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Galois representations attached to moments of Kloosterman sums and conjectures of Evans

Part of: Discontinuous groups and automorphic forms Exponential sums and character sums

Published online by Cambridge University Press:  07 October 2014

Zhiwei Yun  and
Christelle Vincent
Zhiwei Yun
Affiliation:
Department of Mathematics, Stanford University, 450 Serra Mall, Building 380, Stanford, CA 94305, USA email zwyun@stanford.edu
Christelle Vincent
Affiliation:
Department of Mathematics, Stanford University, 450 Serra Mall, Building 380, Stanford, CA 94305, USA email cvincent@stanford.edu
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Abstract

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Kloosterman sums for a finite field $\mathbb{F}_{p}$ arise as Frobenius trace functions of certain local systems defined over $\mathbb{G}_{m,\mathbb{F}_{p}}$. The moments of Kloosterman sums calculate the Frobenius traces on the cohomology of tensor powers (or symmetric powers, exterior powers, etc.) of these local systems. We show that when $p$ ranges over all primes, the moments of the corresponding Kloosterman sums for $\mathbb{F}_{p}$ arise as Frobenius traces on a continuous $\ell$-adic representation of $\text{Gal}(\overline{\mathbb{Q}}/\mathbb{Q})$ that comes from geometry. We also give bounds on the ramification of these Galois representations. All of this is done in the generality of Kloosterman sheaves attached to reductive groups introduced by Heinloth, Ngô and Yun [Ann. of Math. (2) 177 (2013), 241–310]. As an application, we give proofs of conjectures of Evans [Proc. Amer. Math. Soc. 138 (2010), 517–531; Israel J. Math. 175 (2010), 349–362] expressing the seventh and eighth symmetric power moments of the classical Kloosterman sum in terms of Fourier coefficients of explicit modular forms. The proof for the eighth symmetric power moment conjecture relies on the computation done in Appendix B by C. Vincent.

Keywords

Kloosterman sumsGalois representationsmodularity

MSC classification

Primary: 11L05: Gauss and Kloosterman sums; generalizations
Secondary: 11F30: Fourier coefficients of automorphic forms 11F80: Galois representations
Type
Research Article
Information
Compositio Mathematica , Volume 151 , Issue 1 , January 2015 , pp. 68 - 120
Copyright
© The Author(s) 2014 

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