Automorphic forms and rational homology 3–spheres

Calegari, Frank; Dunfield, Nathan M

doi:10.2140/gt.2006.10.295

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Automorphic forms and rational homology 3–spheres

Frank Calegari and Nathan M Dunfield

Geometry & Topology 10 (2006) 295–329

DOI: 10.2140/gt.2006.10.295

arXiv: math.GT/0508271

Abstract

We investigate a question of Cooper adjacent to the Virtual Haken Conjecture. Assuming certain conjectures in number theory, we show that there exist hyperbolic rational homology 3–spheres with arbitrarily large injectivity radius. These examples come from a tower of abelian covers of an explicit arithmetic 3–manifold. The conjectures we must assume are the Generalized Riemann Hypothesis and a mild strengthening of results of Taylor et al on part of the Langlands Program for ${GL}_{2}$ of an imaginary quadratic field.

The proof of this theorem involves ruling out the existence of an irreducible two dimensional Galois representation $ρ$ of $Gal (\bar{ℚ} ∕ ℚ (\sqrt{- 2}))$ satisfying certain prescribed ramification conditions. In contrast to similar questions of this form, $ρ$ is allowed to have arbitrary ramification at some prime $π$ of $ℤ [\sqrt{- 2}]$ .

In the next paper in this volume, Boston and Ellenberg apply pro– $p$ techniques to our examples and show that our result is true unconditionally. Here, we give additional examples where their techniques apply, including some non-arithmetic examples.

Finally, we investigate the congruence covers of twist-knot orbifolds. Our experimental evidence suggests that these topologically similar orbifolds have rather different behavior depending on whether or not they are arithmetic. In particular, the congruence covers of the non-arithmetic orbifolds have a paucity of homology.

Keywords

virtual Haken Conjecture, Cooper's question, rational homology sphere, injectivity radius, automorphic forms, Galois representations

Mathematical Subject Classification 2000

Primary: 57M27, 11F80, 11F75

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Publication

Received: 18 August 2005

Revised: 28 February 2006

Accepted: 2 January 2006

Published: 2 April 2006

Proposed: Walter Neumann

Seconded: David Gabai, Tomasz Mrowka

Authors

Frank Calegari
Department of Mathematics Harvard University One Oxford Street Cambridge MA 02138 USA
http://www.math.harvard.edu/~fcale/
Nathan M Dunfield
Mathematics 253-37 Caltech Pasadena CA 91125 USA
http://www.its.caltech.edu/~dunfield/

Volume 10, issue 1 (2006)