English

The $\omega$-Borel invariant for representations into $SL(n,\mathbb{C}_\omega)$

Geometric Topology 2020-09-28 v1

Abstract

Let Γ\Gamma be the fundamental group of a complete hyperbolic 33-manifold MM with toric cusps. We define the ω\omega-Borel invariant βnω(ρω)\beta_n^\omega(\rho_\omega) associated to a representation ρω:ΓSL(n,Cω)\rho_\omega: \Gamma \rightarrow SL(n,\mathbb{C}_\omega), where Cω\mathbb{C}_\omega is a field which can be constructed as a quotient of a suitable subset of CN\mathbb{C}^\mathbb{N} with the data of a non-principal ultrafilter ω\omega on N\mathbb{N} and a real divergent sequence λl\lambda_l such that λl1\lambda_l \geq 1. Since a sequence of ω\omega-bounded representations ρl\rho_l into SL(n,C)SL(n,\mathbb{C}) determines a representation ρω\rho_\omega into SL(n,Cω)SL(n,\mathbb{C}_\omega), for n=2n=2 we study the relation between the invariant β2ω(ρω)\beta^\omega_2(\rho_\omega) and the sequence of Borel invariants β2(ρl)\beta_2(\rho_l). We conclude by showing that if a sequence of representations ρl:ΓSL(2,C)\rho_l:\Gamma \rightarrow SL(2,\mathbb{C}) induces a representation ρω:ΓSL(2,Cω)\rho_\omega:\Gamma \rightarrow SL(2,\mathbb{C}_\omega) which determines a reducible action on the asymptotic cone Cω(H3,d/λl,O)C_\omega(\mathbb{H}^3,d/\lambda_l,O) with non-trivial length function, then it holds β2ω(ρω)=0\beta^\omega_2(\rho_\omega)=0.

Keywords

Cite

@article{arxiv.1709.07660,
  title  = {The $\omega$-Borel invariant for representations into $SL(n,\mathbb{C}_\omega)$},
  author = {Alessio Savini},
  journal= {arXiv preprint arXiv:1709.07660},
  year   = {2020}
}

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25 pages