1. Introduction
This paper refers to the field of differential geometry, or, more precisely, to the theory of differentiable manifolds equipped with various “geometric structures” [
1], such as connection, curvature and torsion. We use the following methods: the moving frame method (the E. Cartan method [
2] of differential-geometric research) and the G. F. Laptev method of extensions and envelopments, which includes the Cartan moving frame method and gives a universal character to the first one. Universality and efficiency of the Cartan method were shown in many papers. Upon use of this method, a research of geometry of a manifold with geometric structures fixed on it is reduced to study of geometry of other manifolds (total space of frames above the given manifold or subbundles of the bundle). Thus, automatically there is an analytical apparatus that is most adapted to research of the initial structure [
3]. The method of extensions and envelopments is based on the invariant differential-algebraic apparatus of structure differential forms of considered bundles [
4]. In this paper, the Cartan–Laptev method is applied to research of the centered
m-planes space in projective space
.
The connection theory (see, e.g., [
5,
6]) has an important place in differential geometry. A lot of research devoted to the geometry of planes manifolds in classical spaces includes studying connections. The connection theory also plays a fundamental role in physics.
Normalization of a manifold [
7] of centered planes in projective space can be defined by an analogy with the Norden normalization [
8]. A. P. Norden has described the normalization of a surface. A surface
can be considered as an
m-dimensional manifold of
m-dimensional centered planes
[
9]. Yu. G. Lumiste [
10] has entered a similar normalization of a manifold of
m-planes in projective space. An analogue of this normalization is used in this paper. Thus, in our case, the normal of the first kind (the first normal) is a subspace
of
having only one common point with a centered
m-plane
; and the normal of the second kind (the second normal) is a subspace
of the centered
m-plane
not passing through its centre [
11]. Moreover, we will also use a reduction that is frequently applied in geometry (see, e.g., [
4,
12,
13,
14,
15,
16,
17]).
This paper is a continuation of author’s research [
18,
19]. Our purpose is to give the full analysis of the dynamics of changes of bundles, connection, curvature and torsion at transition from the space of centered planes to the normalized space.
The timeliness of the present paper is caused by the facts that the space of centered planes is a set of all
m-dimensional centered planes, which we may say about a communication with the Grassmann manifold (the set of all
m-planes) [
20]. It is important to emphasize that the Grassmann manifold plays a key role in topology and geometry as the base space of an universal vector bundle. Moreover,
is projective space
.
2. Analytical Apparatus
Projective space
can be presented as the quotient space
of a vector space
by equivalence relation (collinearity) ∼ of nonzero vectors, i.e.,
(see, e.g., [
21]). Thus, we can set the quotient map by
As is known [
11], a projective frame in the space
is a system consisting of points
,
, and the unity point
E. In the vector space
, linearly independent vectors
correspond to the points
and the vector
corresponds to the point
E. These vectors are defined up to a common factor in
. The unity point
E is given along with the basis points
, though we might not mention it each time.
It is supposed that a frame in the vector space is normalized, i.e., , where ∧ sets an exterior product.
The equations of infinitesimal displacements of the moving frame in
can be written in the following way:
, with the condition of normalization
. Here,
d denotes ordinary differentiation in
. The forms
are linear differential forms; they depend on parameters
u (defining a location of the frame) and their differentials
.
The forms are connected by the relation . This condition is also necessary for the number of linearly independent forms that became equal to the number of parameters on which the group of projective transformations of space depends.
The structure equations of projective space
have the form
where
D is a symbol of exterior derivative.
By the condition
from the linear group
, it is possible to determine the special linear group
[
21] acting effectively in
.
Introducing the following new forms (see, e.g., [
9,
10])
and fixing the index
,
, we can expand the forms
as
The formulas of infinitesimal displacements can be written in more detail:
where
,
,
, and the form
plays the role of the proportionality coefficient.
Introducing the basis forms
and omitting, for simplicity, the index 0 in the notation of the forms
and
, the Cartan equations can be defined
where
,
,
are the basis forms of the projective group
acting effectively on
.
