Weil sums of binomials, three-level cross-correlation, and a conjecture of Helleseth

  • Authors:
  • Daniel J. Katz

  • Affiliations:
  • Department of Mathematics, Simon Fraser University, 8888 University Drive, Burnaby, BC V5A 1S6, Canada

  • Venue:
  • Journal of Combinatorial Theory Series A
  • Year:
  • 2012

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Abstract

Let q be a power of a prime p, let @j"q:F"q-C be the canonical additive character @j"q(x)=exp(2@piTr"F"""q"/"F"""p(x)/p), let d be an integer with gcd(d,q-1)=1, and consider Weil sums of the form W"q","d(a)=@?"x"@?"F"""q@j"q(x^d+ax). We are interested in how many different values W"q","d(a) attains as a runs through F"q^@?. We show that if |{W"q","d(a):a@?F"q^@?}|=3, then all the values in {W"q","d(a):a@?F"q^@?} are rational integers and one of these values is 0. This translates into a result on the cross-correlation of a pair of p-ary maximum length linear recursive sequences of period q-1, where one sequence is the decimation of the other by d: if the cross-correlation is three-valued, then all the values are in Z and one of them is -1. We then use this to prove the binary case of a conjecture of Helleseth, which states that if q is of the form 2^2^^^n, then the cross-correlation cannot be three-valued.