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.

References

• Brewer, B. W. (1961), "On certain character sums", Transactions of the American Mathematical Society 99: 241–245, doi:10.2307/1993392, ISSN 0002-9947, MR 0120202, Zbl 0103.03205
• Brewer, B. W. (1966), "On primes of the form u²+5v²", Proceedings of the American Mathematical Society 17: 502–509, doi:10.2307/2035200, ISSN 0002-9939, 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 537798, 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