# Bredon, Introduction to compact transformation groups, Academic Press

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1 1 Introduction Outline Section 3: Topology of 2-orbifolds: Compact group actions Compact group actions Orbit spaces. Tubes and slices. Path-lifting, covering homotopy Locally smooth actions Smooth actions Equivariant triangulations Newman s theorem Some helpful references Bredon, Introduction to compact transformation groups, Academic Press Hsiang, Cohomology theory of topological transformation group, Springer, 1975 Soren Illman, Smooth equivariant triangulations of G-manifolds for G a finite group, Math. Ann. 233, (1978) 2 Compact group actions A group action G X X with e(x) = x for all x and gh(x) = g(h(x)). That is, G Homeo(X) so that the product operation becomes compositions. We only need the result for finite group actions. An equivariant map φ : X Y between G-spaces is a map so that φ(g(x)) = g(φ(x)). An isotropy subgroup G x = {g G g(x) = x}. G g(x) = gg x g 1. G x G φ(x) for an equivariant map φ. Tietze-Gleason Theorem: G a compact group acting on X with a closed invariant set A. Let G also act linearly on R n. Then any equivariant φ : A R n extends to φ : X R n.

2 Orbit spaces An orbit of x is G(x) = {g(x) g G}. G/G x G(x) is one-to-one onto continuous function. An orbit type is given by the conjugacy class of G x in G. The orbit types form a partially ordered set. Denote by X/G the space of orbits with quotient topology. For A X, G(A) = g G g(a) is the saturation of A. Properties: π : X X/G is an open, closed, and proper map. X/G is Hausdorff. X is compact iff X/G is compact. X is locally compact iff X/G is locally compact. Orbit spaces: Examples Let X = G Y and G acts as a product. For k, q relatively prime, the action of Z k on S 3 in C 2 generated by a matrix [ e 2πi/k 0 ] 0 e 2πqi/k giving us a Lens space. We can also consider S 1 -actions given by [ e 2πkiθ 0 0 e 2πqiθ Then it has three orbit types. Consider in general the action of torus T n -action on C n given by (c 1,..., c n )(y 1,..., y n ) = (c 1 y 1,..., c n y n ), c i = 1, y i C. ] Orbit spaces: Examples Then there is a homeomorphism h : C n /T n (R + ) n given by sending (y 1,..., y n ) ( y 1 2,..., y n 2 ). The interiors of sides represent different orbit types. 2

3 H a closed subgroup of Lie group G. The left-coset space G/H where G acts on the right also. G/G x G(x) is given by gg x g(x) is a homeomorphism if G is compact. Twisted product: X a right G-space, Y a left G-space. A left action is given by g(x, y) = (xg 1, gy). The twisted product X G Y is the quotient space. p : X B is a principal bundle with G acting on the left. F a right G-space. Then F G X is the associated bundle. Orbit spaces: Bad examples The Conner-Floyd example: There is an action of Z r for r = pq,p, q relatively prime, on an Euclidean space of large dimensions without stationary points. Find a simplicial action Z pq on S 3 = S 1 S 1 without stationary points obtained by joining action of Z p on S 1 and Z q on the second S 1. Find an equivariant simplicial map h : S 3 S 3 which is homotopically trivial. Build the infinite mapping cylinder which is contactible and imbed it in an Euclidean space of high-dimensions where Z pq acts orthogonally. Find the contractible neighborhood. Taking the product with the real line makes it into a Euclidean space. Orbit spaces: Bad examples Hsiang-Hsiang: If G is any compact, connected, nonabelian Lie group, then there is an action of G on any euclidean space of sufficiently high dimension for which the fixed point set F has any given homotopy type. (F could be empty.) Twisted product G a compact subgroup, X right G-space and Y left G-space. X G Y is the quotient space of X Y where [xg, y] [x, gy] for g G. H a closed subgroup of G G H Y is a left G-space by the action g[g, a] = [gg, a]. This sends equivalence classes to themselves. The inclusion A G H A induces a homeomorphism A/H (G H A)/G. The isotropy subgroup at [e, a]: [e, a] = g[e, a] = [g, a] = [h 1, h(a)]. Thus, G [e,a] = H a. Example: Let G = S 1 and A be the unit-disk and H = Z 3 generated by e 2π/3. G and H acts in a standard way in A. Then consider G H A. 3

