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Abstract
We study subgroups of
PU ( 2 , 1 )
generated by two noncommuting unipotent maps
A and
B whose product
A B is also unipotent.
We call
U
the set of conjugacy classes of such groups. We provide a set of coordinates on
U that make it
homeomorphic to ℝ 2 .
By considering the action on complex hyperbolic space
H ℂ 2 of groups in
U , we describe a
two-dimensional disc
Z
in
U that
parametrises a family of discrete groups. As a corollary, we give a proof of a conjecture of Schwartz
for
( 3 , 3 , ∞ ) –triangle
groups. We also consider a particular group on the boundary of the disc
Z where the
commutator
[ A , B ]
is also unipotent. We show that the boundary of the quotient orbifold associated to
the latter group gives a spherical CR uniformisation of the Whitehead link
complement.
Keywords
discrete subgroups of Lie groups, complex hyperbolic
geometry, spherical CR structures, complex hyperbolic
quasi-Fuchsian groups
Mathematical Subject Classification 2010
Primary: 20H10, 22E40, 51M10
Secondary: 57M50
Publication
Received: 2 October 2015
Revised: 17 May 2016
Accepted: 28 June 2016
Published: 31 August 2017
Proposed: Walter Neumann
Seconded: Danny Calegari, Jean-Pierre Otal