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Abstract
In this paper, it has been proved that if a 3-dimensional cosymplectic manifold admits a Yamabe soliton, then either
is locally flat or the potential field is a contact vector field. Some special potential vector fields of Yamabe solitons on 3-dimensional cosymplectic manifolds have been considered and some other results have been obtained. Also, for general
-dimensional case, it will be shown that if an
cosymplectic manifold
admits a contact Yamabe soliton structure, then
is a cosymplectic manifold. Finally, an example of Yamabe soliton on a 3-dimensional cosymplectic manifold is provided.
PUBLIC INTEREST STATEMENT
The notion of Yamabe flow was introduced by R. S. Hamilton in 1988 in order to study Yamabe’s conjecture stating that any metric is conformally related to a metric with constant scalar curvature. In this research article, we discuss the geometry of Yamabe solitons on 3-dimensional cosymplectic manifolds. We prove that if a 3-dimensional cosymplectic manifold M3 admits a Yamabe soliton, then either M3 is locally flat or the potential field is a contact vector field. We consider some special potential vector fields of Yamabe solitons on 3-dimensional cosymplectic manifolds and obtain some other results. Also, for general (2n + 1)-dimensional case, we will show that if an f - cosymplectic manifold M2n + 1 admits a contact Yamabe soliton structure, then M2n + 1 is a cosymplectic manifold.
1. Introduction
The notion of Yamabe flow was introduced by R. S. Hamilton in 1988 to study Yamabe’s conjecture stating that any metric is conformally related to a metric with constant scalar curvature. Also, Yamabe solitons serve as self-similar solutions of Yamabe flow. This notion is the subject of many types of research in the last decade. A Riemannian manifold is said to be a Yamabe soliton if there is a smooth vector field
and a constant
such that
where is the scalar curvature, and
stands for the Lie derivative operator. The soliton is shrinking, steady and expanding according to as
and
. If
for some real-valued smooth function
on
, then it is called the gradient Yamabe soliton, and
is called the potential function. In this case, the EquationEquation 1
(1)
(1) ) can be rewritten as follows:
On the other hand, the geometry of several kinds of almost contact metric manifolds is the subject of many kinds of research in the last decade. Especially, the notion of Ricci soliton as a generalization of the Einstein metric, on some classes of almost contact metric manifolds has been investigated by many authors (Ghosh, Citation2011; Sharma & Ghosh, Citation2011). For instance, it is proved that any dimensional Sasakian manifold admitting a non-trivial Ricci soliton is homothetic to the standard Sasakian structure on a 3-dimensional Heisenberg group (Sharma & Ghosh, Citation2011). Cho proved that a Kenmotsu 3-manifold
admitting a Ricci soliton with a transversal potential vector field (orthogonal to the Reeb vector field) is of constant sectional curvature
. Also he has shown cosymplectic 3-manifold admitting a Ricci soliton with the Reeb potential vector field or a transversal vector field is of constant sectional curvature 0 (Cho, Citation2013). In (Ghosh, Citation2011), Ghosh proved that a 3-dimensional Kenmotsu manifold equipped with a Ricci soliton is necessarily of constant sectional curvature
. Recently, one of the present authors in a joint work has studied Ricci
-soliton on 3-dimensional
-Einstein almost Kenmotsu manifolds (Azami and Fasihi-Ramandi, Citation2020). The results show that if an
-Einstein almost Kenmotsu manifold
admits a
-Ricci soliton, then
is a Kenmotsu manifold with constant sectional curvature
and the soliton is expanding with
. Ricci solitons on 3-dimensional cosymplectic manifolds have been studied by Wang (Wang, Citation2017), and a rigidity theorem on these types of manifolds is obtained.
In this paper, motivated by the recent work of Wang in (Wang, Citation2017), Yamabe solitons on dimensional cosymplectic manifold
have been considered. After preliminaries on contact and cosymplectic manifolds, it has been proved that if the cosymplectic manifold
admits a Yamabe soliton, then either
is locally flat or the potential field is a contact vector field. Some special potential vector fields of Yamabe solitons on 3-dimensional cosymplectic manifolds have been considered and some other results have been acquired. Also, it has been proved if an
cosymplectic manifold
admits a contact Yamabe soliton, then
is a cosymplectic manifold. Finally, an example of Yamabe soliton on a 3-dimensional cosymplectic manifold will be given.
