Journal of Function Spaces

Volume 2018, Article ID 2864263, 13 pages

https://doi.org/10.1155/2018/2864263

## Slant and Semi-Slant Submanifolds in Metallic Riemannian Manifolds

^{1}Stefan cel Mare University of Suceava, Romania^{2}West University of Timisoara, Romania

Correspondence should be addressed to Cristina E. Hretcanu; moc.oohay@banelesirc

Received 11 May 2018; Accepted 11 July 2018; Published 12 September 2018

Academic Editor: Raúl E. Curto

Copyright © 2018 Cristina E. Hretcanu and Adara M. Blaga. 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

The aim of our paper is to focus on some properties of slant and semi-slant submanifolds of metallic Riemannian manifolds. We give some characterizations for submanifolds to be slant or semi-slant submanifolds in metallic or Golden Riemannian manifolds and we obtain integrability conditions for the distributions involved in the semi-slant submanifolds of Riemannian manifolds endowed with metallic or Golden Riemannian structures. Examples of semi-slant submanifolds of the metallic and Golden Riemannian manifolds are given.

#### 1. Introduction

Since B.Y. Chen defined slant submanifolds in complex manifolds ([1, 2]) in the early 1990s, the differential geometry of slant submanifolds has shown an increasing development. Then, many authors have studied slant submanifolds in different kind of manifolds, such as slant submanifolds in almost contact metric manifolds (A. Lotta ([3])), in Sasakian manifolds (J.L. Cabrerizo* et al.* ([4, 5])), in para-Hermitian manifold (P. Alegre, A. Carriazo ([6])), and in almost product Riemannian manifolds (B. Sahin ([7]), M. Atçeken ([8, 9])).

The notion of slant submanifold was generalized by semi-slant submanifold, pseudo-slant submanifold, and bi-slant submanifold, respectively, in different types of differentiable manifolds. The semi-slant submanifold of almost Hermitian manifold was introduced by N. Papagiuc ([10]). A. Cariazzo* et al.* ([11]) defined and studied bi-slant immersion in almost Hermitian manifolds and pseudo-slant submanifold in almost Hermitian manifolds. The pseudo-slant submanifolds in Kenmotsu or nearly Kenmotsu manifolds ([12, 13]), in LCS-manifolds ([14]), or in locally decomposable Riemannian manifolds ([15]) were studied by M. Atçeken* et al.* Moreover, many examples of semi-slant, pseudo-slant, and bi-slant submanifolds were built by most of the authors.

Semi-slant submanifolds are particular cases of bi-slant submanifolds, defined and studied by A. Cariazzo ([11]). The geometry of slant and semi-slant submanifolds in metallic Riemannian manifolds is related by the properties of slant and semi-slant submanifolds in almost product Riemannian manifolds, studied in ([7, 8, 16]).

The notion of Golden structure on a Riemannian manifold was introduced for the first time by C.E. Hretcanu and M. Crasmareanu in ([17]). Moreover, the authors investigated the properties of a Golden structure related to the almost product structure and of submanifolds in Golden Riemannian manifolds ([18, 19]). Examples of Golden and product-shaped hypersurfaces in real space forms were given in ([20]). The Golden structure was generalized as metallic structures, defined on Riemannian manifolds in ([21]). A.M. Blaga studied the properties of the conjugate connections by a Golden structure and expressed their virtual and structural tensor fields and their behavior on invariant distributions. Also, she studied the impact of the duality between the Golden and almost product structures on Golden and product conjugate connections ([22]). The properties of the metallic conjugate connections were studied by A.M. Blaga and C.E. Hretcanu in ([23]) where the virtual and structural tensor fields were expressed and their behavior on invariant distributions was analyzed.

Recently, the connection adapted on the almost Golden Riemannian structure was studied by F. Etayo* et al.* in ([24]). Some properties regarding the integrability of the Golden Riemannian structures were investigated by A. Gezer* et al.* in ([25]).

The metallic structure is a polynomial structure, which was generally defined by S.I. Goldberg* et al.* in ([26, 27]), inspired by the metallic number given by , which is the positive solution of the equation , for positive integer values of and . These numbers are members of the* metallic means family* or* metallic proportions* (as generalizations of the Golden number ), introduced by Vera W. de Spinadel ([28]). Some examples of the members of the metallic means family are* the Silver mean, the Bronze mean, the Copper mean, the Nickel mean,* and many others.

