1. Introduction
A non-symmetric basic tensor was used by several authors as the main axiom of the theory which is nowdays called a non-symmetric gravitational theory [
1]. Reference [
2] formally introduced a generalized Riemannian space as a differentiable manifold endowed with a non-symmetric basic tensor. The non-symmetric linear connection on a generalized Riemannian space
is explicitly determined by the compatibility condition with the symmetric part
of non-symmetric metric
g. References [
3,
4] found new curvature tensors of a non-symmetric linear connection.
A mapping between two generalized Riemannian spaces is said to be a conformal mapping if it preserves angles between curves of these spaces. Some physical characteristics of conformal mappings were given in [
5]. Geodesic mappings and their generalizations is an active research field, see for instance [
6,
7,
8,
9,
10,
11,
12,
13,
14,
15]. Some conformal and projective invariants of Riemannnian manifolds were obtained by Reference [
16,
17]. Recently, some very interesting remarks on the converse of Weyl’s conformal theorem were given by [
18]. On the other hand [
19] had proved conformal invariace of dual projective curvature tensor.
Let us recall that a geodesic circle on a differentiable manifold is a curve which has constant first curvature and vanishing second curvature. A conformal mapping
which preserves the geodesic circles of the manifold
M is said to be a concircular mapping [
20].
Eisenhart’s non-symmetric metric is the fundamental metric tensor in the non-symmetric gravitational theory (NGT). Recently, some problems and hopes related with the non-symmetric gravity were given by [
21]. In the papers [
22,
23] and the papers that follow these ones, the authors studied conformal and concircular mappings of generalized Riemannian spaces with assumption that these mappings were preserving the torsion tensor. In the present paper we studied conformal and concircular mappings of generalized Riemannian spaces without any of the restrictive assumptions and find some tensors that are invariant with respect to these mappings.
In the existing literature there exist various generalizations of symmetric spaces. We define some new kinds of symmetric spaces with torsion by taking into account five curvature tensors of Eisenhart’s generalized Riemannian spaces and four kinds of covariant derivative.
2. Conformal Mappings of Generalized Riemannian Spaces
A generalized Riemannian space in Eisenhart’s sense
is a differentiable manifold
M endowed with a non-symmetric metric
g which can be described thorough its symmetric and skew-symmetric parts as [
2]
where
denotes the symmetric part of
g and
denotes the skew-symmetric part of
g, i.e.,
On a generalized Riemannian space in Eisenhart’s sense a non-symmetric linear connection
which is compatible with the symmetric part
of generalized Riemannian metric
g is explicitly determined by [
2]
The non-symmetric linear connection
can be described thorough its symmetric part
and the torsion tensor
T as
where the symmetric part ∇ and the torsion tensor
T of non-symmetric linear connection
are respectively determined by
A generalized Riemannian space is endowed with a non-symmetric linear connection, so there exist four kinds of covariant differentiation [
24]:
Let
and
be two generalized Riemannian spaces of dimension
. We can consider the manifolds
M and
in the common coordinate system with respect to the mapping
. In what follows we will assume that all mappings under consideration are diffeomorphisms, which particularly mean that these mappings are bijections. In this coordinate system the corresponding points
and
have the same coordinates and we can consider the connection deformation tensor
where
and
are the Levi–Civita connections of metrics
and
that are symmetric parts of generalized Riemannian metrics
g and
, respectively.
The symmetric part
of a generalized Riemmanian metric
g is of a non-degenerate symmetric bilinear form and defines an inner product on the tangent space of a generalized Riemannian space. From the definition of a conformal mapping, one can find that (see page 237 in [
11])
where
x and
y are tangent vectors of two intersecting curves in the intersection point.
From (
2) it follows that [
11]
where
is a function on
M.
In [
22] the authors made the assumption
which further implies (
3) and the same equation is valid for the skew-symmetric parts (which are not metrics) of the metrics
g and
, i.e.,
Obviously, the condition (
3) is weaker than the condition (
4) and it is independent of condition (
5).