The paper is organized as follows: Section 2 concerns the cosymplectic manifolds, the structure of contact manifolds and some aspects have been described. In section 3, the main results will be given and then an example have been provided . Section 4 concerns the conclusions.
2. Cosymplectic manifolds
In this section, some basic definitions of contact manifolds, with emphasis on those aspects that will be needed in the next section, have been summarized. For more details, one can consult (Blair, Citation2010).
An almost contact structure on a -dimensional smooth manifold
is a triple
, where
is a
-type tensor field,
is a global vector field and
a 1-form, such that
where, denotes the identity mapping, which implies that
,
and
. Generally,
is called the characteristic vector field or the Reeb vector field.
A Riemannian metric on
is said to be compatible with the almost contact structure
if for every
, we have
An almost contact structure endowed with a compatible Riemannian metric is said to be an almost contact metric structure. The fundamental 2-form of an almost contact metric manifold
is defined by
for any vector fields ,
on
.
An almost contact metric manifold is defined as an almost cosymplectic manifold, such that and
. In particular, an almost cosymplectic manifold is said to be a cosymplectic manifold if
. Moreover, on a cosymplectic manifold, we have the following relation:
which implies that is a Killing vector field. It follows directly that
where stands for the Ricci operator. An almost contact structure is said to be almost
Kenmotsu if
and
for a non-zero constant
. More generally, if the constant
is any real number, then an almost contact structure is said to be almost
cosymplectic (Öztürk et al., Citation2010). Moreover, Aktan et al. (Citation2014) generalized the real number
to a smooth function
on
and defined an almost
cosymplectic manifold, which is an almost contact metric manifold
such that
and
for a smooth function
satisfying
. Also, if the almost
cosymplectic structure on
is normal, we say that
is an
cosymplectic manifold. Obviously, if
is constant, then an
cosymplectic manifold is either cosymplectic under condition
, or
Kenmotsu
For a
dimensional
cosymplectic manifold the following identity is valid:
3. Main results
In this section, we present our main results and provide their proofs. The following Theorem is one of our main results in this paper.
Theorem 3.1. If a 3-dimensional cosymplectic manifold admits a Yamabe soliton, then either
is locally flat, or the potential field is a contact vector field.
Proof. Let is a Yamabe soliton on a cosymplectic manifold
of dimension 3. Recall that the curvature tensor
of any 3-dimensional Riemannian manifold can be written as
for any vector fields. If we replace both and
by
in the formula (3.5), using
yields
where denotes the scalar curvature. Therefore
is an
-Einstein manifold. It follows directly from (3.6) that
If we contract in (3.7) and use the following well-known formula,
we obtain for any vector field
on
, and this is equivalent to
On the other hand, by (1.1) we obtain
Taking the covariant differentiation from both sides of the above formula along an arbitrary vector field , we obtain the following equality for any vector fields
and
on
.
But we know the following formula from Yano (Citation1970),
Since is the Levi-Civita connection of
we have
and then the above formula becomes
One can easily check that the operator is a symmetric tensor field of type
, i.e.
. In fact, this symmetry is a consequence of Jacobi identity in the Lie algebra of real smooth function on
. Hence, a simple combinatorial argument shows that
Using (3.10) and (3.9) the following formula is obtained,
Taking the covariant differentiation of along an arbitrary vector field
, we obtain
The following tensorial identity is well-known (see (Yano, Citation1970)),
for any vector fields ,
and
. By a straightforward computation we obtain
for any vector fields ,
and
. Applying (2.2) and (3.8) we obtain
. Thus, using this and contracting the tensorial relation (3.11) over
, then a straightforward computation shows
for any . Moreover, keeping in mind that
is an
-Einstein manifold, by (3.7) and a straightforward calculation we obtain that
for any vector fields . Subtracting (3.12) from this equation gives the following equation,
Putting and
orthogonal to
into (3.13), gives
. This implies that either
or
. In the first case, because of (3.6)
is Einstein and according to (2.3), it is easily seen that
is locally flat. If
, then putting
we have
. On the other hand, from (1.1) we have
for any vector field . Putting,
and
orthogonal to
into above equation gives
, which is equivalent to
for any vector field
. Now, we can write
for any vector field , where we have applied the fact that
is invariant along only
. In this context, we say that the potential vector filed
is a contact vector field, that is,
. This completes the proof.
