Generalized Wintgen inequality for BI-SLANT submanifolds in conformal Sasakian space form with quarter-symmetric connection

Abstract

Purpose

In 1979, P. Wintgen obtained a basic relationship between the extrinsic normal curvature the intrinsic Gauss curvature, and squared mean curvature of any surface in a Euclidean 4-space with the equality holding if and only if the curvature ellipse is a circle. In 1999, P. J. De Smet, F. Dillen, L. Verstraelen and L. Vrancken gave a conjecture of Wintgen inequality, named as the DDVV-conjecture, for general Riemannian submanifolds in real space forms. Later on, this conjecture was proven to be true by Z. Lu and by Ge and Z. Tang independently. Since then, the study of Wintgen’s inequalities and Wintgen ideal submanifolds has attracted many researchers, and a lot of interesting results have been found during the last 15 years. The main purpose of this paper is to extend this conjecture of Wintgen inequality for bi-slant submanifold in conformal Sasakian space form endowed with a quarter symmetric metric connection.

Design/methodology/approach

The authors used standard technique for obtaining generalized Wintgen inequality for bi-slant submanifold in conformal Sasakian space form endowed with a quarter symmetric metric connection.

Findings

The authors establish the generalized Wintgen inequality for bi-slant submanifold in conformal Sasakian space form endowed with a quarter symmetric metric connection, and also find conditions under which the equality holds. Some particular cases are also stated.

Originality/value

The research may be a challenge for new developments focused on new relationships in terms of various invariants, for different types of submanifolds in that ambient space with several connections.

Citation

Aslam, M., Siddiqi, M.D. and Siddiqui, A.N. (2022), "Generalized Wintgen inequality for BI-SLANT submanifolds in conformal Sasakian space form with quarter-symmetric connection", Arab Journal of Mathematical Sciences, Vol. ahead-of-print No. ahead-of-print. https://doi.org/10.1108/AJMS-03-2021-0057

1. Introduction

In 1980, I. Vaisman [1] introduced the concept of conformal changes (or deformation) of almost contact metric structures as follows: Let M̃ be a (2n + 1)-dimensional manifold endowed with an almost contact metric structure (φ, ξ, η, g). A conformal change of the metric g leads to a metric which is no more compatible with the almost contact structure (φ, ξ, η). This can be corrected by a convenient change of ξ and η which implies rather strong restrictions. Using this definition, a new type of almost contact metric structure (φ, ξ, η, g) on a (2n + 1)-dimensional manifold M̃ which is said to be a conformal Sasakian structure if the structure (φ, ξ, η, g) is conformal related to a Sasakian structure (φ̃,ξ̃,η̃,g̃).

The Wintgen inequality is a sharp geometric inequality for surfaces in four-dimensional Euclidean space involving Gauss curvature (intrinsic invariants), normal curvature and square mean curvature (extrinsic invariants). P. Wintgen [2] proved that the Gauss curvature K, the normal curvature K^⊥ and the squared mean curvature ‖H‖2 for any surface M̃2 in E4 satisfy the inequality [3] as follows:

‖H‖2≥K+|K⊥|

and the equality holds if and only if the ellipse of curvature of M̃2 in E4 is a circle. Later, it was extended by I. V. Gaudalupe et al. [4] for arbitrary codimension m in real space forms M̃(m+2)(c) as follows:

‖H‖2+c≥K+|K⊥|.

In 1999, De Smet, Dillen, Verstraelen and Vrancken [5] conjectured the generalized Wintgen inequality for submanifolds in real space form. The conjecture is known as DDVV conjecture. It had been proved by Lu [6] and by Ge and Tang [7] independently. In 2014, Ion Mihai [8] established such inequality for Lagrangian submanifold in complex space form. They provided some applications and also stated such an inequality for slant submanifolds in complex space forms. However, the year 2014 is not the stopping point in investigating Wintgen inequality and some additional steps have been taken in the development of the theory. In fact, many remarkable articles were published in the recent years and several inequalities of this type have been obtained for other classes of submanifolds in several ambient spaces for example, for statistical submanifolds in statistical manifolds of constant curvature [9]; for Legendrian submanifolds in Sasakian space forms [10]; for submanifolds in statistical warped product manifolds [11]; for quaternionic CR-submanifolds in quaternionic space forms [12]; for submanifolds in generalized (κ, μ)-space forms [13]; for totally real submanifolds in LCS-manifolds [14] and so on. For more details, see [15].

In the present article, we obtain the generalized Wintgen inequalities for conformal Sasakian space forms. The equality case of the main inequality is investigated. Lastly, we discuss such inequality for various slant cases as an application of the obtained inequality.

2. Preliminaries

2.1. Sasakian manifold

An odd-dimensional Riemannian manifold (M̃,g) is said to be an almost contact metric manifold [16] if there exist a tensor φ of type (1, 1), a vector field ξ (structure vector field) and a 1-form η on M̃ satisfying

(2.1)φ2X=−X+η(X)ξ,η(ξ)=1,

(2.2)φξ=0,η°ξ=0,g(X,ξ)=η(X),

and

(2.3)g(φX,φY)=g(X,Y)−η(X)η(Y),

for any X,Y∈Γ(TM̃). The two-form Φ is called the fundamental two-form in M̃ and the manifold is said to be a contact metric manifold if

Φ=dη.

