1. Introduction
String theory and homotopy theory are closely related. It is a famous hypothesis that orbifold D-brane charges in string theory can be classified in twisted equivariant K-theory, or rather, Real twisted equivariant K-theory. A natural question is whether we have a classification for M-branes in term of cocycles in a generalized cohomology theory.
M-branes are membranes in M-theory, which is a theoretical framework in physics that unifies the five superstring theories and 11-dimensional supergravity. M-branes come in two types, M2-branes and M5-branes. These branes are analogous to the D-branes in string theory but exist in the 11-dimensional spacetime and M-theory. An M2-brane acts as a source for the 3-form field, similar to how an electric charge is a source for the electromagnetic field. In lower dimensions, M2-brane charges can manifest as various types of charges in string theory, such as D2-brane charges or fundamental string charges. In addition, M5-brane is a magnetic dual to the M2-brane and couples to the 6-form field. In string theory, M5-brane charge can appear as NS5-brane charge or D4-brane charge after compactification. M-brane charges play a crucial role in the duality web of string theory and M-theory. They play a key role in unifying string theories, understanding black hole entropy, and studying gauge theories.
A key open problem in M-theory is the identification of the degrees of freedom that are expected to be hidden at ADE-singularities in spacetime. Comparison with the classification of D-branes by K-theories suggests that the answer must come from the right choice of generalized cohomology theory for M-branes. Recently it is believed that the hypothesis has a non-trivial lift to M-branes classified in twisted real equivariant 4-Cohomotopy. Then, it is an evident question how the statement for D-branes and this for M-branes may relate.
As indicated in [
1], cyclification of
∞-stacks is a fundamental and elementary base-change construction over moduli stacks in cohesive higher topos theory. Quasi-elliptic cohomology theory, which is defined as an equivariant cohomology of a cyclification of orbifolds, is a form of twisted equivariant K-theory involving a further
-action, just as one would expect in lifting through the M/II reduction. It is a variant of Tate K-theory. Quasi-elliptic cohomology theory is a generalized cohomology theory that potentially interpolates the two statements, by approximating equivariant 4-Cohomotopy classified by 4-sphere orbifolds. In addition, the universal shifted integral 4-class of equivariant 4-Cohomotopy theory on ADE-orbifolds induces the Platonic 4-twist of ADE-equivariant Tate-elliptic cohomology,
In [
2], the author computes a list of examples explicitly, specifically for the domain being orbifolds of 4-spheres by finite subgroups of
that serve as the transverse geometry to M5-branes probing ADE-singularities. The results in all cases investigated are deformations of familiar twisted equivariant K-theory groups by a further formal variable
q, being the character of the extra
-action and playing the role of a kind of M-theoretic quantum deformation of the K-theory groups.
In this paper, we consider the approximation of the Real version of quasi-elliptic cohomology to Real twisted equivariant 4-Cohomotopy. Young and the author construct a Real version of quasi-elliptic cohomology in [
3]. Since twisted equivariant
-theory appears in physical contexts that are unoriented, such as orientifold string theory [
4] and unoriented topological field theory [
5,
6], it is natural to expect a relation between unoriented conformal field theory and a Real generalization of elliptic cohomology. This expectation suggests the promotion of the group of loop rotations
to the orthogonal group
so as to include loop reflection, an orientation reversing symmetry of loops. The
-equivariant topology of loop spaces has been studied by a number of authors [
7,
8,
9,
10,
11]. In [
3], Real quasi-elliptic cohomology is constructed as the
-equivariant Freed–Moore
K-theory of an inertia groupoid. In this paper, we compute Real and complex quasi-elliptic cohomology of 4-spheres under the specific action of some finite subgroups of
, which aims to give an approximation to the Real equivariant unstable 4th Cohomotopy, which is especially difficult to compute.
To interpret the relation between the computation and cohomotopy, we start the story by classifying spaces for cohomology theories. For a given cohomology theory
with classifying space
E, we have, for any good enough space
X,
Here, we can regard a map
as a “cocycle” for the
E-cohomology, and a homotopy between such maps as a “boundary” in
E-cohomology. Generally, the classifying space of an abelian cohomology theory is its spectrum at level 0. A classical example is complex topological
K-theory
, whose classifying space can be taken to be
.
