Proof of the simplicity conjecture

Abstract

In the 1970s, Fathi, having proven that the group of compactly supported volume-preserving homeomorphisms of the $n$-ball is simple for $n\ge 3$, asked if the same statement holds in dimension $2$. We show that the group of compactly supported area-preserving homeomorphisms of the two-disc is not simple. This settles what is known as the "simplicity conjecture" in the affirmative. In fact, we prove the a priori stronger statement that this group is not perfect. An important step in our proof involves verifying for certain smooth twist maps a conjecture of Hutchings concerning recovering the Calabi invariant from the asymptotics of spectral invariants defined using periodic Floer homology. Another key step, which builds on recent advances in continuous symplectic topology, involves proving that these spectral invariants extend continuously to area-preserving homeomorphisms of the disc. These two properties of PFH spectral invariants are potentially of independent interest. Our general strategy is partially inspired by suggestions of Fathi and the approach of Oh towards the simplicity question. In particular, we show that infinite twist maps, studied by Oh, are not finite energy homeomorphisms, which resolves the "infinite twist conjecture" in the affirmative; these twist maps are now the first examples of Hamiltonian homeomorphisms which can be said to have infinite energy. Another consequence of our work is that various forms of fragmentation for volume preserving homeomorphisms which hold for higher dimensional balls fail in dimension two.

arXiv commentMinor corrections, expository changes: new figures; more history; new section on open questions; immediate corollaries on mass-flow removed --- we have found a gap in published literature on 'Thurston tricks' in this context, see section 7.1; new corollaries to fragmentation and abundance of normal subgroups; clarified that we also prove the "Infinite Twist Conjecture"; 111 pages, 8 figures

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Cited by in corpus (3)

• Quantitative Heegaard Floer cohomology and the Calabi invariant
• $\hat{G}$-invariant quasimorphisms and symplectic geometry of surfaces
• Closed-open morphisms on periodic Floer homology