Realization of the mapping class group of handlebody by diffeomorphisms

Wall's stable realization for diffeomorphisms of definite 4-manifolds

Let $X$ be a smooth simply connected closed 4-manifold with definite intersection form. We show that any automorphism of the intersection form of $X$ is realized by a diffeomorphism of $X \mathbin{\#} S^2 \times S^2$. This extends and completes Wall's foundational result from 1964.

Realization problems for diffeomorphism groups

We discuss recent results and open questions on the broad theme of (Nielsen) realization problems. Beyond realizing subgroups of mapping class groups, there are many other natural instances where one can ask if a surjection from a group of diffeomorphisms of a manifold to another group admits a section over particular subgroups. This survey includes many open problems, and some short proofs of new results that are illustrative of key techniques; drawing attention to parallels between problems arising in different areas.

A note on the Nielsen realization problem for connected sums of $S^2 \times S^1$

We consider finite group-actions on 3-manifolds $\cal H_g$ obtained as the connected sum of $g$ copies of $S^2 \times S^1$, with free fundamental group $F_g$ of rank $g$. We prove that, for $g > 1$, a finite group of diffeomorphisms of $\cal H_g$ inducing a trivial action on homology is cyclic. As a consequence, no non-cyclic subgroup of the twist subgroup of the mapping class group of $\cal H_g$ (generated by Dehn twists along embedded 2-spheres) can be realized by diffeomorphisms (in the sense of the Nielsen realization problem). We also discuss when a finite subgroup of the outer automorphi…

Links and dynamics

Knots naturally appear in continuous dynamical systems as flow periodic trajectories. However, discrete dynamical systems are also closely connected with the theory of knots and links. For example, for Pixton diffeomorphisms, the equivalence class of the Hopf knot, which is the orbit space of the unstable saddle separatrix in the manifold $\mathbb{S}^2\times \mathbb{S}^1$, is a complete invariant of the topological conjugacy of the system. In this paper we distinguish a class of three-dimensional Morse-Smale diffeomorphisms for which the complete invariant of topological conjugacy is the equiv…

流形 $\#_n \mathbb{CP}^2$ 的同调 Nielsen 实现问题

Homological Nielsen realization for the manifolds $\#_n \mathbb{CP}^2$

研究光滑正定4-流形 $M_n=\#_n\mathbb{CP}^2$ 上，有限等距群 $G\leq O(H_2(M_n;\mathbb{Z}))$ 何时可同构提升为有限定向保向微分同胚群，发现随着 $n$ 增大，随机子群和随机奇阶元素几乎必然不可实现。

Nielsen Realization for sphere twists on 3-manifolds

For a 3-manifold M, the twist group Twist(M) is the subgroup of the mapping class group Mod(M) generated by twists about embedded 2-spheres. We study the Nielsen realization problem for subgroups of Twist(M). We prove that a nontrivial subgroup G<Twist(M) is realized by diffeomorphisms if and only if G is cyclic and M is a connected sum of lens spaces. We also apply our methods to the Burnside problem for 3-manifolds and show that Diff(M) does not contain an infinite torsion group when M is reducible and not a connected sum of lens spaces.

The diffeomorphism group of a K3 surface and Nielsen realization

The Nielsen Realization problem asks when the group homomorphism from Diff(M) to pi_0 Diff(M) admits a section. For M a closed surface, Kerckhoff proved that a section exists over any finite subgroup, but Morita proved that if the genus is large enough then no section exists over the entire mapping class group. We prove the first nonexistence theorem of this type in dimension 4: if M is a smooth closed oriented 4-manifold which contains a K3 surface as a connected summand then no section exists over the whole of the mapping class group. This is done by showing that certain obstructions lying i…

Extending $T^p$ automorphisms over $\RR^{p+2}$ and realizing DE attractors

In this paper we consider the realization of DE attractors by self-diffeomorphisms of manifolds. For any expanding self-map $ϕ:M\to M$ of a connected, closed $p$-dimensional manifold $M$, one can always realize a $(p,q)$-type attractor derived from $ϕ$ by a compactly-supported self-diffeomorphsm of $\RR^{p+q}$, as long as $q\geq p+1$. Thus lower codimensional realizations are more interesting, related to the knotting problem below the stable range. We show that for any expanding self-map $ϕ$ of a standard smooth $p$-dimensional torus $T^p$, there is compactly-supported self-diffeomorphism of $…

四维中的Nielsen实现与投影扭转

Nielsen realization in dimension four and projective twists

本文证明大量非自旋4-流形上光滑Nielsen实现问题失败，通过研究多重扭转、投影扭转等映射类，并利用投影扭转证明许多闭辛4-流形的辛Torelli群不由Dehn扭转平方生成。

实有理曲面自同构的映射类

Mapping classes of Real Rational Surface Automorphisms

本文研究实有理曲面上微分同胚族 $F_n$ 的映射类，通过切开不变曲线证明其限制 $\hat F_n$ 为非 Penner 型伪 Anosov 映射，并在 $n=8$ 时给出 Lehmer 数的新几何实现。

Groups of volume-preserving diffeomorphisms of noncompact manifolds and mass flow toward ends

Suppose M is a noncompact connected oriented C^infty n-manifold and omega is a positive volume form on M. Let D^+(M) denote the group of orientation preserving diffeomorphisms of M endowed with the compact-open C^infty topology and D(M; omega) denote the subgroup of omega-preserving diffeomorphisms of M. In this paper we propose a unified approach for realization of mass transfer toward ends by diffeomorphisms of M. This argument together with Moser's theorem enables us to deduce two selection theorems for the groups D^+(M) and D(M; omega). The first one is the extension of Moser's theorem to …

Spectral theorems for random walks on mapping class groups and $\text{Out}(F_N)$

We establish spectral theorems for random walks on mapping class groups of connected, closed, oriented, hyperbolic surfaces, and on $\text{Out}(F_N)$. In both cases, we relate the asymptotics of the stretching factor of the diffeomorphism/automorphism obtained at time $n$ of the random walk to the Lyapunov exponent of the walk, which gives the typical growth rate of the length of a curve -- or of a conjugacy class in $F_N$ -- under a random product of diffeomorphisms/automorphisms. In the mapping class group case, we first observe that the drift of the random walk in the curve complex is also …

有理4-流形中固定最小亏格为0的迷向同调类的映射类

Mapping classes fixing an isotropic homology class of minimal genus $0$ in rational $4$-manifolds

本文研究有理4-流形中本原迷向同调类的稳定化子，证明其任意元素可由几乎保持亏格0全纯Lefschetz纤维化的微分同胚表示，并解决了极大无挠阿贝尔子群的Nielsen实现问题。