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Non-proper Actions of the Fundamental Group of a Punctured Torus

Virginie Charette
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Abstract

Given an affine isometry of $\R^3$ with hyperbolic linear part, its Margulis invariant measures signed Lorentzian displacement along an invariant spacelike line. In order for a group generated by hyperbolic isometries to act properly on $\R^3$, the sign of the Margulis invariant must be constant over the group. We show that, in the case when the linear part is the fundamental group of a punctured torus, positivity of the Margulis invariant over any finite generating set does not imply that the group acts properly. This contrasts with the case of a pair of pants, where it suffices to check the sign of the Margulis invariant for a certain triple of generators.

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Virginie Charette. 2003-11-04. Non-proper Actions of the Fundamental Group of a Punctured Torus. https://arxiv.org/abs/math/0311040

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