The purpose of the present paper is to investigate the properties of slant and semi-slant submanifolds in metallic (or Golden) Riemannian manifolds. We have found a relation between the slant angles of a submanifold in a Riemannian manifold endowed with a metallic (or Golden) structure and the slant angle of the same submanifold of the almost product Riemannian manifold . Moreover, we have found some integrability conditions for the distributions which are involved in such types of submanifolds in metallic and Golden Riemannian manifolds. We have also given some examples of semi-slant submanifolds in metallic and Golden Riemannian manifolds.

#### 2. Preliminaries

First of all we review some basic formulas and definitions for the metallic and Golden structures defined on a Riemannian manifold.

Let be an -dimensional manifold endowed with a tensor field of type . We say that the structure is a* metallic structure* if it verifiesfor , , where is the identity operator on the Lie algebra of vector fields on . In this situation, the pair is called* metallic manifold*.

If one obtains the* Golden structure* ([17]) determined by a -tensor field which verifies . In this case, is called* Golden manifold*.

Moreover, if ( is a Riemannian manifold endowed with a metallic (or a Golden) structure , such that the Riemannian metric is -compatible, i.e.,for any , then is called a* metallic* (or a* Golden*)* Riemannian structure* and is a* metallic* (or a* Golden*)* Riemannian manifold*.

We can remark thatfor any .

Any metallic structure on induces two almost product structures on this manifold ([21]):Conversely, any almost product structure on induces two metallic structures on ([21]):

If the almost product structure is a Riemannian one, then and are also metallic Riemannian structures. Also, on a metallic manifold there are two complementary distributions and corresponding to the projection operators and ([21]), given byand the operators and verify the following relations:and

In particular, if , we obtain that every Golden structure on induces two almost product structures on this manifold and conversely, an almost product structure on induces two Golden structures on ([17, 19]).

#### 3. Submanifolds of Metallic Riemannian Manifolds

In the next issues we assume that is an -dimensional submanifold, isometrically immersed in the -dimensional metallic (or Golden) Riemannian manifold ( with and . We denote by the tangent space of in a point and by the normal space of in . The tangent space of can be decomposed into the direct sum: , for any . Let be the differential of the immersion . The induced Riemannian metric on is given by , for any , where denotes the set of all vector fields of . For the simplification of the notations, in the rest of the paper we shall note by the vector field , for any .

We consider the decomposition into the tangential and normal parts of and , for any and , are given bywhere and , with

We remark that the maps and are -symmetric ([29]):andfor any and .

For an almost product structure , the decompositions into tangential and normal parts of and , for any and , are given by ([7])where , , , , with

The maps and are -symmetric ([16]):for any and .

*Remark 1. *Let be a Riemannian manifold endowed with an almost product structure and let be the metallic structure induced by on . If is a submanifold in the almost product Riemannian manifold , thenfor any and .

*Remark 2. *Let be a Riemannian manifold endowed with an almost product structure and let be the Golden structure induced by on . If is a submanifold in the almost product Riemannian manifold , thenfor any and .

Let be the codimension of in (where ). We fix a local orthonormal basis of the normal space . Hereafter we assume that the indices run over the range .

For any and , the vector fields and can be decomposed into tangential and normal components ([21]):where , is an -tensor field on , are vector fields on , are -forms on , and is an matrix of smooth real functions on .

Using (9) and (21), we remark that

Theorem 3. *The structure induced on the submanifold by the metallic Riemannian structure on satisfies the following equalities ([30]):for any , where is the Kronecker delta and , are positive integers ([21]).*

A structure induced on the submanifold by the metallic Riemannian structure defined on (determined by the -tensor field on , the vector fields on , the -forms on , and the matrix of smooth real functions on ) which verifies the relations (23), (24), (25), and (26) is called *- metallic Riemannian structure* ([30]).

For , the structure is called *-Golden Riemannian structure*.

*Remark 4. *If is the induced structure on the submanifold by the metallic (or Golden) Riemannian structure on , then is an invariant submanifold with respect to if and only if is a metallic (or Golden) Riemannian manifold, whenever is nontrivial ([21]).

Let and be the Levi-Civita connections on and , respectively. The Gauss and Weingarten formulas are given byfor any and , where is the second fundamental form, is the shape operator of . The second fundamental form and the shape operator are related by

*Remark 5. *Using a local orthonormal basis of the normal space , where is the codimension of in and , for any , we obtainfor any .

*Remark 6. *For , the normal connection has the decomposition , for any , where is an matrix of -forms on . Moreover, from , we obtain ([30]): , which is equivalent to , for any and .

The covariant derivatives of the tangential and normal parts of and are given byandfor any , and . From , it follows thatfor any , , . Moreover, if is an isometrically immersed submanifold of the metallic Riemannian manifold , then ([23])for any , , .