A Sasakian manifold is a normal contact metric manifold. In fact, an almost contact metric manifold is Sasakian manifold if and only if we have

(∇¯Xφ)Y=g(X,Y)ξ−η(Y)X,

for any X,Y∈Γ(TM̃), where ∇¯ denotes the Riemannian connection.

A plane section π in TpM̃ is called a φ-section if it is spanned by X and φX, where X is a unit tangent vector orthogonal to ξ. The sectional curvature of a φ-section is called a φ-sectional curvature. A Sasakian manifold with constant φ-sectional curvature c is said to be a Sasakian space form and denoted by M̃(c). The curvature tensor of a Sasakian space form M̃(c) is given by [16].

R(X,Y)Z=(c+3)4{g(Y,Z)X−g(X,Z)Y}+(c−1)4η(Z)η(Y)X−η(X)Y+(g(Y,Z)η(X)−g(X,Z)η(Y))ξ−g(φY,Z)φX+g(φX,Z)φY+2g(φX,Y)φZ,

for any X,Y,Z∈Γ(TM̃).

2.2. Conformal Sasakian manifold

A (2n + 1)-dimensional Riemannian manifold M̃ endowed with the almost contact metric structure (φ, η, ξ, g) called a conformal Sasakian manifold if for a C^∞ function

f:M̃→R

there exists [3].

g̃=exp(f)g,ξ̃=(exp(−f))12ξ,

η̃=(exp(f))12η,φ̃=φ

such that (M̃,φ̃,ξ̃,g̃) is a Sasakian manifold.

Let ∇̃ and ∇¯ denote connections of M̃ related to metrics g̃ and g, respectively. Using Koszul formula, we derive the following relation between the connections ∇̃ and ∇¯:

∇̃XY=∇¯XY+12{ω(X)Y+ω(Y)X−g(X,Y)ω#},

for any X,Y∈Γ(M̃) so that ω(X) = X(f) and ω^# is vector field of metrically equivalent to one form of ω, i.e. g(ω^#, X) = ω(X). The vector field ω^# = grad  f is called the Lee vector field of conformal Sasakian manifold M̃.

The (2n + 1)-dimensional conformal Sasakian manifold with constant sectional curvature c, denoted by M̃(c), is called a conformal Sasakian space form and its curvature tensor is given by [3].

(2.4)g(R¯(X,Y)Z,W)=exp(f)c+34g(Y,Z)g(X,W)−g(X,Z)g(Y,W)+c−14η(X)η(Z)g(Y,W)−η(Y)η(Z)g(X,W)+g(X,Z)g(ξ,W)η(Y)−g(Y,Z)g(ξ,W)η(X)−g(φY,Z)g(φX,W)−g(φX,Z)g(φY,W)−2g(φX,Y)g(φZ,W)}−12B(X,Z)g(Y,W)−B(Y,Z)g(X,W)+B(Y,W)g(Y,Z)−B(X,W)g(Y,Z)−14‖ω#‖2g(X,Z)g(Y,W)−g(X,W)g(Y,Z),

for any X,Y,Z,W,ω#,ξ∈Γ(TM̃), where B≔∇¯ω−12ω⊗ω.

2.3. Quarter-symmetric metric connection

Let M̃ be an (2n + 1)-dimensional Riemannian manifold with Riemannian metric g and ∇ be the Levi-Civita connection on M̃. Let ∇¯̃ be a linear connection defined by [17].

(2.5)∇¯̃XY=∇¯XY+Λ1λ(Y)X−Λ2g(X,Y)V,

for any X,Y∈Γ(M̃), Λ₁ and Λ₂ are real constants and V is the vector field on M̃ such that λ(X)=g(X,V), where λ is 1-form. If ∇¯̃g=0, then ∇¯̃ is known as quarter-symmetric metric connection and ∇¯̃g≠0, then ∇¯̃ is known as quarter-symmetric non-metric connection. Decomposing the vector field V on M̃ uniquely into its tangent and normal components VT and V⊥, respectively.

The special cases of (2.5) can be obtained as follows:

1. when Λ₁ = Λ₂ = 1, then the above connection reduces to semi-symmetric metric connection and

2. when Λ₁ = 1 and Λ₂ = 0, then the above connection reduces to semi-symmetric non-metric connection.

For any X,Y,Z,W∈Γ(TM̃), the curvature tensor with respect to ∇¯̃ is given by

(2.6)R¯̃(X,Y)Z=∇¯̃X∇¯̃YZ−∇¯̃Y∇¯̃XZ−∇¯̃[X,Y]Z.

On using (2.5), the curvature tensor (2.6) takes the form [17] as follows:

(2.7)R¯̃(X,Y,Z,W)=R¯(X,Y,Z,W)+Λ1α(X,Z)g(Y,W)−Λ1α(Y,Z)g(X,W)+Λ2α(Y,W)g(X,Z)−Λ2α(X,W)g(Y,Z)+Λ2(Λ1−Λ2)g(X,Z)β(Y,W)−Λ2(Λ1−Λ2)g(Y,Z)β(X,W),

where α and β are (0, 2)-tensors and defined as follows:

α(X,Y)=(∇¯Xλ)(Y)−Λ1λ(X)λ(Y)+Λ22g(X,Y)λ(V),

and

β(X,Y)=λ(V)2g(X,Y)+λ(X)λ(Y).