In addition, instead of using the whole spectrum of
E, with only the classifying space, we can define a generalized non-abelian cohomology theory
which makes good sense. One issue is that computing such cohomology theories is generally difficult. One method is approximating the cohomology theory
E by another one
, which is better understood and easier to compute. The method is clearly possible whenever there is a map of classifying spaces
because it induces, evidently, a cohomology operation
which provides an image of the less-understood
E-cohomology in the better-understood
-cohomology.
The archetypical example here is the Chern-Dold character map [
12,
13], which approximates any generalized cohomology theory by a rational cohomology theory. For instance, the ordinary Chern character on
with
is represented by a map of classifying spaces
This map of classifying spaces is itself a cocycle in the rational cohomology of the classifying space
. In other words, the Chern character itself can be viewed as an element in
Generally, a map of classifying spaces , inducing a cohomology operation , is itself a cocycle in the -cohomology of the classifying space E. Thus, in order to understand E-cohomology, we may try to understand the -cohomology of its classifying space E for suitable alternative cohomology theories .
Now, we consider the cohomology theory, the
n-th Cohomotopy theory
whose classifying space is an
n-sphere
. Each cocycle in the
-cohomology
is represented by a map
. From it, we obtain a cohomology operation
which provides us images of
in
-cohomology similarly to how the Chern character provides images of
K-cohomology in ordinary rational cohomology.
It is suggested by
Hypothesis H [
14,
15,
16] that, specifically,
-twisted equivariant unstable 4
classifies the charges carried by M-branes in M-theory in a way that is analogous to the traditional idea that
classifies the charges of D-branes in string theory. Therefore, it is essential to compute the
of spacetime domains relevant in M-theory. This can be hard, in particular, once we remember that all of these need to be performed in twisted equivariant generality. Thus, we apply the idea to approximate
of spacetime by using the cocycles
in
for some suitable cohomology theory
. Instead of
itself, we will study the image of the corresponding cohomology operation
Some information of the actual
may be lost but what is retained can still be valuable and is expected to be better understandable.
Specifically, the classifying spaces for equivariant
are orbifolds
of the 4-sphere acted by a group
G, i.e., the orbifolds of representation 4-spheres. Hence, the elements of the
G-equivariant
-cohomology
serve, in the above way, as “generalized equivariant characters” on equivariant
, namely, as the equivariant cohomology operation
As conjectured in [
1], the choice
should be a particularly suitable approximation to equivariant
for the purpose of computing M-brane charge. One motivation for this is that the Witten elliptic genus, which was originally discussed for string [
17], actually makes sense for M5-branes [
18,
19,
20,
21,
22], so that one should expect that it is actually part of the charges carried by M5-branes. But these charges should also be in
, and hence, it is conjectured in [
1] that there is a useful approximation of
by elliptic cohomology, and specifically by quasi-elliptic cohomology.
This is the motivation for computing the quasi-elliptic cohomology for representation 4-spheres. Moreover, as indicated in [
13], the particular choice of equivariance groups
G as finite subgroups of
comes from the fact that these are the most interesting isotropy groups for the orbifolds on which these M5-branes may sit. We describe the interesting groups and their action on 4-spheres below.
The space
of quaternions is isomorphic to
as a real vector space. In addition, the group of the unit quaternions is isomorphic to the special unitary group
, which is isomorphic to
. It can be identified with a subgroup of
via the composition
where the first homomorphism is the inclusion into the first factor. Under quaternion multiplication, there are two choices of group action by
on
that we are especially interested in.
The group action can extend to
by keeping the north pole and the south pole fixed. In ([
2] Section 6), we compute complex quasi-elliptic cohomology of
under the first group action in (
1). In
Section 4, we compute the Real quasi-elliptic cohomology for that. Moreover, in
Section 5, we compute some examples of complex and Real quasi-elliptic cohomology of
under the second group action in (
1). In addition, in
Section 6.1, we discuss the relation between the computation and physics a little; in
Section 6.2, we give a future problem that we are interested in.
In the computation of Real quasi-elliptic cohomology theories, we pick a Real structure on each equivariance group, with which the corresponding theories are computable. Real quasi-elliptic cohomology is defined as a Freed–Moore K-theory of a Real version of an inertia groupoid. The computation in
Section 4 shows how the classification of M-brane charges is described by equivariant complex K-theories when the conjugacy classes are free under the involution, and how the classification is described by equivariant KR-theories when the conjugacy classes are fixed by the involution. In
Section 5.2, we compute some Real and complex quasi-elliptic cohomology theories whose equivariance groups are some finite product subgroups of
. The conjugacy classes of the equivariance group determine the distinct types of localized charges, while the Real conjugacy classes describe how these charges behave under the involution. The connection is formalized using equivariant K-theories and equivariant KR-theories, which provide a deep link between geometry, algebraic topology and physics.
