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Diffeomorphism groups of balls and spheres Kathryn Mann Abstract. In this paper we discuss the relationship between groups of diffeomorphisms of spheres and balls. We survey results of a topological nature and then address the relationship as abstract (discrete) groups. We prove that the identity component of the group of smooth diffeomorphisms of an odd dimensional sphere admits no nontrivial homomorphisms to the group of diffeomorphisms of a ball of any dimension. This result generalizes theorems of Ghys and Herman. We also examine finitely generated subgroups of diffeomorphisms of spheres, and produce an example of a finitely generated torsion free group with an action on the circle by smooth diffeomorphisms that does not extend to a C1 action on the disc. Contents 1. Introduction 1 2. Topological sections: known results 3 3. Group-theoretic sections 4 4. Actions of torsion free groups 5 5. Exotic homomorphisms: non-existence 8 References 10 1. Introduction r r Let M be a manifold and let Diff0(M) denote the group of isotopically trivial C -diffeomorphisms of M. If M has boundary @M, there is a natural map r r π : Diff0(M) ! Diff0(@M) given by restricting the domain of a diffeomorphism to the boundary. The map π is surjective, as any isotopically trivial diffeomorphism f of the boundary can be extended to a diffeomorphism F ∼ of M supported on a collar neighborhood N = @M × I of @M by taking a smooth isotopy ft from f to the identity, and defining F to agree with ft on @M × ftg. r r One way to measure the difference between the groups Diff0(M) and Diff0(@M) is to ask whether π admits a section. By section, we mean a map r r φ : Diff0(@M) ! Diff0(M) r such that π ◦ φ is the identity on Diff0(@M). There are several categories in which to ask this, namely i) Topological: Require φ to be continuous, ignoring the group structure. ii) (Purely) group-theoretic: Only require φ to be a group homomorphism, ignoring the r topological structure on Diff0(M). 1991 Mathematics Subject Classification. 57S25, 57S05, 57R50. Key words and phrases. diffeomorphism groups; group actions on manifolds; spheres. 1 2 KATHRYN MANN iii) Extensions of group actions: In the case where no group-theoretic section exists, we ask the following local (in the sense of group theory) question. For which finitely generated groups Γ r r and a homomorphisms ρ :Γ ! Diff0(@M) does there exist a homomorphism φ :Γ ! Diff0(M) such that π ◦ φ = ρ? If such a homomorphism exists, we say that φ extends the action of Γ on @M to a Cr action on M. In this paper, we treat the case of the ball M = Bn+1 with boundary Sn. Note in the category of homeomorphisms rather than diffeomorphisms, there is a natural way to extend homeomorphisms of Sn to homeomorphisms of Bn+1. This is by \coning off” the sphere to the ball and extending each homeomorphism to be constant along rays. The result is a continuous group homomorphism n n+1 φ : Homeo0(S ) ! Homeo0(B ) n+1 n which is also a section of π : Homeo0(B ) ! Homeo0(S ) in the sense above. We will see, however, that the question of sections for groups of diffeomorphisms is much more interesting! Summary of results. Our goal in this work is to paint a relatively complete picture of known and new results for the ball Bn. Here is an outline. Topological sections. In Section 2 we give brief survey of known results on existence and nonexis- tence of topological sections, and the relationship between topological sections and exotic spheres. The reader may skip this section if desired; it stands independent from the rest of this paper. Group-theoretic sections. In contrast with the topological case, it is a theorem of Ghys that no r n r n+1 group theoretic sections φ : Diff0(S ) ! Diff0(B ) exist for any n or r. A close reading of 2n−1 Ghys' work in [Ghy91] produces finitely generated subgroups of Diff0(S ) that fail to extend 2n to Diff0(B ) and we give an explicit presentation of such a group in Section 3. These examples rely heavily on the dynamics of finite order diffeomorphisms. Extending actions of torsion free groups. Building on Ghys' work and using results of Franks and Handel involving distorted elements in finite groups, in Section 4 we explicitly construct a group Γ to prove the following. Theorem 1.1. There exists a finitely generated, torsion-free group Γ and a homomorphism ρ :Γ ! Diff1(S1) that does not extend to a C1 action of Γ on B2. Note that in contrast to Theorem 1.1, any action of Z, of