1 Introduction
where k ≥ 3 is a given integer and e(y) = e2πiy.
if prime p > 3 satisfying (3, p − 1) = 1.
where χ be the Dirichlet character mod p.
it seems that it has not been studied before or at least we have not found any relevant results. The problem is meaningful for it can reflect the upper bound approximation of C(m, n, k; p). It is easy to find that the mean value (1) is the best possible. Thus we believe that (2) has similar asymptotic properties to (1). In fact, we can use the analytic method and the qualities of the congruence equation to propose an exact computational formula for (2) with a special integer k = 3. That is to say, we shall prove the following identity:
From this theorem we may immediately deduce the following estimate:
are two open problems, we will further study.
2 Some simple lemmas
In this section, we will put up several simple Lemmas which are necessary to prove the theorem. Hereunder, we will use many properties of trigonometric sums and congruence equation, which can be found in references [6] and [13], so we will not repeat them here. First we have the following:
This proves Lemma 2.1.□
This proves Lemma 2.2.□
3 Proof of the theorem
For all integers 1 ≤ a, b, c, d, e ≤p − 1, we compute the number of the solutions in (9). We separate the solutions in (9) into three cases:
- : a = 1, 2 ≤ b, c ≤ p − 1; b = 1, 2 ≤ a, c ≤ p − 1; c = 1, 2 ≤ a, b ≤ p − 1.
- : a = b = 1, 2 ≤ c ≤ p − 1; a = c = 1, 2 ≤ b ≤ p − 1; b = c = 1, 2 ≤ a ≤ p − 1.
- : a = b = c = 1.
In the case (C), the equation (9) becomes d + e ≡ 2 mod p and d2 + e2 ≡ 2 mod p, so d = e = 1. That is, in this case, the equation (9) has only one solution (a, b, c, d, e) = (1,1,1,1,1).
In the case (B), if a = b = 1 and 2 ≤ c ≤ p − 1, then the equation (9) becomes 2 ≤ c ≤ p − 1, d + e ≡ c + 1 mod p, d2 + e2 ≡ c2 + 1 mod p and de ≡ c mod p, or (d−1)(e−1)≡ 0 mod p and (d2−1)(e2−1) ≡ 0 mod p, 2 ≤ c ≤ p − 1. In this case, the number of the solutions of the congruence equation is 2(p−2). So in the case (B), the number of all solutions of the congruence equation (9) is 3 × 2(p−2) = 6(p−2).
It is clear that the number of the solutions of the congruence equation d + e ≡ b + c mod p, d2 + e2 ≡ b2 + c2 mod p, 1 ≤ b, c, d, e ≤ p − 1 is (p−1)(2p−3). In fact, this congruence equation is equivalent to the congruence equation d + e ≡ b + 1 mod p, d2 + e2 ≡ b2 + 1 mod p, 1 ≤ b, c, d, e ≤ p − 1. So from (B) we know that the number of the solutions is (p − 1)(2p−3). In the case (A), the congruence equation (9) becomes 2 ≤ b, c ≤ p − 1, d + e ≡ b + c mod p, d2 + e2≡ b2 + c2 mod p. So from (B) and (C) we know that the number of the solutions of the congruence equation (9) in case (A) is 3 × [(p − 1)(2p − 3)− 4(p − 2) − 1].
This completes the proof of our theorem.
The authors would like to thank the referees for their very helpful and detailed comments, which have significantly improved the presentation of this paper.
This work is supported by N. S. F. (11371291), P. N. S. F. (2013JM1017), N. S. B. R. (2016JQ1013) and S. R. P. F. (16JK1373) of P. R. China.
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