The curvature tensor of conformal Saasakian space form M̃(c) with a quarter-symmetric connection ∇¯̃ is given by

(2.8)g(R¯̃(X,Y)Z,W)=exp(f)c+34g(Y,Z)g(X,W)−g(X,Z)g(Y,W)+c−14η(X)η(Z)g(Y,W)−η(Y)η(Z)g(X,W)+g(X,Z)g(ξ,W)η(Y)−g(Y,Z)g(ξ,W)η(X)−g(φY,Z)g(φX,W)−g(φX,Z)g(φY,W)−2g(φX,Y)g(φZ,W)−12B(X,Z)g(Y,W)−B(Y,Z)g(X,W)+B(Y,W)g(Y,Z)−B(X,W)g(Y,Z)−14‖ω#‖2g(X,Z)g(Y,W)−g(X,W)g(Y,Z)+Λ1α(X,Z)g(Y,W)−Λ1α(Y,Z)g(X,W)+Λ2g(X,Z)α(Y,W)−Λ2g(Y,Z)α(X,W)+Λ2(Λ1−Λ2)g(X,Z)β(Y,W)−Λ2(Λ1−Λ2)g(Y,Z)β(X,W).

For simplicity, we have put tr(α) = a and tr(β) = b.

Let M be an m-dimensional submanifold of a (2n + 1)-dimensional conformal Saasakian space form M̃(c). We consider the induced quarter-symmetric connection on M represented by ∇̃M and the induced Levi-Civita connection denoted by ∇M. Let R and RM be the curvature tensors of ∇̃M and ∇M. Then, the Gauss equation is given by

(2.9)R¯̃(X,Y,Z,W)=R(X,Y,Z,W)−g(h(X,W),h(Y,Z))+g(h(Y,W),h(X,Z))+(Λ1−Λ2)g(h(Y,Z),V⊥)g(X,W)+(Λ2−Λ1)g(h(X,Z),V⊥)g(Y,W),

where h is the second fundamental form of M in M̃ with respect to ∇¯̃ and defined as follows:

h(X,Y)=h′(X,Y)−Λ2g(X,Y)V⊥.

Here, h′ is the second fundamental form of M in M̃ with respect to ∇¯ and g denotes the Riemannian metric on M.

For any X∈Γ(TM), we can write φX = PX + SX, where the PX (respectively, SX) is the tangential component (respectively normal component) of φX. If P = 0, then the submanifold is anti-invariant and if S = 0, then the submanifold is invariant. The squared norm of P at p∈M is given as follows:

(2.10)‖P‖2=∑i,j=1mg2(φei,ej),

where {e₁, …, e_m} is any orthonormal basis of TpM and p∈M. The structure vector field ξ can be decomposed as ξ = ξ^T + ξ^⊥, where ξ^T and ξ^⊥ are tangential and normal components of ξ.

The notion of bi-slant submanifolds was introduced by A. Carriazo et al. as a natural generalization of CR, slant, semi-slant and hemi-slant submanifolds (see [18–20]). Recently, S. Uddin and B.-Y. Chen studied bi-slant and pointwise bi-slant submanifolds for their warped products in [21, 22]. A submanifold M of an almost contact-metric manifold M̃ is called bi-slant submanifolds, whenever we have

1. TMm=Dθ1⊕Dθ1⊕⟨ξ⟩ and

2. φDθ1⊥Dθ2 and φDθ2⊥Dθ1

where Dθ1 and Dθ2 are two orthogonal distributions of M with slant angle θ₁ and θ₂, respectively.

Let M be a bi-slant submanifold of a conformal Sasakian space form M̃. We assume that dim(M)=m=2m1+2m2+1, where dim(Dθ1)=m1 and dim(Dθ2)=m2. Let {e₁, …, e_m = ξ} be an orthonormal basis of TpM at p in M with

e1,e2=secθ1Pe1,…,e2m1−1,e2m1=secθ1Pe2m1−1,e2m1+1,e2m1+2=secθ2Pe2m1+1,…,e2m1+2m2−1,e2m1+2m2=secθ2Pe2m1+2m2−1,e2m1+2m2+1=ξ,

from which we have [23] as follows:

g2(φei+1,ei)=cos2θ1,fori=1,2,…,2m1−1cos2θ2,fori=2m1+1,…,2m1+2m2−1.

Thus, we have

∑i,j=1mg2(φei,ej)=2{m1⁡cos2θ1+m2⁡cos2θ2}.

In fact, semi-slant, pseudo-slant, CR and slant submanifolds can be obtained from bi-slant submanifolds in particular. We can see the cases in the following Table 1:

The special case of slant submanifold are invariant and anti-invariant if θ = 0 and θ=π2, respectively. The slant submanifold is said to be proper slant and proper bi-slant submanifold, if 0<θ<π2 and θ_i lies between 0 and π2.

