#### Volume 17, issue 4 (2017)

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The diagonal slice of Schottky space

### Caroline Series, Ser Peow Tan and Yasushi Yamashita

Algebraic & Geometric Topology 17 (2017) 2239–2282
##### Abstract

An irreducible representation of the free group on two generators $X,Y$ into $SL\left(2,ℂ\right)$ is determined up to conjugation by the traces of $X,Y$ and $XY$. If the representation is faithful and discrete, the resulting manifold is in general a genus-$2$ handlebody. We study the diagonal slice of the representation variety in which $Tr\phantom{\rule{0.3em}{0ex}}X=Tr\phantom{\rule{0.3em}{0ex}}Y=Tr\phantom{\rule{0.3em}{0ex}}XY$. Using the symmetry, we are able to compute the Keen–Series pleating rays and thus fully determine the locus of faithful discrete representations. We also computationally determine the “Bowditch set” consisting of those parameter values for which no primitive elements in $〈X,Y〉$ have traces in $\left[-2,2\right]$, and at most finitely many primitive elements have traces with absolute value at most $2$. The graphics make clear that this set is both strictly larger than, and significantly different from, the discreteness locus.

##### Keywords
Schottky group, nondiscrete group, primitive element, Bowditch condition
Primary: 30F40
Secondary: 57M50
##### Publication
Received: 17 April 2016
Revised: 17 October 2016
Accepted: 31 October 2016
Published: 3 August 2017
##### Authors
 Caroline Series Mathematics Institute University of Warwick Leeman Building Coventry CV4 7AL United Kingdom http://homepages.warwick.ac.uk/~masbb/ Ser Peow Tan Department of Mathematics National University of Singapore Block S17, 10 Lower Kent Ridge Rd. Singapore 119076 http://www.math.nus.edu.sg/~mattansp/ Yasushi Yamashita Department of Information and Computer Sciences Nara Women’s University Kitauoyanishi-machi, Nara 630-8506 Japan http://vivaldi.ics.nara-wu.ac.jp/~yamasita/