Remark 1. We employ the inhomogeneous analytic apparatus with the derivation formulas (
1)
and the structure equations (
2)
. By contrast to the homogeneous case, this apparatus is more convenient for investigation of centered planes; and it was used in the previous author’s papers [22,23,24,25]. 3. The Space of Centered Planes
For the purpose of this paper, the term “space of centered planes”, denoted by , will be taken to mean a space of all m-dimensional centered planes in projective space . The space is a differentiable manifold and its points are m-dimensional centered planes.
Putting the vertice
A of the moving frame on a
m-plane
and fixing it as a centre, we get a centered plane
. Putting the vertices
of the frame on the plane
, we fix index ranges
and
. From the derivation formulas (
1), we immediately get stationarity equations for the centered plane. These equations have the form
,
,
. The forms
,
,
are the basis forms of the space
; the rest forms
,
,
,
,
are secondary.
Remark 2. The dimension of the space Π
of centered planes differs from the dimension of the Grassmann manifold [26] by the size m [22], i.e., . 4. Principal Bundle of the Space
The specification of the moving frame to the space
yields the principal bundle
, its typical fiber is a stationary subgroup
G of the centered plane
and base space is the space
; in addition, thereto, dim
. Total space of the bundle
[
3] is the projective group
and the projection
associates with each element of the group
the plane
in
, which is invariant under the action of this element.
The basis forms
,
,
satisfy the Cartan structure equations
The exterior differentials of the secondary forms are as follows:
Remark 3. The principal bundle of the space Π contains the following five quotient bundles [22]: - 1.
is the quotient bundle of linear plane frames belonging to the planes , its typical fiber is the linear quotient group acting on the pencil of lines on the plane with the structure Equations (
3)
and (
4)
; - 2.
is the quotient bundle of normal linear frames; it is dual to the quotient bundle of linear plane frames; the typical fiber is the linear quotient group acting on the quotient space with the structure Equations (3) and (6); - 3.
is the quotient bundle of plane co-affine frames belonging to the plane ; its typical fiber is the co-affine quotient group acting on the plane and . This quotient bundle has the structure Equations (3)–(5); - 4.
is the affine quotient bundle whose typical fiber () is an affine quotient group [27] of the group acting on the pencil of lines through A with the structure Equations (3), (4), (6) and (7); - 5.
the maximal quotient bundle is made from the quotient bundle of plane co-affine frames and the affine quotient bundle with the structure Equations (3)–(7).
Normalization of the space
is made by the fields of the following geometric patterns: the first kind normal, i.e., an
-plane
intersecting the plane
only at the point
A and the second kind normal, i.e., an
-plane
contained in the centered plane
and not passing through its centre
A (see, e.g., [
11,
28]).
Let us now analyze the dynamics of changes of the bundle at the consecutive canonizations:
by placing the vertices on the first normal (the 1st canonization);
by placing the vertices on the second normal (the 2nd canonization);
by simultaneous placing the vertices on the corresponding normals (full canonization).
Remark 4. The space or is said to be a semi-normalized space in the first or second case, respectively, and the space is a normalized space in the third case.
4.1. The Bundle
We put the vertices
on the first normal
. Then, the following relations must hold:
with the differential congruences
Here, and subsequently, the differential operator
acts in the standard way (see, e.g., [
29])
Taking into account (
8), from the structure Equation (
2), we have
From Equations (
10)–(
13), it can be argued that, at the first canonization, the principal bundle
is narrowed to the principal bundle
; its typical fiber is the stationary subgroup
of a pair of the affine additional planes
. There are four quotient bundles in the subbundle
:
the quotient bundle of plane linear frames with the structure Equations (
3) and (
10);
the quotient bundle of normal linear frames (
3) and (
12);
the quotient bundle of plane co-affine frames (
3), (
10), and (
11);
the quotient bundle of normal co-affine frames (
3), (
12), and (
13).
4.2. The Bundle
If we do not use the previous canonization and place the vertices
on the second normal
, then
and
Then, from the structure Equation (
2), we have
In fact, according to Equations (
16)–(
19), we can make a conclusion that, at the second canonization, the principal bundle
is narrowed to the principal bundle
; its typical fiber is the stationary subgroup
of the pair
. There are four quotient bundles in the subbundle
:
the quotient bundle of plane linear frames with the structure Equations (
3) and (
16);
the quotient bundle of normal linear frames (
3) and (
17);
the bundle
(
3), (
16)–(
18) whose typical fiber
H is an affine quotient group of
;
the bundle of normal co-affine frames (
3), (
17), and (
19).