3. Main inequalities

In [10], Mihai discussed the generalized Wintgen inequality for Legendrian submanifolds in Sasakian space forms. He also stated such an inequality for contact slant submanifolds in Sasakian space forms. Thus, in this section, we obtain such an inequality in terms of the invariant ρM (called normalized scalar-normal curvature) for bi-slant submanifolds of dimension m in a (2n + 1)-dimensional conformal Sasakian space form M̃. Consider the local orthonormal tangent frame {e₁, …, e_m} of the tangent bundle TM of M and a local orthonormal normal frame {e_m+1, …, e_2n+1} of the normal bundle T⊥M of M in M̃. At any p∈M, the scalar curvature τ at that point is given by

(3.1)T=∑1≤i<j≤mR(ei,ej,ej,ei).

The mean curvature H of submanifold is given by

H=1m∑i=1mh(ei,ei).

Conveniently, let us put

hijr=g(h(ei,ei),er),

for any i, j = {1, …, m} and r = {m + 1, …, 2n + 1}.

We denote by K and R^⊥, the sectional curvature function and the normal curvature tensor on M, respectively. Then the normalized scalar curvature ρ is given by [8].

(3.2)ρ=2τm(m−1)=2m(m−1)∑1≤i<j≤mκ(ei∧ej).

In term of the components of the second fundamental form, we can express the scalar normal curvature κM of M by the formula [8].

(3.3)κM=∑1≤r<s≤2n−m+1∑1≤i<j≤m∑t=1mhjtrhits−hjtrhits2,

and the normalized scalar normal curvature is given by [8].

(3.4)ρM=2m(m−1)κM.

Theorem 3.1.

Let M be an m-dimensional bi-slant submanifold in conformal Sasakian space form M̃(c) of dimension (2n + 1) endowed with a quarter-symmetric connection, then we have

(3.5) ρM+ρ≤‖H‖2+exp(f)c+34−c−12m+3(c−1)4m(m−1)(m1⁡cos2θ1+m2⁡cos2θ2)+tr(B)m+14‖ω#‖2−(Λ1+Λ2)am+Λ2(Λ1−Λ2)bm−(Λ2−Λ1)λ(H).

Moreover, the equality case holds in the above inequality at a point p∈M if and only if, with respect to some suitable orthonormal basis e1,…,em of TpM and ξ1,…,ξ2n−m+1 of Tp⊥M, the shape operators S_γ, γ = 1, …, 2n − m + 1, take the forms as follows:

(3.6)S1=μ1ζ0…0ζμ10…000μ1…0⋮⋮⋮⋱⋮000…μ1,

(3.7)S2=μ2+ζ00…00μ2−ζ0…000μ2…0⋮⋮⋮⋱⋮000…μ2,

(3.8)S3=μ300…00μ30…000μ3…0⋮⋮⋮⋱⋮000…μ3,S4=⋯=S2n−m+1=0,

where μ₁, μ₂, μ₃, and ζ are real numbers.

Proof.

Let {e₁, …, e_m} and {e_m+1, …, e_2n+1} be orthonormal tangent frame and orthonormal normal frame on M, respectively. Putting X = W = e_i, Y = Z = e_j, i ≠ j in (2.9) and using (2.8), we obtain

(3.9) R¯̃(ei,ej,ej,ei)=exp(f)c+34g(ej,ej)g(ei,ei)−g(ei,ej)g(ej,ei)+c−14η(ei)η(ej)g(ej,ei)−η(ej)η(ej)g(ei,ei)+g(ei,ej)g(ξ,ei)η(ej)−g(ej,ej)g(ξ,ei)η(ei)−g(φej,ej)g(φei,ei)−g(φei,ej)g(φej,ei)−2g(φei,ej)g(φej,ei)}−12B(ei,ej)g(ej,ei)−B(ej,ej)g(ei,ei)+B(ej,ei)g(ei,ej)−B(ei,ei)g(ej,ej)−14‖ω#‖2g(ei,ej)g(ej,ei)−g(ej,ej)g(ei,ei)+Λ1α(ei,ej)g(ej,ei)−Λ1α(ej,ej)g(ei,ei)+Λ2g(ei,ej)α(ej,ei)−Λ2g(ej,ej)α(ei,ei)+Λ2(Λ1−Λ2)g(ei,ej)β(ej,ei)−Λ2(Λ1−Λ2)g(ej,ej)β(ei,ei),−(Λ1−Λ2)g(h(ej,ej),P⊥)g(ei,ei)−(Λ2−Λ1)g(h(ei,ej),P⊥)g(ej,ei).

By taking summation 1 ≤ i < j ≤ m of (3.9) and using (2.7), we have

(3.10)∑1≤i<j≤mR(ei,ej,ej,ei)=exp(f)2(c+3)m(m−1)4+(c−1)42−2m+3‖P‖2+(m−1)2tr(B)+18m(m−1)‖ω#‖2+12(Λ1+Λ2)(1−m)a+Λ2(Λ1−Λ2)(1−m)b+(Λ2−Λ1)m(m−1)λ(H)+∑r=12n−m+1∑1≤i<j≤m[hiirhjjr−(hijr)2],

where

λ(H)=1m∑j=1mλ(h(ej,ej))=g(V⊥,H).

Using (2.10) and (3.1), we obtain

(3.11)τ=exp(f)2(c+3)m(m−1)4+(c−1)42−2m+3(m1⁡cos2θ1+m2⁡cos2θ2)+(m−1)2tr(B)+18m(m−1)‖ω#‖2+12(Λ1+Λ2)(1−m)a+Λ2(Λ1−Λ2)(1−m)b+(Λ2−Λ1)m(m−1)λ(H)+∑r=12n−m+1∑1≤i<j≤m[hiirhjjr−(hijr)2].

