Boundedness of diffeomorphism groups of manifold pairs -- Circle case --

TL;DR

This paper investigates the boundedness of conjugation-invariant norms on diffeomorphism groups of manifold pairs, especially when the submanifold is a union of circles, revealing conditions for boundedness and unboundedness.

Contribution

It establishes new relations among various norms on diffeomorphism groups and characterizes boundedness based on the rank of a subgroup A derived from rotation quasimorphisms.

Findings

Group is weakly simple when dimension conditions are met.

Boundedness depends on the rank of subgroup A.

Unbounded when the rank of A is less than m.

Abstract

In this paper we study boundedness of conjugation invariant norms on diffeomorphism groups of manifold pairs. For the diffeomorphism group ${\mathcal D} \equiv {\rm Diff}(M,N)_0$ of a closed manifold pair $(M, N)$ with $\dim N \geq 1$, first we clarify the relation among the fragmentation norm, the conjugation generated norm, the commutator length $cl$ and the commutator length with support in balls $clb$ and show that ${\mathcal D}$ is weakly simple relative to a union of some normal subgroups of ${\mathcal D}$. For the boundedness of these norms, this paper focuses on the case where $N$ is a union of $m$ circles. In this case, the rotation angle on $N$ induces a quasimorphism $\nu : {\rm Isot}(M, N)_0 \to {\Bbb R}^m$, which determines a subgroup $A$ of ${\Bbb Z}^m$ and a function $\widehat{\nu} : {\mathcal D} \to {\Bbb R}^m/A$. If ${\rm rank}\,A = m$, these data leads to an upper bound of $clb$ on ${\mathcal D}$ modulo the normal subgroup ${\mathcal G} \cong {\rm Diff}_c(M - N)_0$. Then, some upper bounds of $cl$ and $clb$ on ${\mathcal D}$ are obtained from those on ${\mathcal G}$. As a consequence, the group ${\mathcal D}$ is uniformly weakly simple and bounded when $\dim M \neq 2,4$. On the other hand, if ${\rm rank}\,A < m$, then the group ${\mathcal D}$ admits a surjective quasimorphism, so it is unbounded and not uniformly perfect. We examine the group $A$ in some explicit examples.