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A remark on algebraic curves derived from convolution sums
Journal of Inequalities and Applications volume 2013, Article number: 58 (2013)
Abstract
Hahn (Rocky Mt. J. Math. 37:1593-1622, 2007) established three differential equations according to , , and , which allows us to obtain the values of the formulas for
etc. Finally, by using the above equations, we derive the algebraic curves.
MSC:11A67.
1 Introduction
Let â„• denote the sets of positive integers. In number theory, divisor functions are defined as
for . The convolution sum is special one of divisor functions begun by Ramanujan’s effort and expanded by many authors; e.g., see [1]. For example,
is the result of Ramanujan and also Huard, Ou, Spearman, and Williams [[1], (3.10)]. We can see
in [[1], (3.11), (3.12)], respectively. Moreover, there are
in [[1], (4.4)] and
in [[1], Theorem 6]. In addition, Lahiri [2] gave the value of the sum
where the sum is over all positive integers satisfying , , and . In this paper we substitute for and obtain the formulas as Lahiri’s evaluation. So, we refer to Hahn’s paper [3]. Officially, we look into Hanh’s definition of three functions for ,
Three functions (6), (7), and (8) satisfy the following differential equations:
The paper is organized as follows. In Section 2, we obtain the values of the formulas for
etc. and insist on the following. Let
If N is an odd integer and , then there exist satisfying
with (see Theorem 2.9).
In Section 2, we derive the convolution sums of the restricted divisor functions. In Section 3, we obtain the algebraic curves by using the convolution sums in Section 2.
2 Convolution sum
Theorem 2.1 Let N (≥3) be any positive integer. Then we have
Proof Multiplying on both sides in (9), we obtain
Employing the definition of and , we can rewrite (12) as
Thus, we have
Then we refer to
in [[3], (4.4)],
in [[3], (4.9)] and by direct calculation we get
This completes the proof of the theorem. □
Corollary 2.2 We obtain
where as (8).
Proof By separating the variables in (11), we can have
Thus,
We note that is positive because
according to
in [[3], (1.12), (1.13)]. Therefore, we can exchange the summation and the integral. This completes the proof of the corollary. □
We introduce the following Lemma 2.3 to deduce Theorem 2.6.
Lemma 2.3 Let N (≥2) be any positive integer. Then we have
-
(a)
.
-
(b)
.
-
(c)
.
Proof (a) The proof is the same as Remark in [[3], p.13]. Also, Hahn showed that
in [[3], Theorem 4.2] derived by the identity , which is the same result (a).
-
(b)
The convolution sum can be written as
Then we obtain
Therefore, we have proved (b) by using (3) and (5).
-
(c)
We obtain the proof by direct calculation of the index m. □
Corollary 2.4 Let be an odd prime in Lemma 2.3 and let (). Then we have
-
(a)
.
-
(b)
.
-
(c)
.
Proof
From Lemma 2.3(a), (b), and (c), we obtain
This completes the proof of the corollary. □
Example 2.5 The first eleven values of and are listed in Table 1.
Theorem 2.6 Let N (≥3) be any positive integer. Then we have
Proof Multiplying in (10), we obtain
So, we have
Therefore
Lastly, we use Lemma 2.3. □
Theorem 2.7 Let N (≥3) be any positive integer. Then we have
Proof
Let us consider
by Lemma 2.3(a). Then (13) becomes
by . □
Remark 2.8 Let be an odd prime in Theorem 2.7. Then we have
Proof It is obvious. □
Importantly, we propose the following theorem.
Theorem 2.9 Let
If N is an odd integer and , then there exist satisfying
with .
Proof It is satisfied by Theorem 2.1, Lemma 2.3, Theorem 2.6, and Theorem 2.7. □
The expressions are shown in Table 2.
3 Algebraic curves derived from convolution sums
To obtain the result of this section, we need a general theory and it is this that we describe. Suppose that the two polynomials
have common zero, say . Then each of the equations
is of the form , where p is a polynomial of degree at most , and as each of these equations is satisfied when , the determinant of the coefficients must vanish. This determinant is the resultant of and and
where the omitted elements are zero, and the diagonal of contains m occurrences of and n of [[4], p.206]. We can obtain Table 3 from the results of Section 2.
Corollary 3.1 Let
There exists a polynomial such that with where p is an odd prime. We abbreviate to and it is also applied to the other values. Then we get Table 4.
Proof We illustrate the proof for the first in Table 4. In Table 3 we consider and with an odd prime x put by . As
the polynomials
have common zero, namely, . We deduce that for each odd prime p (),
and this simplifies to give , so we can find the irreducible polynomial . Other results in Table 4 can also be obtained by using the resultant. □
Remark 3.2 The plane curves in Corollary 3.1 all have zero-genus since x, y are polynomials of p, which leads to a morphism from the projective line to plane curves . Then it follows easily from the Riemann-Hurwitz theorem (e.g., see Corollary 1 on page 91 of [5]).
Example 3.3 We suggest Figure 1, Figure 2 and Figure 3 for results of and .
References
Huard JG, Ou ZM, Spearman BK, Williams KS: Elementary evaluation of certain convolution sums involving divisor functions. II. Number Theory for the Millennium 2002, 229–274.
Lahiri DB:On Ramanujan’s function and the divisor function , I. Bull. Calcutta Math. Soc. 1946, 38: 193–206.
Hahn H: Convolution sums of some functions on divisors. Rocky Mt. J. Math. 2007, 37: 1593–1622. 10.1216/rmjm/1194275937
Beardon AF: Sums of powers of integers. Am. Math. Mon. 1996, 103: 201–213. 10.2307/2975368
Rosen M Graduate Texts in Mathematics 210. In Number Theory in Function Fields. Springer, Berlin; 2002.
Acknowledgements
Dedicated to Professor Hari M Srivastava.
This work was supported by the Kyungnam University Foundation Grant, 2013.
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Kim, D., Kim, A. & Kim, MS. A remark on algebraic curves derived from convolution sums. J Inequal Appl 2013, 58 (2013). https://doi.org/10.1186/1029-242X-2013-58
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DOI: https://doi.org/10.1186/1029-242X-2013-58