On the other hand, we have

(3.12)m2‖H‖2=∑r=12n−m+1∑i=1mhiir2=1m−1∑r=12n−m+1∑1≤i<j≤m(hiir−hjjr)2+2mm−1∑r=12n−m+1∑1≤i<j≤mhiirhjjr.

Further, from [6], we have

(3.13)∑r=12n−m+1∑1≤i<j≤m(hiir−hjjr)2+2m∑r=12n−m+1∑1≤i<j≤m(hiir)2≥2m∑1≤r<s≤2n−m+1∑1≤i<j≤mhjkrhiks−hikrhjks212.

Now, combining (3.3), (3.12) and (3.14), we have

(3.14)m2‖H‖2−m2ρM≥2mm−1∑r=12n−m+1∑1≤i<j≤m[hiirhjjr−(hijr)2].

Taking into account (3.2), (3.11) and (3.14), we obtain the required inequality.

Finally, by investigating the equality case of (3.5), the equality sign holds in (3.5) at a point p∈M if and only if the shape operators take the forms (3.6)–(3.8) with respect to some suitable tangent and normal orthonormal bases. □

An immediate consequence of Theorem 3.1 yields the following:

Corollary 3.2.

Let M be a minimal m-dimensional bi-slant submanifold in conformal Sasakian space form M̃(c) of dimension (2n + 1) endowed with a quarter-symmetric connection, then we have

(3.15) ρM+ρ≤exp(f)c+34−c−12m+3(c−1)4m(m−1)(m1⁡cos2θ1+m2⁡cos2θ2)+tr(B)m+14‖ω#‖2−(Λ1+Λ2)am+Λ2(Λ1−Λ2)bm−(Λ2−Λ1)λ(H).

Corollary 3.3.

Let M be an m-dimensional submanifold in conformal Sasakian space form M̃(c) of dimension (2n + 1) endowed with a quarter-symmetric connection, then we have Table 2:

For the semi-symmetric metric connection Λ₁ = Λ₂ = 1, we have

Theorem 3.4.

Let M be an m-dimensional bi-slant submanifold in conformal Sasakian space form M̃(c) of dimension (2n + 1) endowed with a semi-symmetric metric connection, then we have

(3.16) ρM+ρ≤‖H‖2+exp(f)c+34−c−12m+3(c−1)4m(m−1)(m1⁡cos2θ1+m2⁡cos2θ2)+tr(B)m+14‖ω#‖2−2am.

Corollary 3.5.

Let M be an m-dimensional submanifold in conformal Sasakian space form M̃(c) of dimension (2n + 1) endowed with a semi-symmetric metric connection, then we have Table 3:

For the semi-symmetric non-metric connection Λ₁ = 1 and Λ₂ = 0, we have

Theorem 3.6.

Let M be an m-dimensional bi-slant submanifold in conformal Sasakian space form M̃(c) of dimension (2n + 1) endowed with a semi-symmetric non-metric connection, then we have

(3.17) ρM+ρ≤‖H‖2+exp(f)c+34−c−12m+3(c−1)4m(m−1)(m1⁡cos2θ1+m2⁡cos2θ2)+tr(B)m+14‖ω#‖2−am−λ(H).

Corollary 3.7.

Let M be an m-dimensional submanifold in conformal Sasakian space form M̃(c) of dimension (2n + 1) endowed with a semi-symmetric non-metric connection, then we have Table 4 as follows:

4. Some examples of conformal Sasakian manifolds

In this segment, we provide some examples of a conformal Sasakian manifolds which is not Sasakian.

Example 4.1.

Let us consider a three-dimensional manifold

M̃=(x1,x2,x3)∈R3:x3>0,

where (x₁, x₂, x₃) are standard coordinates in R3. We choose the vector fields

v1=2∂∂x1+x2∂∂x3,v2=2∂∂x2,v3=2(ex3/2)∂∂x3,

which are linearly independent at each point of M̃. Let g be the Riemannian metric defined by

g(vi,vj)=0,i≠j,i,j=1,2,3,g(v1,v1)=g(v2,v2)=e−x3,g(v3,v3)=1.

Let η be the an 1-form defined by

η(v3)=1,η(v2)=0,η(v1)=0,η(U)=g(U,v3),∀U∈Γ(TM̃).

We define the (1, 1) tensor field φ as

φ(v1)=v2,φ(v2)=−v1,φ(v3)=0.

The linear property of g and φ yield that

η(v3)=1,φ2(U)=−U+η(U)v3

g(φU,φV)=g(U,V)−η(U)η(V),

for any U,V∈Γ(M̃). Thus, (M̃,φ,ξ,η,g) defines an almost contact metric manifold with ξ = v₃ [24]. Then we have

[v1,v2]=−2e−z1/2.

Similarly,

[v1,ξ]=x2v3,[v2,v3]=0.

The Riemannian connection ∇¯ of the metric g is given by

2g(∇¯XY,Z)=Xg(Y,Z)+Yg(Z,X)−Zg(X,Y)

−g(X,[Y,Z])−g(Y,[X,Z])+g(Z,[X,Y]).