Using an analogy of a locally product manifold ([31]), we can define* locally metallic (or locally Golden) Riemannian manifold* as follows ([30]).

*Definition 7. *If is a metallic (or Golden) Riemannian manifold and is parallel with respect to the Levi-Civita connection on (i.e., ), we say that is a* locally metallic (or locally Golden) Riemannian manifold*.

Proposition 8. *If is a submanifold of a locally metallic (or locally Golden) Riemannian manifold , thenandfor any , where is the Levi-Civita connection on .*

*Proof. *From locally metallic (or locally Golden) Riemannian manifold, we have , for any .

Thus, , which is equivalent to Taking the normal and the tangential components of this equality, we getandInterchanging and and subtracting these equalities, we obtain the tangential and normal components of , which give us (34) and (35).

From (30), (31), (37), and (38) we obtain the following.

Proposition 9. *If is a submanifold of a locally metallic (or Golden) Riemannian manifold , then the covariant derivatives of and verifyandfor any , and .*

Proposition 10. *If is an -dimensional submanifold of codimension in a locally metallic (or locally Golden) Riemannian manifold , then the structure induced on by the metallic (or Golden) Riemannian structure has the following properties ([30]):for any .*

*Proof. *From we obtain , for any . Using (27)(i), (29), and (21)(ii), we get for any . Identifying the tangential and normal components, respectively, of the last two equalities, we get (41) and (42).

Using (34), (35), (41), and (42), we obtain the following.

Proposition 11. *If is a submanifold of a locally metallic (or locally Golden) Riemannian manifold , thenfor any , where is the Levi-Civita connection on .*

#### 4. Slant Submanifolds in Metallic or Golden Riemannian Manifolds

Let be an -dimensional submanifold, isometrically immersed in an -dimensional metallic (or Golden) Riemannian manifold (, where and . Using the Cauchy-Schwartz inequality ([6]), we have for any . Thus, there exists a function , such that for any and any nonzero tangent vector . The angle between and is called the* Wirtinger angle* of and it verifies

*Definition 12 (see [29]). *A submanifold in a metallic (or Golden) Riemannian manifold ( is called slant submanifold if the angle between and is constant, for any and . In such a case, is called the* slant angle* of in , and it verifiesThe immersion is named slant immersion of in .

*Remark 13. *The invariant and anti-invariant submanifolds in the metallic (or Golden) Riemannian manifold ( are particular cases of slant submanifolds with the slant angle and , respectively. A slant submanifold in , which is neither invariant nor anti-invariant, is called proper slant submanifold and the immersion is called proper slant immersion.

Proposition 14. *([29]) Let be an isometrically immersed submanifold of the metallic Riemannian manifold . If is a slant submanifold with the slant angle , then, for any , we get**Moreover, we havewhere is the identity on and*

*Remark 15. *Let be the identity on . From (23) and (52), we have

Proposition 16. *If is an isometrically immersed slant submanifold of the Golden Riemannian manifold with the slant angle , thenfor any , . If is the identity on , we have*

*Definition 17 (see [8]). *A submanifold in an almost product Riemannian manifold ( is a slant submanifold if the angle between and is constant, for any and . In such a case, is called the slant angle of the submanifold in and it verifies

Proposition 18 (see [16]). *If is a slant submanifold isometrically immersed in an almost product Riemannian manifold with the slant angle then, for any , we get*

In the next proposition we find a relation between the slant angles of the submanifold in the metallic Riemannian manifold and the slant angle of the submanifold in the almost product Riemannian manifold .

Theorem 19. *Let be a submanifold in the Riemannian manifold ) endowed with an almost product structure on and let be the induced metallic structure by on ). If is a slant submanifold in the almost product Riemannian manifold with the slant angle and ( is the identity on ) and , then is a slant submanifold in the metallic Riemannian manifold with slant angle given by*

*Proof. *From (17)(ii), we obtain , for any . From (51) and (60)(ii) and , we getfor any . Using , we havefor any . Replacing by and using , for any , we obtainfor any . Summing equalities (63) and (64), we obtainfor any . Using , , and in (65), we get (61).

In particular, for , we obtain the relation between slant angle of the immersed submanifold in a Golden Riemannian manifold ) and the slant angle of immersed in the almost product Riemannian manifold .

Proposition 20. *Let be a submanifold in the Riemannian manifold ) endowed with an almost product structure on and let be the induced Golden structure by on ). If is a slant submanifold in the almost product Riemannian manifold with the slant angle and ( is the identity on ) and , then is a slant submanifold in the Golden Riemannian manifold with slant angle given bywhere is the Golden number.*

#### 5. Semi-Slant Submanifolds in Metallic or Golden Riemannian Manifolds

We define the slant distribution of a metallic (or Golden) Riemannian manifold, using a similar definition as for Riemannian product manifold ([7, 16]).