The above theorem has been proved about Ricci soliton on 3- dimensional cosymplectic manifolds by Wang (Citation2017). Now we get the following result:
Corollary 3.2. If a 3-dimensional cosymplectic manifold admits a Yamabe soliton with the potential vector field or a unit transversal potential vector field
then the potential vector field is Killing.
Proof. Suppose that a 3-dimensional cosymplectic manifold admits a Yamabe soliton with the potential vector field
. It is obviously Killing vector field. When
is orthogonal to
and of unit length, from above theorem, we have
. On the other hand, putting
and
into EquationEquation 14
(14)
(14) ) gives
Consequently, , which completes the proof.
Note that the above corollary when a 3-dimensional cosymplectic manifold admits a Ricci soliton, reduced to this fact that the soliton is steady (Cho, Citation2013). Yano has proved that if the holonomy group of a Riemannian
-manifold leaves a point invariant, then there exists a vector field
on
which satisfies
for any vector tangent to
(Yano & Chen, Citation1971). The vector field
mentioned above is called concurrent vector field. We consider the concurrent vector field
and obtain the following corollary:
Corollary 3.3. If a 3-dimensional cosymplectic manifold admits a Yamabe soliton with concurrent potential vector field
, then either the soliton is expanding with
or
is an infinitesimal contact transformation.
Proof. By (3.14) we can write
for any vector field and
. Hence,
. If
, then
and consequently, the soliton is expanding. Otherwise,
for any vector field . In this context, we say that the potential vector field
is an infinitesimal contact transformation (see (Tanno, Citation1962)), that is,
.
Finally, we consider cosymplectic manifold
and get the following theorem:
Theorem 3.4. If an cosymplectic manifold
admits a contact Yamabe soliton, then
is cosymplectic manifold.
Proof. In view of (2.4), we have
Therefore it implies from the Yamabe soliton (1.1) with that
therefore for any vector field on
, we have
By taking and using (3.15), we get
thus It follows that
. Consequently,
and
is Killing vector field.
It is well known that any Euclidean space of odd dimension admits a cosymplectic structure. In the following example, we construct a gradient Yamabe soliton on 3-dimensional cosymplectic manifold (for more details see (Wei & Wylie, Citation2009): example 2.1).
Example 3.5. Let be the Euclidean 3-space equipped with the usual flat metric
. Set
, where the potential function
for some non-zero constant
. One can easily check that
, that is
, is a gradient Yamabe soliton.
4. Conclusions
In this paper, we considered Yamabe solitons on 3-dimensional cosymplectic manifold . We proved that if a cosymplectic manifold
admits a Yamabe soliton structure, then either
is locally flat or the potential vector field is a contact vector field. Also, we considered Yamabe soliton structure on cosymplectic manifold with concurrent potential field and obtained some interesting results. Moreover, we proved that if an
-cosymplectic manifold
admits a contact Yamabe soliton, then
is cosymplectic manifold. In this paper, we concentrate on Yamabe solitons on 3- dimensional manifolds. Studying this structure for general dimension manifold could be the topic of another article.
Acknowledgements
The authors would like to thank the anonymous referee(s) and editor for reading the manuscript carefully and giving valuable suggestions on it.
Additional information
Funding
Notes on contributors
Hajar Ghahremani-Gol
The first author has received his Ph.D. degree in the joint supervision Dr Boroojerdian at Amirkabir University of Technology. Dr Fasihi is an Assistance professor of mathematics and faculty member at Imam Khomeini International University. He has published some research papers in widespread international journals. His interested are differential geometry, theoritical and Mathematical physics.
The second author has received her Ph.D. degree in the joint supervision Professor Razavi and Dr Didehvar at Amirkabir University of Technology. Dr Ghahremani-Gol currently works as an Assistant Professor at Shahed University. The research area of Ghahremani-Gol is Riemannian geometry, Mathematical fluid dynamics and she has published some research papers around them.
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