By Koszul’s formula, we obtain the following

∇¯v1v2=−x3v2−e−x31/2v3,∇¯v1v3=−ex31/2(v1−v2),∇¯v1v1=−x2v1e−x31/2v3,

∇¯v2v1=−x2v2e−x31/2v3,∇¯v2v2=x2v1e−x31/2v3,∇¯v2v3=−e−x31/2(v1+v2),

∇¯v3v3=x2v1ex3v1,∇¯v3v2=x2v1+e−x31/2v3,∇¯v3v1=−ex31/2(v1−v2)−x2v3.

Using contact transformation

(4.1)φ̃=φ,ξ̃=e−x1/2ξ,η̃=ex1/2η,g=ex1g.

(M̃,φ̃,ξ̃,η̃,g) is Sasakian manifold. So M̃ is a conformal Sasakian manifold but not Sasakian. Since by the definition, we have

(4.2)∇¯v2v3≠φv2,

for any v1,v2∈Γ(M̃) (for instance ∇¯v2v3≠0). By using the above results, we can find the non-vanishing components of Riemannian curvature, Ricci curvature tensor and scalar curvature as follows:

(4.3)R¯(v1,v2)v2=−4v1+x2e−x31/2v3,

R¯(v1,v3)v3=−v1+3v2+x2e−x31/2v3,R¯(v2,v3)v3=ex31/2v1+e−x31/2v2.

In view of above expressions, we turn up the following:

K¯(v1,v2)=−4ex3,K¯(v1,v3)=−1,K¯(v2,v3)=ex3(1−x2).

Note that the sectional curvature of manifold M̃ with almost contact-metric structure (M̃,φ̃,ξ̃,η̃,g) is

K¯(v1,v2)=−3,K¯(v1,v3)=−1,K¯(v2,v3)=−1.

Moreover, the non-vanishing components of Ricci curvature tensor, and scalar curvature are given by

(4.4)Ric¯(v2,v2)=−4,Ric¯(v3,v3)=−1+e−x3/2.

ρ¯=−5+e−x3/2.

Example 4.2

Let us consider a three-dimensional manifold

M̃=(x1,x2,x3)∈R3:x3>0,

where (x₁, x₂, x₃) are standard coordinates in R3. We choose the vector fields

v1=x1∂∂x3,v2=x1∂∂x2,v3=−(e−x1/2)x1∂∂x1,

which are linearly independent at each point of M̃. Let g be the Riemannian metric defined by

g(vi,vj)=0,i≠j,i,j=1,2,3,g(v1,v1)=g(v2,v2)=e−x1,g(v3,v3)=1.

Let η be the an 1-form defined by

η(v3)=1,η(v2)=0,η(v1)=0,η(U)=g(U,v3),∀U∈Γ(TM̃).

We define the (1, 1) tensor field φ by

φ(v1)=v2,φ(v2)=−v1,φ(v3)=0.

The linear property of g and φ yield

η(v3)=1,φ2(U)=−U+η(U)v3,

g(φU,φV)=g(U,V)−η(U)η(V),

for any U,V∈Γ(M̃). Thus, (M̃,φ,ξ,η,g) defines an almost contact metric manifold with ξ = v₃. Then, we have

[v1,v2]=0.

Similarly,

[v1,ξ]=ex1/2v1,[v2,v3]=ex1/2.

The Riemannian connection ∇¯ of the metric g is given by

2g(∇¯XY,Z)=Xg(Y,Z)+Yg(Z,X)−Zg(X,Y)

−g(X,[Y,Z])−g(Y,[X,Z])+g(Z,[X,Y]).

By Koszul’s formula, we obtain the following:

∇¯v1v2=0,∇¯v1v3=12ex1/2(x1+2)v1,∇¯v1v1=−12ex1/2(x1+2)v3,

∇¯v2v1=0,∇¯v2v2=−12e−x1/2(x1+2)v3,∇¯v2v3=12ex1/2x1v2,

∇¯v3v3=0,∇¯v3v2=12ex1/2x1v2,∇¯v3v1=−12ex1/2x1v1.

Adopting contact transformation

(4.5)φ̃=φ,ξ̃=e−x1/2ξ,η̃=ex1/2η,g̃=ex1g.

(M̃,φ̃,ξ̃,η̃,g̃) is Sasakian manifold. Therefore, M̃ is a conformal Sasakian manifold but not Sasakian, since by the definition, we have

(4.6)(∇¯v2φ)v1≠g(v1,v2)ξ−η(v2)v1,

for any v1,v2∈Γ(M̃). By using the above results, we find the non-vanishing components of Riemannian curvature, Ricci curvature tensor and scalar curvature.

(4.7)R¯(v1,v2)v2=−14(x1+2)2v1,R¯(v2,v3)v3=−ex1v2,R¯(v1,v2)v3=0,

R¯(v1,v3)v3=−ex1v1,R¯(v3,v1)v1=−v3,R¯(v3,v1)v2=0,

R¯(v2,v1)v1=−14(x1+2)2v2,R¯(v3,v2)v2=−v3.

(4.8)Ric¯(v1,v1)=Ric¯(v2,v2)=14(x1+2)2e−x1,Ric¯(v3,v3)=2e−2x1.

ρ¯=e−x112(−x1+2)2+2e−x1.

Example 4.3.