4 Tubes and slices X a G-space. P an orbit of type G/H. A tube about P is a G-equivariant imbedding G H A X onto an open neighborhood of P where A is a some space where H acts on. Every orbit passes the image of e A. P = G(x) for x = [e, a] where a is the stationary point of H in A. In general G x = H a H for x = [e, a]. A slice: x X, A set x S such that G x (S) = S. Then S is a slice if G Gx S X so that [g, s] g(s) is a tube about G x. S is a slice iff S is the image of e A for some tube. Tubes and slices Let x S and H = G x. Then the following are equivalent: There is a tube φ : G H A X about G(x) such that φ([e, A]) = S. S is a slice at x. G(S) is an open neighborhood of G(x) and there is an equivariant retraction f : G(S) G(x) with f 1 (x) = S. It would be a good exercise to apply these theories to above examples... The existence of tubes Let X be a completely regular G-space. There is a tube about any orbit of a complete regular G-space with G compact. (Mostow) Let x 0 have an isotropy group H in G. Find an orthogonal representation of G in R n with a point v 0 whose isotropy group is H. There is an equivalence G(x 0 ) and G(v 0 ). Extend this to a neighborhood. For R n, we can find the equivariant retraction. Transfer this on X. If G is a finite group acting on a manifold, then a tube is a union of disjoint open sets and a slice is an open subset where G x acts on. 4

5 Path-lifting and covering homotopy theorem. Let X be a G-space, G a compact Lie group, and f : I X/G any path. Then there exists a lifting f : I X so that π f = f. Let f : X Y be an equivariant map. Let f : X/G Y/G be an induced map. Let F : X/G I Y/G be a homotopy preserving orbit types that starts at f. Then there is an equivariant F : X I Y lifting F starting at f. If G is finite and X a smooth manifold with a smooth G-action and if the functions are smooth, then the lifts can be chosen to be also smooth. Locally smooth actions M a G-space, G a compact Lie group, P an orbit of type G/H. V a vector space where H acts orthogonally. Then a linear tube in M is a tube of the form φ : G H V M. Let S be a slice. S is a linear slice if G Gx S M given by [g, s] g(s) is equivalent to a linear slice. (If G x -space S is equivalent to the orthogonal G x -space.) If there is a linear tube about each orbit, then M is said to be locally smooth. Locally smooth actions There exists a maximum orbit type G/H for G. (That is, H is conjugate to a subgroup of each isotropy group.) Near each tube, we find the maximal orbit types has to be dense and open. The maximal orbits so obtained are called principal orbits. If M is a smooth manifold and compact Lie G acts smoothly, this is true. Manifold M a smooth manifold, G a compact Lie group acting smoothly on M. If G is finite, then this is equivalent to the fact that each i g : M M given by x g(x) is a diffeomorphism. Let n be the dimension of M and d the dimension of the maximal orbit. M = M/G is a manifold with boundary if n d 2. 5

6 Manifold Let k = n d, the codimension of the principal orbits. Consider a linear tube G K V. The orbit space (G K V ) = V. Let S be the unit sphere in V. Then V is a cone over S. dim M = dim V = dim S + 1. If k = 0, then M is discrete. If M is a sphere, then M is one or two points. If k = 1, then M is locally a cone over one or two points. Hence M is a 1-manifold. If k = 2, then M is locally a cone over an arc or a circle. (as S is a 1-manifold by the previous step.) Example: Z 2 action on R 3 generated by the antipodal map. The result is not a manifold. Smooth actions Recall smooth actions. G-compact Lie group acting smoothly on M. Then there exists an invariant Riemannian metric on M. G(x) is a smooth manifold. G/G x G(x) is a diffeomorphism. Exponential map: For X T p M, there is a unique geodesic γ X with tangent vector at p equal to X. The exponential map exp : T p M M is defined by X γ X (1). If A is an invariant smooth submanifold, then A has an open invariant tubular neighborhood. The smooth action of a compact Lie group is locally smooth. Use the fact that orbits are smooth submanifolds and the above item. Some facts needed later The subspace M (H) of same orbit type G/H is a smooth locally-closed submanifold of M. A a closed invariant submanifold. Then any two open (resp. closed) invariant tubular neighborhoods are equivariantly isotopic. 6

7 Newman s theorem Let M be a connected topological n-manifold. Then there is a finite open covering U of the one-point compactification of M such that there is no effective action of a compact Lie group with each orbit contained in some member of U. (Proof: algebraic topology) If G is a compact Lie group acting effectively on M, then M G is nowhere dense. Equivariant triangulations Sören Illman proved: Let G be a finite group. Let M be a smooth G-manifold with or without boundary. Then we have: There exists an equivariant simplicial complex K and a smooth equivariant triangulation h : K M. If h : K M and h 1 : L M are smooth triangulations of M, there exist equivariant subdivisions K and L of K and L, respectively, such that K and L are G-isomorphic. This result was widely used once a proof by Yang (1963) was given. But an error was discovered by Siebenmann (1970) and proved in

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