*Definition 21. *Let be an immersed submanifold of a metallic (or Golden) Riemannian manifold . A differentiable distribution on is called a slant distribution if the angle between and the vector subspace is constant, for any and any nonzero vector field . The constant angle is called the slant angle of the distribution .

Proposition 22. *Let be a differentiable distribution on a submanifold of a metallic (or Golden) Riemannian manifold . The distribution is a slant distribution if and only if there exists a constant such thatfor any , where is the orthogonal projection on . Moreover, if is the slant angle of , then it satisfies .*

*Proof. *If the distribution is a slant distribution on , by using we get , for any and we obtain (67).

Conversely, if there exists a constant such that (67) holds for any , we obtain and Thus, , and using we get . Thus, is constant and is a slant distribution on .

*Definition 23. *Let be an immersed submanifold in a metallic (or Golden) Riemannian manifold . We say that is a* bi-slant submanifold* of if there exist two orthogonal differentiable distributions and on such that and , are slant distributions with the slant angles and , respectively.

For a differentiable distribution on , we denote by the orthogonal distribution of in (i.e., ). Let and be the orthogonal projections on and . Thus, for any , we can consider the decomposition of , where and .

If is a bi-slant submanifold of a metallic Riemannian manifold with the orthogonal distribution and and the slant angles and , respectively, then , for any . In a similar manner as in ([16]), we can prove the following.

Proposition 24. *If is a bi-slant submanifold in a metallic (or Golden) Riemannian manifold , with the slant angles and , for any and , then is a slant submanifold in the metallic Riemannian manifold with the slant angle .*

*Proof. *From , for any and , it follows that . Thus, we obtain , for any and , for any . Moreover, using the projections of any on and , respectively, we obtain the decomposition , where and .

From (for ) and using , we obtain , for any . Thus, is a slant submanifold in the metallic (or Golden) Riemannian manifold with the slant angle .

If is a* bi-slant submanifold* of a manifold , for particular values of the angles and , we obtain the following.

*Definition 25. *An immersed submanifold in a metallic (or Golden) Riemannian manifold is a semi-slant submanifold if there exist two orthogonal distributions and on such that (1) admits the orthogonal direct decomposition ;(2)The distribution is invariant distribution (i.e., );(3)The distribution is slant with angle .Moreover, if , then is a proper semi-slant submanifold.

*Rema rk 5.6. *If is a semi-slant submanifold of a metallic Riemannian manifold with the slant angle of the distributions , then we get that (1) is an invariant submanifold if ;(2) is an anti-invariant submanifold if and ;(3) is a semi-invariant submanifold if is anti-invariant (i.e., ).

If is a semi-slant submanifold in a metallic (or Golden) Riemannian manifold then, for any ,Moreover, we have and the cosine of the slant angle of the distribution is constant, for any nonzero . If , for any nonzero we get

Proposition 27. *If is a semi-slant submanifold of the metallic Riemannian manifold with the slant angle of the distribution then, for any , , we get*

*Proof. *Taking in (71) we have , for any , and using (70)(iii) we get (72). From (70)(ii) we get , for any . Thus, we obtain , for any , and it implies (73).

*Remark 28. *A semi-slant submanifold of a Golden Riemannian manifold with the slant angle of the distribution verifies for any , .

Proposition 29. *Let be a semi-slant submanifold of a metallic Riemannian manifold with the slant angle of the distribution . Thenwhere is the identity on and*

*Proof. *Using , for any , and (72), we obtain (76). Moreover, we have , for any and . For the identity on we have ; thus, we get (77).

*Remark 30. *A semi-slant submanifold of a Golden Riemannian manifold with the slant angle of the distribution verifieswhere is the identity on and

Proposition 31. *Let be an immersed submanifold of a metallic Riemannian manifold . Then is a semi-slant submanifold in if and only if exists a constant such that is a distribution and , for any orthogonal to , where and are given in (1).*

*Proof. *If we consider a semi-slant submanifold of the metallic Riemannian manifold then, in (72) we put . Thus, we obtain and we get . For a nonzero vector field , let , where and . Because is invariant, then and using the property of the metallic structure (1), we obtain , which implies . Because , we obtain and we get ( because and are nonzero natural numbers). Thus, we obtain and , which implies . Therefore, .

Conversely, if there exists a real number such that we have , for any , it follows that which implies that does not depend on . We can consider the orthogonal direct sum . For and (with ), we have From and , we obtain and this implies and . Thus,