Let R2n+1 endowed with an almost contact structure (φ, ξ, η, g) given by [3]

g=e−2tη⊗η+14∑i=1n(dxi)2+(dyi)2,ξ=et2∂∂z,

φ∑i=1nXi∂∂xi+Yi∂∂yi+Z∂∂z=∑i=1nYi∂∂xi−Xi∂∂yi+∑i=1nYiyi∂∂z,

where t=12∑i=1n(xi)2+(yi)2+z2.

Then, (R2n+1,φ,ξ,η,g) is not Sasakian manifold, but (R2n+1,φ̃,ξ̃,η̃,g̃) is Sasakian space form with constant φ̃-sectional curvature, where

φ̃=φ,ξ̃=2∂∂z,η̃=12dz−∑i=1nyidxi,

g̃=η⊗η+14∑i=1n(dxi)2+(dyi)2.

Therefore, (M̃,φ̃,ξ̃,η̃,g̃) is the conformal Sasakian space form so that (M̃,φ̃,ξ̃,η̃,g̃) is Sasakian space form of constant φ̃-sectional curvature c = − 3.

Example 4.4.

In [25], it was shown that the warped product R×fCn is a generalized Sasakian space form with

f1=−(f′)2f2,f2=0,f3=−(f′)2f2+f′′f,

where f = f(t), t∈R and f′ denotes the derivative of f with respect to t. If we choose n = 4 and f(t) = e^t, then M̃ is a five-dimensional conformal Sasakian space form.

Example 4.5.

Let us consider a 11-dimensional manifold M̃=(x1,…,x10,z)∈R11:z≠0, where (x₁, x₂, x₃, x₄, x₅, x₆, x₇, x₈, x₉, x₁₀, z) are standard coordinates in R11. We choose the vector fields Ei=e−z∂∂xi,i=1,2,…,10,E11=e−z∂∂z, which are linearly independent at each point of M̃. We define g̃ by

g̃=e2zG,

where G is the Euclidean metric on R11. Hence Eii=1,2,…,11 is an orthonormal basis of M̃.

We consider an one-form η̃ defined by

η̃=ezdz,η̃(X)=g̃(X,E11),∀X∈Γ(TM̃).

We define the (1, 1) tensor field φ̃ by

φ̃∑i=15xi∂∂xi+xi+5∂∂xi+5+z∂∂z=∑i=15xi∂∂xi+5−xi+5∂∂xi.

Thus, we have

φ̃(Ei)=Ei+5,φ̃(Ei+5)=−Ei,φ̃(E11)=0,1≤i≤5.

The linear property of g and φ̃ yield that

η̃(E11)=1,φ̃2(X)=−X+η̃(X)E11

g̃(φ̃X,φ̃Y)=g̃(X,Y)−η̃(X)η̃(Y),

for any X,Y∈Γ(TM̃). Thus, (M̃,φ̃,ξ̃,η̃,g̃) defines an almost contact-metric manifold with ξ̃=E11. Then, we have [E₁, E₂] = 0. Similarly, [Ei,ξ̃]=e−zEi, [E_i, E_j] = 0, 1 ≤ i ≠ j ≤ 10.

2g̃(∇¯XY,Z)=Xg̃(Y,Z)+Yg̃(Z,X)−Zg̃(X,Y)

−g̃(X,[Y,Z])−g̃(Y,[X,Z])+g̃(Z,[X,Y]).

By Koszul’s formula, we obtain the equations as follows:

∇¯EiEi=−e−zξ̃,∇¯ξ̃ξ̃=0,∇¯ξ̃Ei=0,∇¯Eiξ̃=e−zEi,1≤i≤10.

Thus, we see that M̃ is the conformal Sasakian manifold.

Now, we define a submanifold M of M̃ by the immersion γ as follows:

γ(u1,u2,u3,u4,u5,u6,u7)=e−zu1,u3,0,12u5,12u6,u2,u4⁡cos⁡θ,u4⁡sin⁡θ,12u5,12u6,u7

for 0<θ<π2.

It is easy to check that tangent bundle TM=Span{X1,X2,X3,X4,X5,X6,X7}, where

X1=e−z∂∂x1,X2=e−z∂∂x6,X3=e−z∂∂x2,X4=e−zcos⁡θ∂∂x7+sin⁡θ∂∂x8,

X5=e−z12∂∂x4+∂∂x9,X6=e−z12∂∂x5+∂∂x10,X7=e−z∂∂z.

Using the almost contact structure φ, we obtain

φ̃X1=e−z∂∂x6,φ̃X2=−e−z∂∂x1,φ̃X3=e−z∂∂x7,

φ̃X4=−e−zcos⁡θ∂∂x2+sin⁡θ∂∂x3,φ̃X5=e−z12−∂∂x4+∂∂x9,

φ̃X6=e−z12−∂∂x5+∂∂x10,φ̃X7=0.

If we consider the distributions as follows:

Dθ1=Span{X1,X2},Dθ2=Span{X3,X4},ξ=Span{X5,X6}.

Then, we have TM=Dθ1⊕Dθ2⊕ξ. By some computations, it can be verified that M is bi-slant submanifold of M̃.