In addition, we would like to mention some other methods approximating 4-Cohomotopy, which is a challenging task in equivariant homotopy theory. Generally, a G-equivariant 4-Cohomotopy of a space X is difficult to compute and depends heavily on the equivariance group G and the topological space X. Tools from equivariant homotopy theory, such as the Borel construction, equivariant cohomology, and spectral sequences, are often used to study these groups. We list a few of them below.
- (1)
One method is to use obstruction theory to determine whether a
G-equivariant map
exists and classify such maps up to homotopy [
23]. This method works well for spaces that can be decomposed into
G-CW complexes. However, obstruction classes can be difficult to compute explicitly, especially for higher dimensional spaces.
- (2)
In addition, we can apply Atiyah–Hirzebruch spectral sequence or the Serre spectral sequence to approximate equivariant 4-Cohomotopy whose
-page involves terms like
[
24]. This method breaks down the problem into simpler pieces using cohomology and homotopy groups. It can handle a wide range of spaces
X and equivariance groups
G. But higher differentials in the spectral sequence are often difficult to compute and the spectral sequences may not always converge cleanly.
- (3)
Another method is that we stabilize the problem first by replacing
with its stabilization
and use equivariant Adams spectral sequence to approximate 4-Cohomotopy [
24]. Reducing the problem to stable homotoy groups certainly simplifies the problem and Adams spectral sequence is a powerful tool. However, stabilization loses some geometric information about
X and the 4-sphere, and the unstable problem of interest may not directly be solved.
- (4)
In addition, when the space
X has a smooth structure and the
G-action is geometric, one can study
G-equivariant instantons or harmonic maps to
[
25,
26]. Using these methods, the equivariant maps can be constructed explicitly, which may connect to physical applications. But the method may not be generalized to more abstract or singular spaces.
The method we use in this paper is a novel one. It works well especially when the cohomology theory approximating 4-Cohomotopy is suitably chosen and computable. In this paper, we implemented this idea.
In the
Appendix A and
Appendix B, we give some corollaries of the decomposition formula for complex equivariant
K-theories in [
27] and the Mackey decomposition formula for Freed–Moore
K-theories in [
3]. They are used in the computation in
Section 4 and
Section 5, respectively.
In addition, before we present the computation of quasi-elliptic cohomology, we review, in
Section 2 and
Section 3, quasi-elliptic cohomology and twisted Real quasi-elliptic cohomology, respectively.
3. Twisted Real Quasi-Elliptic Cohomology
In this section, we review the definition and properties of twisted Real quasi-elliptic cohomology. For more details, please refer to [
3].
We will use -graded groups to define the Real structure on groups.
Definition 5. Let G be a finite group. A is a group homomorphism . The ungraded group of is . When π is non-trivial, is called a Real structure on G. The group acts on G by Real conjugation. The Real centralizer of isThe group is -graded with ungraded group the centralizer . Example 2. The terminal -graded group is and is denoted simply by . If acts on a group , then so does any -graded group and the resulting semi-direct product is naturally -graded.
Example 3. The dihedral group is a Real structure on . The subgroup is a normal subgroup of and we have the short exact sequencewith a generator of mapped to the rotation r. Example 4. As computed in ([3] Example 1.8), the Real representation ring with respect to the Real structure is isomorphic to complex representation ring Example 5. For any , the Real centralizer is -graded with ungraded group the centralizer . It is a Real Structure on .
In addition, the element is Real central and so generates a normal subgroup isomorphic to . This leads to the definition of the enhanced Real centralizer
of g.It is a Real structure on the group . The set of connected components
of the conjugation quotient groupoid is the set of conjugacy classes of
G. Given a Real structure
, Real conjugation defines an involution of
. This defines a partition
with
the fixed point set of the involution. The conjugacy class of
is fixed by the involution if and only if
. The set
of Real conjugacy classes of
G inherits from (
8) a partition
Let
X be a
-space. Note that for each
, the fixed point space
is a
-space. In addition, the
-action on
as defined in (
2) can extend to an action by
:
for any element
, any
.
The Real loop groupoid
, as defined in ([
3] Definition 2.7), adds the involution as morphisms into the groupoid
, and it is a double cover of the groupoid
. In addition, we have the Real version of the decomposition (
4), i.e., the decomposition of the groupoid
corresponding to the partition (
9).
