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Harmonic measures and rigidity for surface group actions on the circle

Masanori Adachi, Yoshifumi Matsuda and Hiraku Nozawa

Algebraic & Geometric Topology 25 (2025) 2391–2412
Abstract

We study rigidity properties of actions of a torsion-free lattice of PSU (1,1) on the circle S1. We follow the approaches of Frankel and Thurston proposed in preprints via foliated harmonic measures on the suspension bundles. Our main results are a curvature estimate and a Gauss–Bonnet formula for the S1 connection obtained by taking the average of the flat connection with respect to a harmonic measure. As consequences, we give a precise description of the harmonic measure on suspension foliations with maximal Euler number and an alternative proof of rigidity theorems of Matsumoto and Burger, Iozzi and Wienhard.

Keywords
group action, surface group, Euler number, harmonic measure, Gauss–Bonnet formula, rigidity
Mathematical Subject Classification
Primary: 22D50, 53C12
Secondary: 31A15, 57M60, 57R20, 58D19
References
Publication
Received: 1 October 2023
Revised: 15 April 2024
Accepted: 12 May 2024
Published: 11 August 2025
Authors
Masanori Adachi
Department of Mathematics
Shizuoka University
Shizuoka
Japan
Yoshifumi Matsuda
Department of Mathematical Sciences
Aoyama Gakuin University
Sagamihara
Japan
Hiraku Nozawa
Department of Mathematical Sciences
Ritsumeikan University
Kusatsu
Japan

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