Table 1

Different types of submanifolds

S N	Mm	Dθ1	Dθ2	θ1	θ2
(1)	Bi-slant	Slant distribution	Slant distribution	Slant angle	Slant angle
(2)	Semi-slant	Invariant distribution	Slant distribution	0	Slant angle
(3)	Pseudo-slant	Slant distribution	Anti-invariant distribution	Slant angle	π2
(4)	Contact CR	Invariant distribution	Anti-invariant distribution	0	π2
(5)	Slant	Either Dθ1=0 or Dθ2=0		Either θ1 = θ2 = θ or θ1 = θ2 ≠ θ

Table 2

Inequalities for different submanifolds in a conformal Sasakian space form endowed with a quarter-symmetric connection

SN	M	Inequality
(1)	Semi-slant	ρM+ρ≤‖H‖2+exp(f)c+34−c−12m+3(c−1)4m(m−1)(m1+m2⁡cos2θ2)+tr(B)m+14‖ω#‖2−(Λ1+Λ2)am+Λ2(Λ1−Λ2)bm−(Λ2−Λ1)λ(H)
(2)	Pseudo-slant	ρM+ρ≤‖H‖2+exp(f)c+34−c−12m+3(c−1)4m(m−1)(m1⁡cos2θ1)+tr(B)m+14‖ω#‖2−(Λ1+Λ2)am+Λ2(Λ1−Λ2)bm−(Λ2−Λ1)λ(H)
(3)	Contact CR	ρM+ρ≤‖H‖2+exp(f)c+34−c−12m+3(c−1)4m(m−1)(m1)+tr(B)m+14‖ω#‖2−(Λ1+Λ2)am+Λ2(Λ1−Λ2)bm−(Λ2−Λ1)λ(H)
(4)	Slant	ρM+ρ≤‖H‖2+exp(f)c+34−c−12m+3(c−1)4m(m−1)(m1+m2)cos2⁡θ+tr(B)m+14‖ω#‖2−(Λ1+Λ2)am+Λ2(Λ1−Λ2)bm−(Λ2−Λ1)λ(H)
(5)	Invariant	ρM+ρ≤‖H‖2+exp(f)c+34−c−12m+3(c−1)4m(m−1)(m1+m2)+tr(B)m+14‖ω#‖2−(Λ1+Λ2)am+Λ2(Λ1−Λ2)bm−(Λ2−Λ1)λ(H)
(6)	Anti-nvariant	ρM+ρ≤‖H‖2+exp(f)c+34−c−12m+tr(B)m+14‖ω#‖2−(Λ1+Λ2)am+Λ2(Λ1−Λ2)bm−(Λ2−Λ1)λ(H)

Table 3

Inequalities for different submanifolds in a conformal Sasakian space form endowed with a semi-symmetric metric connection

SN	M	Inequality
(1)	Semi-slant	ρM+ρ≤‖H‖2+exp(f)c+34−c−12m+3(c−1)4m(m−1)(m1+m2⁡cos2θ2)+tr(B)m+14‖ω#‖2−2am
(2)	Pseudo-slant	ρM+ρ≤‖H‖2+exp(f)c+34−c−12m+3(c−1)4m(m−1)(m1⁡cos2θ1)+tr(B)m+14‖ω#‖2−2am
(3)	Contact CR	ρM+ρ≤‖H‖2+exp(f)c+34−c−12m+3(c−1)4m(m−1)(m1)+tr(B)m+14‖ω#‖2−2am
(4)	Slant	ρM+ρ≤‖H‖2+exp(f)c+34−c−12m+3(c−1)4m(m−1)(m1+m2)cos2⁡θ+tr(B)m+14‖ω#‖2−2am
(5)	Invariant	ρM+ρ≤‖H‖2+exp(f)c+34−c−12m+3(c−1)4m(m−1)(m1+m2)+tr(B)m+14‖ω#‖2−2am
(6)	Anti-invariant	ρM+ρ≤‖H‖2+exp(f)c+34−c−12m+tr(B)m+14‖ω#‖2−2am

Table 4

Inequalities for different submanifolds in a conformal Sasakian space form endowed with a semi-symmetric non-metric connection

SN	M	Inequality
(1)	Semi-slant	ρM+ρ≤‖H‖2+exp(f)c+34−c−12m+3(c−1)4m(m−1)(m1+m2⁡cos2θ2)+tr(B)m+14‖ω#‖2−2am−λ(H)
(2)	Pseudo-slant	ρM+ρ≤‖H‖2+exp(f)c+34−c−12m+3(c−1)4m(m−1)(m1⁡cos2θ1)+tr(B)m+14‖ω#‖2−2am−λ(H)
(3)	Contact CR	ρM+ρ≤‖H‖2+exp(f)c+34−c−12m+3(c−1)4m(m−1)(m1)+tr(B)m+14‖ω#‖2−2am−λ(H)
(4)	Slant	ρM+ρ≤‖H‖2+exp(f)c+34−c−12m+3(c−1)4m(m−1)(m1+m2)cos2⁡θ+tr(B)m+14‖ω#‖2−2am−λ(H)
(5)	Invariant	ρM+ρ≤‖H‖2+exp(f)c+34−c−12m+3(c−1)4m(m−1)(m1+m2)+tr(B)m+14‖ω#‖2−2am−λ(H)
(6)	Anti-invariant	ρM+ρ≤‖H‖2+exp(f)c+34−c−12m+tr(B)m+14‖ω#‖2−2am−λ(H)