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Some estimate of character sums and its applications
Journal of Inequalities and Applications volume 2013, Article number: 328 (2013)
Abstract
The main purpose of this paper is using the elementary methods and the properties of Gauss sums to give a sharp estimate for some character sums. Then using this estimate to prove the existence of some special primitive roots , an odd prime, and to prove that for any integer n with , if p is large enough, then there exist two primitive roots α and β of p such that both and are also primitive roots of p, where satisfies . Let denote the number of all pairs of primitive roots of p such that both and are also primitive roots of p. Then we can give an interesting asymptotic formula for .
MSC:11M20.
1 Introduction
Let be an integer. For any integer n with , from the well-known Euler-Fermat theorem, we have , where is Euler ϕ-function. That is, denotes the number of all integers with . Let k be the smallest positive integer such that . If , then n is called a primitive root of q. If q has a primitive root, then each reduced residue system can be expressed as a geometric progression. This gives a powerful tool that can be used in problems involving reduced residue systems. Unfortunately, not all modulo q have primitive roots. In fact primitive roots exist only for the following several cases:
where p is an odd prime and .
Many people have studied the properties of primitive roots and related problems, and obtained many interesting results; see [1–6] and [7]. For example, Juping Wang [4] proved that Golomb’s conjecture is true for almost all . That is, there exist two primitive elements α and β in finite fields such that . Cohen and Mullen [2] established a generalization of Golomb’s conjecture by proving the existence of such that, whenever , there exist with , where γ, δ and ε are arbitrary non-zero elements of .
In this paper, we consider the existence of some special primitive roots of p, such as all α, β, and are primitive roots of p, where satisfies . Furthermore, for any integer n with , are there primitive roots α and β of p such that both and are also two primitive roots of p? Let denote the number of all pairs of primitive roots of p such that both and are also primitive roots of p. How about the asymptotic properties of ?
In this paper, we shall use the elementary methods and estimate for character sums to study this problem, and prove the following conclusion.
Theorem Let p be an odd prime, then for any integer n with , we have the asymptotic formula
where , denotes the number of all distinct prime divisors of m, denotes the product over all distinct prime divisors of .
Taking , from our theorem we may immediately deduce the following two corollaries.
Corollary 1 Let p be a prime large enough, then there exist two primitive roots α and β of p such that both and are also primitive roots of p.
Corollary 2 Let p be a prime large enough, then there exist two primitive roots α and β of p such that both and are also primitive roots of p.
2 Several lemmas
In this section, we shall give several lemmas, which are necessary in the proof of our theorem. Dirichlet characters and Gauss sums are used in this paper, please refer to [8] for more details. First we have the following lemma.
Lemma 1 Let p be an odd prime, and let and be (not all principal characters). Then, for any m with , we have the estimate
Proof If is the principal character , then we have
If , then we have
If neither nor is the principal character , then from the properties of Gauss sums we have
Note that both and are non-principal characters , for any character , and . So from (2.3) we may immediately deduce the inequality
Now Lemma 1 follows from identities (2.1), (2.2) and estimate (2.4). □
Lemma 2 Let p be an odd prime, and let , , and be four non-principal characters with (or ). Then, for any integer n with , we have the estimate
Proof From the properties of reduced residue system , we have
It is clear that for any character , we have
So, if , the principal character , then from (2.5) and (2.6) we have the identity
If , note that and not all principal characters , so from (2.5), (2.6) and Lemma 1 we have the estimate
Combining (2.7) and (2.8), we may immediately deduce Lemma 2. □
Lemma 3 Let p be an odd prime. Then, for any integer c with , we have the identity
where indc denotes the index of c relative to some fixed primitive root of p, is the Möbius function.
Proof See Proposition 2.2 of reference [9]. □
3 Proof of the theorem
In this section, we shall complete the proof of our theorem. First we write . It is clear that is a Dirichlet character . For any integer n with , from Lemma 3 we have
Now we estimate each term in (3.1) respectively. It is clear that for any integer and , we have the identity
For any three non-principal characters , and , from Lemma 2 we have
From Lemma 1 we know that for any two non-principal characters and , we have the estimates
If four characters , , and in the last term of (3.1) do not satisfy the condition of Lemma 2, this time from Lemma 2 and the orthogonality of characters we have
Note that the identity , and implies , and . So from (3.1)-(3.12) we have
where we have used the identity , and . This completes the proof of our theorem.
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Acknowledgements
The authors would like to thank the referee for carefully examining this paper and providing a number of important comments. This work is supported by the N.S.F. (11071194, 61202437) of P.R. China, N.S.F. of Shaanxi Province (2012K06-43) and Foundation of Shaanxi Educational Committee (12JK0874).
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Authors’ contributions
JL carried out the sharp estimate for some character sums and give an asymptotic formula for . DH participated in the research and summary of the study.
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Li, J., Han, D. Some estimate of character sums and its applications. J Inequal Appl 2013, 328 (2013). https://doi.org/10.1186/1029-242X-2013-328
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DOI: https://doi.org/10.1186/1029-242X-2013-328