Group actions on disks
This page has not been refereed. The information given here might be incomplete or provisional.
 1 Topological actions
 2 Smooth actions
 2.1 Fixed point free
 2.1.1 History
Floyd and Richardson [Floyd&Richardson1959] have constructed for the first time a smooth fixed point free action of on a disk for , the alternating group on five letters (see [Bredon1972, pp. 55-58] for a transparent description of the construction). Next, Greever [Greever1960] has described plenty of finite solvable groups which can act smoothly on disks without fixed points. Then, Oliver [Oliver1975], [Oliver1976] has answered completely the question of which compact Lie groups admit smooth fixed point free actions on disks.
 2.1.2 Oliver number
Let be a finite group not of prime power order. Oliver [Oliver1975] has proven that the set
Lemma 2.1 (Oliver Lemma). For a finite group not of prime power order, if and only if there does not exist a sequence of normal subgroups such that is a -group, is a -group, and is cyclic for two (possibly the same) primes and .
Moreover, the work [Oliver1977, Theorem 7] yields the following proposition.
Proposition 2.2. For a finite nilpotent group not of prime power order, the following conclusions hold:
- if has at most one non-cyclic Sylow subgroup.
- for two distinct primes and , if has just one non-cyclic -Sylow and -Sylow subgroups.
- if has three or more non-cyclic Sylow subgroups.
The notion of the Oliver number extends to compact Lie groups as follows.
- if is abelian and is not of prime power order.
- if is non-abelian (see [Oliver1976]).
 2.1.3 Oliver group
Definition 2.3. A finite group not of prime power order is called an Oliver group if (cf. Oliver Lemma).
Examples of finite Oliver groups include:
- for three distinct primes , , and .
- the solvable groups and of order 72.
- all non-solvable groups, e.g., and for .
 2.1.4 Results
Theorem 2.4. A compact Lie group has a smooth fixed point free action on some disk if and only if the identity connected component of is non-abelian, or the quotient group is not of prime power order and .
Theorem 2.5. Let be a compact Lie group such that the identity connected component of is non-abelian, or the quotient group is not of prime power order. Let be a CW-complex. Then the following three statements are equivalent.
- is compact and the Euler-Poincaré characteristic .
- There exists a finite contractible -CW-complex such that the fixed point set is homeomorphic to .
- There exists a smooth action of on a disk such that the fixed point set is homotopy equivalent to .
 2.2 Fixed point sets
 2.2.1 History
 2.2.2 Definitions
For a compact space , between the reduced real, complex, and quaternion -theory groups , , and , respectively, consider
- the induction (complexification and quaternization) homomorphisms ,
- and the forgetful (complexification and realification) homomorphisms .
 2.2.3 Results
Theorem 2.6 ([Oliver1996]). Let be a finite group not of prime power order, and let denote a -Sylow subgroup of . Let be a smooth manifold. Then there exists a smooth action of on some disk such that the fixed point is diffeomorphic to if and only if the following two statements hold.
- is compact and .
- The class of satisfies the following condition depending on .
- is arbitrary, if is in the class of finite groups with dihedral subquotient of order for two distinct primes and .
- , if has a composite order element conjugate to its inverse and .
- , if has a composite order element but never conjugate to its inverse and .
- , i.e., is stably complex, if has a composite order element but never conjugate to its inverse and .
- , if has no composite order element and .
- , if has no composite order element and .
 3 References
- [Bredon1972] G. E. Bredon, Introduction to compact transformation groups, Academic Press, New York, 1972. MR0413144 (54 #1265) Zbl 0484.57001
- [Floyd&Richardson1959] E. E. Floyd and R. W. Richardson, An action of a finite group on an -cell without stationary points. , Bull. Amer. Math. Soc. 65 (1959), 73–76. MR0100848 (20 #7276) Zbl 0088.15302
- [Greever1960] J. Greever, Stationary points for finite transformation groups, Duke Math. J 27 (1960), 163–170. MR0110094 (22 #977) Zbl 0113.16505
- [Laitinen&Morimoto1998] E. Laitinen and M. Morimoto, Finite groups with smooth one fixed point actions on spheres, Forum Math. 10 (1998), no.4, 479–520. MR1631012 (99k:57078) Zbl 0905.57023
- [Oliver1975] R. Oliver, Fixed-point sets of group actions on finite acyclic complexes, Comment. Math. Helv. 50 (1975), 155–177. MR0375361 (51 #11556) Zbl 0304.57020
- [Oliver1976] R. Oliver, Smooth compact Lie group actions on disks, Math. Z. 149 (1976), no.1, 79–96. MR0423390 (54 #11369) Zbl 0334.57023
- [Oliver1977] R. Oliver, -actions on disks and permutation representations. II, Math. Z. 157 (1977), no.3, 237–263. MR0646085 (58 #31126) Zbl 0386.20002
- [Oliver1978] R. Oliver, -actions on disks and permutation representations, J. Algebra 50 (1978), no.1, 44–62. MR0501044 (58 #18508) Zbl 0386.20002
- [Oliver1996] B. Oliver, Fixed point sets and tangent bundles of actions on disks and Euclidean spaces, Topology 35 (1996), no.3, 583–615. MR1396768 (97g:57059) Zbl 0861.57047