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A Classification of subgroups of SL(4,R) Isomorphic to R^3 and Generalized Cusps in Projective 3 Manifolds

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 Added by Arielle Leitner
 Publication date 2015
  fields
and research's language is English




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This paper uses work of Haettel to classify all subgroups of PGL(4,R) isomorphic to (R^3 , +), up to conjugacy. We use this to show there are 4 families of generalized cusps up to projective equivalence in dimension 3.



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A generalized cusp $C$ is diffeomorphic to $[0,infty)$ times a closed Euclidean manifold. Geometrically $C$ is the quotient of a properly convex domain by a lattice, $Gamma$, in one of a family of affine groups $G(psi)$, parameterized by a point $psi$ in the (dual closed) Weyl chamber for $SL(n+1,mathbb{R})$, and $Gamma$ determines the cusp up to equivalence. These affine groups correspond to certain fibered geometries, each of which is a bundle over an open simplex with fiber a horoball in hyperbolic space, and the lattices are classified by certain Bieberbach groups plus some auxiliary data. The cusp has finite Busemann measure if and only if $G(psi)$ contains unipotent elements. There is a natural underlying Euclidean structure on $C$ unrelated to the Hilbert metric.
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In this paper, a generalized cusp is a properly convex manifold with strictly convex boundary that is diffeomorphic to $M times [0, infty)$ where $M$ is a closed Euclidean manifold. These are classified in [2]. The marked moduli space is homeomorphic to a subspace of the space of conjugacy classes of representations of $pi_1(M)$. It has one description as a generalization of a trace-variety, and another description involving weight data that is similar to that used to describe semi-simple Lie groups. It is also a bundle over the space of Euclidean similarity (conformally flat) structures on $M$, and the fiber is a closed cone in the space of cubic differentials. For 3-dimensional orientable generalized cusps, the fiber is homeomorphic to a cone on a solid torus.
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