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On sums of two and three roots of unity

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Abstract Let α ≠ 0 be a sum of some k distinct Nth roots of unity, where 2 ≤ k N . In 1986, Myerson raised the following two problems.… Click to show full abstract

Abstract Let α ≠ 0 be a sum of some k distinct Nth roots of unity, where 2 ≤ k N . In 1986, Myerson raised the following two problems. How small can | α | be? How large can the modulus of the product of all conjugates of α lying in the disc | z | 1 be? A simple Liouville type argument gives the lower bound k − N + 2 for these quantities, so the problem is to find appropriate upper bounds. As for the first question, for k ≥ 5 , it remains a huge gap between lower and the best known upper bound N − d k . In this note, we give a complete answer to the second question of Myerson for k = 2 . For k = 3 and N large prime, we show that a positive proportion of the conjugates of any such α lie in the disc | z | ≤ ϱ , where ϱ 1 . This implies a naturally expected upper bound.

Keywords: sums two; two three; three roots; roots unity

Journal Title: Journal of Number Theory
Year Published: 2018

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