a free group, or any action of any group that is conjugate into the standard action of PSL(2; R) on S1 will extend to an acton by diffeomorphisms on B2. r n+1 r n Exotic homomorphisms. In Section 5, we show that the failure of π : Diff0(B ) ! Diff0(S ) to admit a section is due (at least in the case where n is odd) to a fundamental difference between the algebraic structure of groups of diffeomorphisms of spheres and groups of diffeomorphisms of balls. We prove 1 2k−1 1 m Theorem 1.2. There is no nontrivial group homomorphism Diff0 (S ) ! Diff0(B ) for any m, k ≥ 1. This generalizes a result of M. Herman in [Her]. Theorem 1.2 also stands in contrast to situation with homeomorphisms of balls and spheres { any continuous foliation of Bn+l by n-spheres can be n n+l used to construct a continuous group homomorphism Homeo0(S ) ! Homeo0(B ). DIFFEOMORPHISM GROUPS OF BALLS AND SPHERES 3 Acknowledgements. The author would like to thank Christian Bonatti, Danny Calegari, Benson Farb, John Franks and Amie Wilkinson for helpful conversations and their interest in this project, Kiran Parkhe and Bena Tshishiku for their comments on drafts of this paper, and Igor Belegradek for a correction in the section on exotic spheres. 2. Topological sections: known results In order to contrast our work on group-theoretic sections with the (fundamentally different) question of topological sections, we present a brief summary of known results in the topological case. Let Diff(Bn rel @) denote the group of smooth diffeomorphisms of Bn that restrict to the identity on @Bn = Sn−1. The natural restriction map Diff(Bn) !π Diff(Sn−1) is a fibration with fiber Diff(Bn rel @). Hence, asking for a topological section of π amounts to asking for a section of this fibration. In low dimensions (n ≤ 3), it is known that the fiber Diff(Bn rel @) is contractible, so a topolog- ical section exists. The n = 2 case is a classical theorem of Smale [Sma59], and the n = 3 case a 1 highly non-trivial theorem of Hatcher [Hat83]. Incidentally, Diff0(B rel @) is also contractible and this is quite elementary { an element of Diff(B1 rel @) is a nonincreasing or nondecreasing function of the closed interval, and we can explicitly define a retraction of Diff(B1 rel @) to the identity via r : Diff(B1 rel @) × [0; 1] ! Diff(B1 rel @) r(f; t)(x) = tf(x) + (1 − t)x: Whether Diff(B4 rel @) is contractible is an open question. To the best of the author's knowl- 4 π 3 edge, whether Diff0(B ) ! Diff0(S ) has a section is also open. However, in higher dimensions Diff(Bn rel @) is not always contractible, giving a first obstruction to a section. This is related to the existence of exotic smooth structures on spheres. Exotic spheres. Let f 2 Diff(Bn rel @) be a diffeomorphism. We can extend f by the identity to n n+1 n+1 a homeomorphism of S , and glue two copies of B together using f; producing a sphere Sf n n+1 with a smooth structure. If f lies in the identity component of Diff(B rel @), then Sf will be n+1 n+1 smoothly isotopic to the standard n+1-sphere S . If not, there is no reason that Sf need even be diffeomorphic to Sn+1. In fact, it follows from the pseudoisotopy theorem of Cerf in [Cer70] n that, for n ≥ 5, the induced map from π0(Diff(B rel @)) to the group of exotic n + 1-spheres is injective. Moreover { and more pertinent to our discussion { it follows from Smale's h-cobordism theorem n (proved in [Sma61]) that the map from π0(Diff(B rel @)) to exotic n + 1-spheres is surjective. In n particular, this means that in any dimension n+1 where exotic spheres exist, π0(Diff(B rel @)) 6= 0. Let us now return to the fibration π : Diff(Bn) ! Diff(Sn−1) and look at the tail end of the long exact sequence in homotopy groups. If we consider the restriction of π to the identity components n π n−1 Diff0(B ) ! Diff0(S ) we have n n−1 n ::: ! π1(Diff0(B )) ! π1(Diff0(S )) ! π0(Diff(B rel @)) ! 0 Thus, whenever exotic spheres exist, the connecting homomorphism n−1 n π1(Diff0(S )) ! π0(Diff(B rel @)) is nonzero, and so no section of the bundle exists. Question 2.1. Does this bundle have a section in any dimensions n ≥ 5 where exotic spheres do not exist? 4 KATHRYN MANN We remark that for all n ≥ 5, it is known that Diff(Bn rel @) has some nontrivial higher homotopy groups. In fact the author learned from Allen Hatcher that recent work of Crowey and Schick [CS13] shows that Diff(Bn rel @) has infinitely many nonzero higher homotopy groups whenever n ≥ 7.
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