Proposition 2. There is an equivalence of -graded groupoids The twisted Real quasi-elliptic cohomology is defined in ([
3] Definition 3.2, Proposition 3.3) in terms of Freed–Moore K-theories.
Definition 6 ([
3] Definition 3.2 Proposition 3.3)
.where is a fixed element in and is the Real transgression map. By the property of the Freed–Moore K-theory [
5], if the Real structure
splits, each factor in (
12) is the equivariant
-theory defined by Atiyah and Segal [
31].
In addition, using the partition (
9), the isomorphism (
12) can be written as
The -graded morphism which tracks loop rotation and reflection makes into a -algebra and, in particular, a module over .
Theorem 1 ([
3] Theorem 3.15)
. Assume that is non-trivially -graded. The relation between twisted Real quasi-elliptic cohomology and twisted Real equivariant Tate K-theory is In addition, we give an example computing Real quasi-elliptic cohomology, which is ([
3] Example 3.7). The conclusions in Example 6 are applied in the computation of
Section 4 and
Section 5.
Example 6. Let and . The -action on is trivial. By the isomorphism (13),As discussed in ([3] Example 3.7), 4. Real Quasi-Elliptic Cohomology of Acted by a Finite Subgroup of Spin(3)
In this section, we compute all the Real quasi-elliptic cohomology theories
where
G goes over all the finite subgroups of
.
First, we explain how the group
G acts on
. We have the standard orthogonal
-action on
and also on the subspace
. The covering map
makes
a well-defined
-space. The
G-action on
is induced by the composition
where
is the projection to the first factor of the product group.
We give the explicit formula of the
G-action below. The group
of unit quaternions is isomorphic to
via the correspondence
In view of this,
can be described as the group
and
can be identified with the quaternionic unitary group. Thus, as indicated in ([
32] p. 263), the inclusion from
is given by the formula
In addition, as shown in ([
32] p. 151), the rotation of
represented by
is given by the map
where
is identified with the linear space
Then, the group
acts on
via the composition
and the standard orthogonal action.
In the rest part of the paper, we will use the symbol
to denote the matrix
and the symbol
to denote the matrix
First, we need to pick a Real structure
on the group
as well as on all its finite subgroups by equipping the group with a reflection
s. The choice is definitely not unique. Next, we define the reflection action on
and, thus, together with (
18), we define the action on
by
.
Example 7. Motivated by the Real structureof the cyclic group , we want to pick a Real structure on making the diagrams below commute.where the horizontal sequences are all exact. In the left column, the generator r of the rotation group is mapped to the rotation in . The lower left vertical map can be chosen to map the rotation to . In addition, the reflection in can be mapped toIt is straightforward to check that is the identity map for any θ. In addition, we can take the action of s on to beNote that under the reflection (21), the north and south poles of are still fixed. On the -plane, the two pairs of pointsare switched by the reflection, respectively. It is straightforward to check that acts as the identity map on for any θ. Thus, it is reasonable to take the Real structure to be the subgroupof and take the projection to be the determinant map det. Instead, we can map the rotation r to the matrix , which is a conjugation of . We havewhere and θ is any real number. In addition, the reflection s is fixed under the conjugation. The corresponding Real structure of is still and the diagram (20) still commutes. Moreover, we would like to mention a different choice of the Real structure . In the diagram (20), we map the rotation in to the same matrix in but map the reflection toNote that , i.e., is not a fixed point under the conjugation taking to . We can check, for any θ, . The action of on can be defined asUnder the reflection (22), the north and south poles are also fixed. On , the two pairs of pointsare switched by the reflection, respectively. It is straightforward to check that acts as identity on for any θ. Thus, it is reasonable to take the Real structure to be the subgroupof and the projection π to be the determinant det. Since is a normal subgroup of , both Real structures, and , split.
Example 8. For any finite subgroup G of ,is the restriction of the Real structureof to G. It defines a Real structure on G. Similarly,defines a Real structure on G. Remark 1. We give in Example 7 some reasonable choices of reflection on the representation sphere , which all keep the north pole and the south pole fixed. We did not find a canonical choice of reflection that switches the north pole and the south pole.