If there exists a conformal mapping
between generalized Riemannian spaces
and
, then the connection deformation tensor
takes form [
11]
where
is a gradient vector field.
S. M. Minčić cosidered four kinds of covariant derivatives
,
and examined various Ricci type identities [
24]. Also, he showed that among the twelve curvature tensors which appeared in the Ricci type identities, there exist five which are linearly independent [
4]:
Corresponding Ricci tensors are defined by
where
.
Reference [
3] gave the geometric meaning of the curvature tensors
,
by taking into account parallel displacement with respect to covariant derivatives
,
. The geometric meaning of the fifth curvature tensor
was given by S. M. Minčić.
Definition 1. The scalar curvature of kind of a generalized Riemannian space is defined bywhere is the Ricci tensor of kind given by (
8)
. In the same manner as it was done for the Riemannian curvature tensors in [
11] we can find the relation between the components
and
of curvature tensors
and
,
, given by (
7) with respect to the conformal mapping
between generalized Riemannian spaces
and
[
11]:
where
Contracting (
9) on the indices
h and
k we obtain that [
11]
where
and
are the Ricci tensors and
.
By contracting (
11) with
we get [
11]
where
and
are the scalar curvatures of metrics
g and
, respectively.
From (
9), (
11) and (
12) it follows that the tensors
,
given below are invariant with respect to the conformal mapping
between generalized Riemannian spaces
and
:
where the components
of tensors
,
are determined by
Theorem 1. Let be a conformal mapping between two generalized Riemannian spaces and of dimension . Then the tensors , defined by (
13)
are invariant with respect to the mapping f. The tensors
,
are analogous to the Weyl conformal curvature tensor. In Theorems 2 and 3 we will prove that the tensors
,
given by (
13) have the same properties as the curvature tensors
,
.
Theorem 2. Let and be two generalized Riemannian spaces of dimension and be a conformal mapping. Then the tensors , determined by (
13)
satisfy: Proof. First, let us give the proof for the case
. In this case formula (
13) reads
which further implies
where we used the following properties of the curvature tensor
and the tensor
[
25]
This completes the proof of part
. Part
can be proved analogously. To prove part
let us observe that the curvature tensor
does not have the same properties as the curvature tensors
and
, i.e., [
25]
or more precisely
Therefore,
which completes the proof of part
. Parts
and
can be proved in the same manner. □
Let us denote
where
A is an arbitrary
tensor field.
Theorem 3. Let and be two generalized Riemannian spaces of dimension and be a conformal mapping. Then the tensors , determined by (
13)
satisfy Concircular Mappings of Generalized Riemannian Spaces
We shall consider concircular mappings between generalized Riemannian spaces in Eisenhart’s sense. In what follows we shall prove that there exist some tensors invariant with respect to these mappings. Let
and
be two generalized Riemannian spaces of dimension
. If a conformal mapping
is concircular then the tensor
determined in Equation (
10) satisfies [
11]
where
is a function.
Theorem 4. Let and be two n-dimensional generalized Riemannian spaces and be a concircular mapping. Then the tensors and , given below are invariant with respect to the mapping fwhere . Proof. Since the mapping
between two generalized Riemannian spaces
and
is concircular the formulas (
9), (
11) and (
12) respectively become
Consequently, we obtain that
where the tensor
is defined by (
14) in the space
and the tensor
is defined in the same manner in the space
.
From (
16) it follows that
where the tensor
is defined by (
15) in the space
and the tensor
is defined in the same manner in the space
. □
The tensors
and
,
determined in Theorem 4 by (
14) and (
15) are analogous to Yano’s tensor of concircular curvature and the Einstein tensor [
11], respectively.
Theorem 5. If there exists such that the scalar curvature of a generalized Riemannian space is constant, then the tensor determined by (
14)
satisfieswhere . Remark 1. The result given in Theorem 5
is a generalization of the well-known result which is valid in a Riemannian space:where is a tensor of concircular curvature and is a Riemannian tensor of a Riemannian space.