Kloosterman sum

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In mathematics, a Kloosterman sum is a particular kind of exponential sum. For a prime number p we have

$K(a,b;p)=\sum e_p(ax+bx^*)$ ,

where

e p (t) = e 2π i t / p

and x* is the inverse of x modulo p. The summation is taken for 1 ≤ x ≤ p − 1. They are named for the Dutch mathematician Hendrik Kloosterman (1900-1968), who introduced them in 1926.

These sums, which can also be defined for a composite number modulus, are part of the theory of harmonic analysis over finite fields, being in a rough sense analogues there of Bessel functions. They turn out to have close connections with modular forms, and various analytic number theory techniques are used to provide estimates for the coefficients of modular forms starting with estimates for Kloosterman sums.

A fundamental technique of André Weil reduces the estimate

|K(a,b;p)| ≤ 2√p

when ab ≠ 0 to his results on local zeta-functions. Geometrically the sum is taken along a 'hyperbola'

XY = ab

and we consider this as defining an algebraic curve over the finite field with p elements. This curve has a ramified Artin-Schreier covering C, and Weil showed that the local zeta-function of C has a factorization; this is the Artin L-function theory for the case of global fields that are function fields, for which Weil gives a 1938 paper of J. Weissinger as reference (the next year he gave a 1935 paper of Hasse as earlier reference for the idea; given Weil's rather denigratory remark on the abilities of analytic number theorists to work out this example themselves, in his Collected Papers, these ideas were presumably 'folklore' of quite long standing). The non-polar factors are of type

1 − Kt

where K is a Kloosterman sum. The estimate then follows from Weil's basic work of 1940.

This technique in fact shows much more generally that complete exponential sums 'along' algebraic varieties have good estimates, depending on the Weil conjectures in dimension > 1. It has been pushed much further by Pierre Deligne, Gérard Laumon, and Nicholas Katz.