As indicated in ([33] p. 215), for V, a real vector space equipped with a linear G-action, stereographic projection exhibits a G-equivariant homeomorphism between the representation sphere (the one-point compactification) and the unit sphere (where the -summand is equipped with the trivial G-action): Another reasonable choice of the reflection on is the one sending a point to . The map corresponding to that on , which iswhere is the length of the vector. The map preserves the angle but not the length of the vector when it is not 1, and, especially, it is not linear. Thus, this is not the right choice of reflection for our computation. Remark 2. We would like to mention that, other than the choice of the Real structure, the choice of the action of on is also not unique where G is any finite subgroup of . Though different choices of the Real structure may lead to different , different choices of the action of the reflection may lead to little difference.
In the computation of with G a finite subgroup of , for most elements , the fixed point spaces consist of only the north pole and the south pole, where the reflections, those in Example 7, etc., act trivially. In addition, for the identity element , is a representation sphere of the group . Thus, by ([34] Theorem 5.1), the computation of the corresponding factor can be reduced to that of the Real representation ring of . To compute the Real quasi-elliptic cohomology of 4-spheres
acted by a finite subgroup
we need to find all the fixed points in
G under the involution, i.e., the Real conjugation. Below is a conclusion that we will apply in the computation later.
Proposition 3. If we take the Real structure on a finite subgroup G of , for any element β in G, we have the conclusions below.
- (1)
The element β is a fixed point under the involution if and only if is in the conjugacy class of β in G.
- (2)
If β is a unit quaternion whose coefficient of i is zero, then and β is a fixed point under the involution.
Proof. A given element
is a fixed point under the involution if and only if the set
is nonempty, i.e., there is an element
for some
satisfying
So we obtain the first conclusion.
Since
is an element in
, thus, it has a quaternion representation
. In (ii), we discuss a very special case that
. We start the computation below.
The right hand side should be the inverse of
. So we establish the equation
Solving the equation, we obtain
i.e.,
if and only if
is a unit quaternion with
. □
Similarly, we have the conclusion.
Proposition 4. If we take the Real structure on a finite subgroup G of , for any element β in G, we have the conclusions below.
- (1)
The element β is a fixed point under the involution s if and only if is in the conjugacy class of β in G.
- (2)
If β is a unit quaternion whose coefficient of k is zero, then we have and β is a fixed point under the involution.
The proof is analogous to that of Proposition 3.
Next, we will compute
with
G a finite subgroup of
one by one. Before that, we recall the classification of the finite subgroups of
. There are many references for the classification, including ([
35] Chapter XIII), [
36,
37] etc. The finite subgroups of
are classified as follows:
the cyclic group of order
nthe dicyclic group of order
the binary tetrahedral group ;
the binary octahedral group ;
the binary icosahedral group ;
where n is any positive integer.
Example 9. In this example, we compute where is the finite cyclic subgroupWe take the Real structure as defined in Example 8, i.e., the group below together with the determinant map detIt is isomorphic to the dihedral group . The involution on is trivial. Thus, by ([3] Example 3.7), we obtain directly thatwhere consists of the fixed points, i.e., the south pole and the north pole of . Thus,and by ([3] Example 3.7), the right hand side is isomorphic to In addition, by ([34] Theorem 5.1), Example 10. In this example, we compute where is the dicyclic groupwhere τ is the reflectionThe Real structure on is taken to be , as that defined in Example 8. In , there are conjugacy classes. They are as follows:
- (1)
,
- (2)
,
- (3)
, , ⋯, ,
- (4)
,
- (5)
,
where the first two classes form the centre of the group.
By Proposition 4, all the conjugacy classes are fixed points under the reflection s. Thus, all the factors in are equivariant KR-theories. The factor in corresponding to each conjugacy class is computed below one by one.
- (1)
First, we consider the Real conjugacy class represented by I. The centralizer and the Real centralizer is Then, the group By ([34] Theorem 5.1), Note that is a Real structure on .
- (2)
Then, we consider the Real conjugacy class represented by . In this case, the centralizer and the Real centralizer In this case, we have the Real central extension By Corollary A1,where is the sign representation of . - (3)
Then, we compute the factor in corresponding to which is not .
