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**Group invariant Peano curves.**
*(English)*
Zbl 1136.57009

Let \(S\) be a hyperbolic surface, whose universal cover is the hyperbolic plane \(\mathbb{H}^2\). A discrete faithful representation of the fundamental group of \(S\) in the group \(\text{Isom}(\mathbb{H}^3)\) (or the image of such a representation) is called doubly degenerate if the limit set of the induced group action on the compactification \(\mathbb{H}^3\cup S^2_{\infty}\) is equal to the sphere \(S^2_{\infty}\).

One of the aims of this paper is to describe some doubly degenerate groups. The main result is that if \(M\) is a closed hyperbolic \(3\)-manifold which fibers over the circle with pseudo-Anosov monodromy, then the lift of the inclusion map of the fiber \(S\) in \(M\) to the hyperbolic universal covers extends continuously to a map between the compactifications of the covering spaces, and induces at the boundary an equivariant \(S^2_{\infty}\)-filling Peano curve. In this situation, \(S\) is a closed surface, and the authors conjecture that the result extends to the case where \(S\) is a once-punctured hyperbolic surface. Evidence for this conjecture is provided by the case of a figure-eight knot complement, which the authors analyze in detail.

The study of sphere-filling curves is based on a theorem by R. L. Moore which gives a condition under which the quotient of the \(2\)-sphere by an equivalence relation induced by a cellular decomposition is homeomorphic to the \(2\)-sphere. In the main example considered, the \(2\)-sphere decomposition is obtained by collapsing two laminations.

The paper under review contains several fundamental ideas and techniques of \(3\)-dimensional geometry and topology, and it has been circulated as a preprint for several years.

One of the aims of this paper is to describe some doubly degenerate groups. The main result is that if \(M\) is a closed hyperbolic \(3\)-manifold which fibers over the circle with pseudo-Anosov monodromy, then the lift of the inclusion map of the fiber \(S\) in \(M\) to the hyperbolic universal covers extends continuously to a map between the compactifications of the covering spaces, and induces at the boundary an equivariant \(S^2_{\infty}\)-filling Peano curve. In this situation, \(S\) is a closed surface, and the authors conjecture that the result extends to the case where \(S\) is a once-punctured hyperbolic surface. Evidence for this conjecture is provided by the case of a figure-eight knot complement, which the authors analyze in detail.

The study of sphere-filling curves is based on a theorem by R. L. Moore which gives a condition under which the quotient of the \(2\)-sphere by an equivalence relation induced by a cellular decomposition is homeomorphic to the \(2\)-sphere. In the main example considered, the \(2\)-sphere decomposition is obtained by collapsing two laminations.

The paper under review contains several fundamental ideas and techniques of \(3\)-dimensional geometry and topology, and it has been circulated as a preprint for several years.

Reviewer: Athanase Papadopoulos (Strasbourg)

### MSC:

57M60 | Group actions on manifolds and cell complexes in low dimensions |

57M50 | General geometric structures on low-dimensional manifolds |

57N05 | Topology of the Euclidean \(2\)-space, \(2\)-manifolds (MSC2010) |

57N60 | Cellularity in topological manifolds |

20F65 | Geometric group theory |

### Keywords:

hyperbolic 3-manifold; figure-eight knot complement; invariant Peano curve; doubly degenerate group; pseudo-Anosov monodromy
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\textit{J. W. Cannon} and \textit{W. P. Thurston}, Geom. Topol. 11, 1315--1355 (2007; Zbl 1136.57009)

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### References:

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