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Character sum : ウィキペディア英語版
Character sum
In mathematics, a character sum is a sum
:\Sigma \chi(n)\,
of values of a Dirichlet character χ ''modulo'' ''N'', taken over a given range of values of ''n''. Such sums are basic in a number of questions, for example in the distribution of quadratic residues, and in particular in the classical question of finding an upper bound for the least quadratic non-residue ''modulo'' ''N''. Character sums are often closely linked to exponential sums by the Gauss sums (this is like a finite Mellin transform).
Assume χ is a nonprincipal Dirichlet character to the modulus ''N''.
==Sums over ranges==
The sum taken over all residue classes mod ''N'' is then zero. This means that the cases of interest will be sums \Sigma over relatively short ranges, of length ''R'' < ''N'' say,
:M \le n < M + R.
A fundamental improvement on the trivial estimate \Sigma = O(N) is the Pólya–Vinogradov inequality (George Pólya, I. M. Vinogradov, independently in 1918), stating in big O notation that
:\Sigma = O(\sqrt\log N).
Assuming the generalized Riemann hypothesis, Hugh Montgomery and R. C. Vaughan have shown〔Montgomery and Vaughan (1977)〕 that there is the further improvement
:\Sigma = O(\sqrt\log\log N).

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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