The centralizer is the cyclic group ; and the Real centralizer is the dihedral group of order . In this case, by ([3] Example 3.7), - (4)
Then, we compute the factor corresponding to the conjugacy class represented by τ. The centralizer and the Real centralizer - (5)
For the conjugacy class represented by , the centralizer and the Real centralizer . Then, the factor corresponding to is
In conclusion,where is the sign representation of . Example 11. In this example, we compute where is the binary tetrahedral group . We take the Real structure on it, i.e.,The quaternion representation of is given explicitly in [38,39]. We can compute the conjugacy classes in explicitly. A multiplication table for the binary tetrahedral group is given in [40]. For the convenience of the readers, we apply the same symbols of the elements as those in [39,40]. A list of representatives are given in Figure 1. This list can be obtained by direct computation. In addition, by Proposition 3, an element in represents a fixed point in if and only if it is , , or . Note that, for , if we take the Real structure , we will obtain the same set of fixed points under the reflection. Below, we compute the factors of corresponding to each conjugacy class respectively.
- (1)
For the conjugacy class represented by I, the Real centralizer . By ([34] Theorem 5.1), we have - (2)
For the conjugacy class represented by , we have .
Let denote the group . We have the short exact sequence Especially, we have the commutative diagram below: Note that we have the short exact sequence By Corollary A1,where is the sign representation of . - (3)
For the conjugacy class represented by j, . The centralizer and the Real centralizer - (4)
For the conjugacy class represented by a, we have - (5)
For the conjugacy class represented by , we have - (6)
For the conjugacy class represented by , we have - (7)
For the conjugacy class represented by , we have
Thus, in conclusion,where is the sign representation of . Example 12. In this example, we compute where is the binary octahedral group. We take the Real structure on it, i.e., .
A presentation of is given asWe can obtain immediately that . Equivalently, there is a quaternion presentation of given by the embeddingsending θ to , t to , and r to . By [41] and direct computation, we obtain Figure 2, which provides a list of the representatives of the conjugacy classes of , the centralizers of each representative, and the corresponding fixed point spaces. Below, we give the factor of corresponding to each conjugacy class.
- (1)
For the conjugacy class represented by I, the Real centralizer . The factor corresponding to I - (2)
For the conjugacy class represented by , the Real centralizer . Let denote the chiral octahedral group and the Real structure , and we have the commutative diagram below. Thus, by Corollary A1,where is the sign representation of . - (3)
For the conjugacy class represented by j is , its Real centralizer Thus, is isomorphic to - (4)
For the conjugacy class represented by , the Real centralizer Note that and . Then is isomorphic to - (5)
For the conjugacy class represented , the Real centralizer Then, is isomorphic to - (6)
For the conjugacy class represented by , the Real centralizer Thus, is isomorphic to - (7)
For the conjugacy class represented by , its Real centralizer Thus, is isomorphic to - (8)
For the conjugacy class represented by , its Real centralizer Thus, is isomorphic to
Thus, in conclusion,where is the sign representation of . Example 13. In this example, we compute where is the binary icosahedral group. A presentation of this group isThe cardinality of is 120. In this example, we use τ to denote and σ to denote the number . We take the Real structure on , i.e., . By ([42] p. 7635, Table 1) and direct computation, we obtain a list of the representatives of the conjugacy classes of , the centralizers of each representative, whether it is fixed under the involution or not, and the corresponding fixed point spaces in Figure 3. Next, we compute each factor of corresponding to each conjugacy class of .
- (1)
For the conjugacy class , the Real centralizer . Thus, by ([34] Theorem 5.1), - (2)
For the conjugacy class , the Real centralizer . Thus, by Corollary A1,where is the sign representation of . - (3)
For the conjugacy class represented by , its Real centralizer Thus, is isomorphic to - (4)
For the conjugacy class represented by , the Real centralizer Thus, is isomorphic to - (5)
For the conjugacy class represented by , the Real centralizer Thus, the factor is isomorphic to - (6)
For the conjugacy class represented by , the Real centralizer Thus, the factor is isomorphic to - (7)
For the conjugacy class represented by , the Real centralizer Thus, the factor is isomorphic to - (8)
For the conjugacy class represented by , the Real centralizer Thus, the factor is isomorphic to - (9)
For the conjugacy class represented by , the Real centralizer Thus, the corresponding factor is isomorphic to
In conclusion,where is the sign representation of . Remark 3. As we can see in the examples of this section, most computations lead to the equivariant KR-theory of a single point. The whole data of the equivariant KR-theory, by the computation in ([31] Section 8) and ([43] Proposition 3.1), are given aswhere and are the forgetful functors, the map ρ is given explicitly in ([43] Proposition 2.17), and the map η is given explicitly in ([43] Proposition 2.24). In addition, there is a graded ring isomorphism (see [31] Section 8)