4.3. The Bundle
Now, suppose that we have already made canonizations considered in items 4.1 and 4.2 simultaneously, that is,
and
. In this case, conditions (
8) and (
14) are satisfied and the structure Equation (
2) will become
Obviously,
is the stationary subgroup of the centered
pair
,
[
30]—then at the full canonization from narrowing
of the principal bundle
. The following three quotient bundles are allocated:
the bundle of plane linear frames (
3) and (
20);
the bundle of normal linear frames (
3) and (
21);
the bundle of normal co-affine frames (
3), (
21), and (
22).
5. A Connection on the Bundle Associated with the Space
Using the Laptev–Lumiste method (see, e.g., [
4,
31]), on the principal bundle
, we define a fundamental-group connection by the forms
The components of the connection object [
4]
satisfy the following differential congruences modulo the basis forms
,
,
:
Remark 5. The connection object Γ contains the following five geometric subobjects , , , , and . These subobjects determine connections on the corresponding (see Remark 3) quotient bundles.
Let us consider the dynamics of changes of the connection at consecutive canonizations and we will be convinced that the connection is not uniquely induced at the normalization of the space .
5.1. The Connection Object at Adaptation of the Moving Frame to the First Normal
By placing the vertices
on the first normal
, condition (
8) is satisfied, that is, the forms
become principal and the connection object
is narrowed to the object
; and differential congruences for its components have the form
All of this points to the fact that the following theorem holds.
Theorem 1. At an adaptation of the moving frame to a field of the first normals the connection object Γ is reduced to the object . The object contains three subobjects , , that set connections on the quotient bundles of plane linear frames, normal linear frames, and plane co-affine frames, respectively.
5.2. Connection Object at Adaptation of the Moving Frame to the Second Normal
Without using the previous canonization and placing the vertices
on the second normal
, we get condition (
14). The connection object
is narrowed to the object
with the following congruences for its components:
The arguments given above prove Theorem 2.
Theorem 2. At an adaptation of the moving frame to a field of the second normals, the connection object Γ is reduced to the object . The object contains three subobjects , , that set connections on the quotient bundles of plane linear frames, normal linear frames, and affine quotient bundle, respectively.
5.3. Connection Object at Normalization
By making both canonizations simultaneously, that is, placing the vertices
on the first normal
and the vertices
on the second normal
, the differential congruences for object’s
components will become
In addition, we have the following theorem.
Theorem 3. At an adaptation of the moving frame to normalization of the space Π, the connection object Γ is reduced to the object . The object contains two subobjects , that set connections on the quotient bundles of plane and normal linear frames.
5.4. Reduced Connection Objects
With the help of conditions (
8), the forms
become principal, and, therefore, congruences for the components
,
,
in (
24) will be carried out identically and they can be omitted. Using conditions (
8) in the rest of the differential congruences (
24), the components of the reduced connection object
will satisfy (
25) if the following conditions hold:
Theorem 4. The reduced object coincides with the object only if conditions (28) hold, where the object gives a connection on the reduced bundle that arises at the adaptation of the moving frame to a field of the first normals. By substituting conditions (
14) into the differential congruences for components of the connection object
, we get congruences (
26) with conditions
for the components of the reduced connection object
.
Theorem 5. The reduced object coincides with the object only in the case (29), where the object gives a connection on the reduced bundle that arises at the adaptation of the moving frame to a field of the second normals. Taking into account conditions (
8) and (
14) in the first six and last three differential congruences (
24), we have that the components of the reduced connection object
satisfy the differential congruences (
27).
Theorem 6. The reduced connection object coincides with the object , which gives a connection on the reduced bundle at the adaptation of the moving frame to the normalization of the space Π.
Remark 6. Adaptations of the moving frame cause the reductions of associated bundle and differential congruences for components of group connection object. Semi-canonizations (the first and second canonizations) lead to reductions of bundle and connection object, but, according to Theorems 4 and 5, the reduced connection objects can differ from the objects specifying connections on the reduced bundles.