Brewer sum

In mathematics, Brewer sums are finite character sum introduced by Brewer (1961, 1966) related to Jacobsthal sums.

Definition

The Brewer sum is given by

\Lambda _{n}(a)=\sum _{x{\bmod {p}}}{\binom {D_{n+1}(x,a)}{p}}

where D_n is the Dickson polynomial (or "Brewer polynomial") given by

D_{0}(x,a)=2,\quad D_{1}(x,a)=x,\quad D_{n+1}(x,a)=xD_{n}(x,a)-aD_{n-1}(x,a)

and () is the Legendre symbol.

The Brewer sum is zero when n is coprime to q²−1.

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References

Brewer, B. W. (1961), "On certain character sums", Transactions of the American Mathematical Society, 99 (2): 241–245, doi:10.2307/1993392, ISSN 0002-9947, JSTOR 1993392, MR 0120202, Zbl 0103.03205
Brewer, B. W. (1966), "On primes of the form u²+5v²", Proceedings of the American Mathematical Society, 17 (2): 502–509, doi:10.2307/2035200, ISSN 0002-9939, JSTOR 2035200, MR 0188171, Zbl 0147.29801
Berndt, Bruce C.; Evans, Ronald J. (1979), "Sums of Gauss, Eisenstein, Jacobi, Jacobsthal, and Brewer", Illinois Journal of Mathematics, 23 (3): 374–437, ISSN 0019-2082, MR 0537798, Zbl 0393.12029
Lidl, Rudolf; Niederreiter, Harald (1997), Finite fields, Encyclopedia of Mathematics and Its Applications, 20 (2nd ed.), Cambridge University Press, ISBN 0-521-39231-4, Zbl 0866.11069

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