5,710 research outputs found
On invertible generating pairs of fundamental groups of graph manifolds
We study invertible generating pairs of fundamental groups of graph
manifolds, that is, pairs of elements (g,h) for which the map g --> g^{-1}, h
--> h^{-1} extends to an automorphism. We show in particular that a graph
manifold is of Heegaard genus 2 if and only if its fundamental group has an
invertible generating pair.Comment: This is the version published by Geometry & Topology Monographs on 29
April 200
Nielsen equivalence in a class of random groups
We show that for every there exists a torsion-free one-ended
word-hyperbolic group of rank admitting generating -tuples
and such that the -tuples
are not
Nielsen-equivalent in . The group is produced via a probabilistic
construction.Comment: 34 pages, 2 figures; a revised final version, to appear in the
Journal of Topolog
Generating pairs of 2-bridge knot groups
We study Nielsen equivalence classes of generating pairs of Kleinian groups
and HNN-extensions. We establish the following facts: - Hyperbolic 2-bridge
knot groups have infinitely many Nielsen classes of generating pairs. - For any
natural number N there is a closed hyperbolic 3-manifold whose fundamental
group has N distinct Nielsen classes of generating pairs. - Two pairs of
elements of a fundamental group of an HNN-extension are Nielsen equivalent iff
they are so for the obvious reasons.Comment: Final version to appear in Geometriae Dedicat
Central Stars of Planetary Nebulae: New spectral classifications and catalogue
Context. There are more than 3000 true and probable known Galactic Planetary
Nebulae (PNe), but only for 13% of them there is central star spectroscopic
information available. Aims. To contribute to the knowledge of central stars of
planetary nebulae and star evolution. Methods. We undertook a spectroscopic
survey of central stars of PNe in low resolution and compiled a large list of
central stars for which information was dispersed in the literature. Results.
We complete a catalogue of 492 true and probable CSPN and we provide a
preliminary spectral classification for 45 central star of PNe, This made it
possible to update the proportion of CSPN with atmosphere poor in hydrogen with
regard to the whole in at least 30% and contribute with statistical information
that allow to infer the origin of H-poor stars.Comment: 19 pages, 1 figure, accepted to be published in A&A (October 24,
2010
Kleinian groups and the rank problem
We prove that the rank problem is decidable in the class of torsion-free
word-hyperbolic Kleinian groups. We also show that every group in this class
has only finitely many Nielsen equivalence classes of generating sets of a
given cardinality.Comment: Published by Geometry and Topology at
http://www.maths.warwick.ac.uk/gt/GTVol9/paper12.abs.htm
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