## Jørgensen number and arithmeticity

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- by Jason Callahan
- Conform. Geom. Dyn.
**13**(2009), 160-186 - DOI: https://doi.org/10.1090/S1088-4173-09-00196-9
- Published electronically: July 23, 2009
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## Abstract:

The Jørgensen number of a rank-two non-elementary Kleinian group $\Gamma$ is \[ J(\Gamma ) = \inf \{|\mathrm {tr}^2 X - 4| + |\mathrm {tr} [X, Y] - 2| : \langle X, Y \rangle = \Gamma \}. \] Jørgensen’s Inequality guarantees $J(\Gamma ) \geq 1$, and $\Gamma$ is a Jørgensen group if $J(\Gamma ) = 1$. This paper shows that the only torsion-free Jørgensen group is the figure-eight knot group, identifies all non-cocompact arithmetic Jørgensen groups, and establishes a characterization of cocompact arithmetic Jørgensen groups. The paper concludes with computations of $J(\Gamma )$ for several non-cocompact Kleinian groups including some two-bridge knot and link groups.## References

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## Bibliographic Information

**Jason Callahan**- Affiliation: Department of Mathematics, The University of Texas at Austin, 1 University Station C1200, Austin, Texas 78712 and Department of Mathematics, St. Edward’s University, 3001 South Congress Avenue, Austin, Texas 78704
- MR Author ID: 877083
- Email: callahan@math.utexas.edu; jasonc@stedwards.edu
- Received by editor(s): May 14, 2009
- Published electronically: July 23, 2009
- © Copyright 2009
American Mathematical Society

The copyright for this article reverts to public domain 28 years after publication. - Journal: Conform. Geom. Dyn.
**13**(2009), 160-186 - MSC (2000): Primary 30F40; Secondary 57M05, 57M25, 57M50
- DOI: https://doi.org/10.1090/S1088-4173-09-00196-9
- MathSciNet review: 2525101