coddazi equation

(4.2)

Now,

(4.3)

The first term in the right-hand side of Equation (4.3) on using Equation (2.25), decomposition (2.25) and part (ii) of Proposition (3.1) becomes,

In view of assumption , the above equation gives

(4.4)

Where as, the second term of Equation (4.3) with the help of Equations (2.5) and (2.25) can be written as

By (i) and (ii) parts of Proposition 3.1, the above equation becomes

Applying Equation (3.14), (i) and (ii) parts of Proposition 3.1, we get

(4.5)

On substituting Equations (4.4) and (4.5) in Equation (4.3), we find

(4.6)

Similarly, we obtain

(4.7)

By formula (2.25) and part (i) of Proposition 3.1, we have

(4.8)

and

(4.9)

On using (i) and (ii) parts of Proposition 3.1 and the fact NT is totally geodesic, we have

and

The right-hand side of above equation, on making use of the fact that NT is totally geodesic in M and the formula (2.25) reduced to -g(∇ZϕXϕX, Z), thus by using Gauss formula, we find

(4.10)

Let {X0 = ξ, X1, X2,..., Xp, Xp+1 = ϕX1,..., X2p = Xp} and {Z1, Z2,..., Zq} be a local orthonormal frame of vector fields on NT and N, respectively. Choosing X, Z as basic vector fields and substituting from Equations (4.3)-(4.10) into Equation (4.2), we obtain

Summing both side over i = 1, 2,..., p, r = 1, 2,..., q and making use of Equation (4.1), we obtain

(4.11)

Here we use

Finally, on the same line of the proof of Equation (3.15), we can prove

(4.12)

The result follows immideatly from Equations (4.11) and (4.12).

### Competing interests

The authors declare that they have no competing interests.

### Authors' contributions

All authors read and approved the final manuscript.

### Acknowledgements

Authors are thankful to the referee for his valuable suggestion and comments. This work is supported by the Research Grant number 0136-1432-S, Deanship of Scientific research, University of Tabuk, K.S.A.

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