Ramanujan's sum

In number theory, Ramanujan's sum, usually denoted cq(n), is a function of two positive integer variables q and n defined by the formula

where (a, q) = 1 means that a only takes on values coprime to q.

Srinivasa Ramanujan mentioned the sums in a 1918 paper.[1] In addition to the expansions discussed in this article, Ramanujan's sums are used in the proof of Vinogradov's theorem that every sufficiently large odd number is the sum of three primes.[2]

Notation

For integers a and b, is read "a divides b" and means that there is an integer c such that Similarly, is read "a does not divide b". The summation symbol

means that d goes through all the positive divisors of m, e.g.

is the greatest common divisor,

is Euler's totient function,

is the Möbius function, and

is the Riemann zeta function.


Formulas for cq(n)

Trigonometry

These formulas come from the definition, Euler's formula and elementary trigonometric identities.

and so on (OEIS: A000012, OEIS: A033999, OEIS: A099837, OEIS: A176742,.., OEIS: A100051,...). cq(n) is always an integer.

Kluyver

Let Then ζq is a root of the equation xq − 1 = 0. Each of its powers,

is also a root. Therefore, since there are q of them, they are all of the roots. The numbers where 1 ≤ nq are called the q-th roots of unity. ζq is called a primitive q-th root of unity because the smallest value of n that makes is q. The other primitive q-th roots of unity are the numbers where (a, q) = 1. Therefore, there are φ(q) primitive q-th roots of unity.

Thus, the Ramanujan sum cq(n) is the sum of the n-th powers of the primitive q-th roots of unity.

It is a fact[3] that the powers of ζq are precisely the primitive roots for all the divisors of q.

Example. Let q = 12. Then

and are the primitive twelfth roots of unity,
and are the primitive sixth roots of unity,
and are the primitive fourth roots of unity,
and are the primitive third roots of unity,
is the primitive second root of unity, and
is the primitive first root of unity.

Therefore, if

is the sum of the n-th powers of all the roots, primitive and imprimitive,

and by Möbius inversion,

It follows from the identity xq − 1 = (x − 1)(xq−1 + xq−2 + ... + x + 1) that

and this leads to the formula

published by Kluyver in 1906.[4]

This shows that cq(n) is always an integer. Compare it with the formula

von Sterneck

It is easily shown from the definition that cq(n) is multiplicative when considered as a function of q for a fixed value of n:[5] i.e.

From the definition (or Kluyver's formula) it is straightforward to prove that, if p is a prime number,

and if pk is a prime power where k > 1,

This result and the multiplicative property can be used to prove

This is called von Sterneck's arithmetic function.[6] The equivalence of it and Ramanujan's sum is due to Hölder.[7][8]

Other properties of cq(n)

For all positive integers q,

For a fixed value of q the absolute value of the sequence is bounded by φ(q), and for a fixed value of n the absolute value of the sequence is bounded by n.

If q > 1

Let m1, m2 > 0, m = lcm(m1, m2). Then[9] Ramanujan's sums satisfy an orthogonality property:

Let n, k > 0. Then[10]

known as the Brauer - Rademacher identity.

If n > 0 and a is any integer, we also have[11]

due to Cohen.

Table

Ramanujan sum cs(n)
n
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
s 1 1 111111111 1111111111 1111111111
2 1 1 11111111 1111111111 1111111111
3 11 2 112 112112 112112 112112 112112
4 0202 0202 0202 0202 0202 0202 0202 02
5 11114 11114 11114 11114 11114 11114
6 112112 112112 112112 112112 112112
7 1111116 1111116 1111116 1111116 11
8 00040004 00040004 00040004 000400
9 003003006 003003006 003003006 003
10 1111411114 1111411114 1111411114
11 111111111110 111111111110 11111111
12 020204020204 020204020204 020204
13 11111111111112 11111111111112 1111
14 11111161111116 11111161111116 11
15 112142112412118 112142112412118
16 0000000800000008 00000008000000
17 111111111111111116 1111111111111
18 003003006003003006 003003006003
19 11111111111111111118 11111111111
20 0202020208 0202020208 0202020208
21 1121126121121621121112 112112612
22 111111111110 111111111110 11111111
23 111111111111111111111122 1111111
24 0004 0004 0008 0004 0004 0008 0004 00
25 00005 00005 00005 00005 000020 00005
26 11111111111112 11111111111112 1111
27 000000009 000000009 0000000018 000
28 020202020202012 020202020202012 02
29 111111111111111111111111111128 1
30 112142112412118 112142112412118

Ramanujan expansions

If f(n) is an arithmetic function (i.e. a complex-valued function of the integers or natural numbers), then a convergent infinite series of the form:

or of the form:

where the akC, is called a Ramanujan expansion[12] of f(n).

Ramanujan found expansions of some of the well-known functions of number theory. All of these results are proved in an "elementary" manner (i.e. only using formal manipulations of series and the simplest results about convergence).[13][14][15]

The expansion of the zero function depends on a result from the analytic theory of prime numbers, namely that the series

converges to 0, and the results for r(n) and r(n) depend on theorems in an earlier paper.[16]

All the formulas in this section are from Ramanujan's 1918 paper.

Generating functions

The generating functions of the Ramanujan sums are Dirichlet series:

is a generating function for the sequence cq(1), cq(2), ... where q is kept constant, and

is a generating function for the sequence c1(n), c2(n), ... where n is kept constant.

There is also the double Dirichlet series

σk(n)

σk(n) is the divisor function (i.e. the sum of the k-th powers of the divisors of n, including 1 and n). σ0(n), the number of divisors of n, is usually written d(n) and σ1(n), the sum of the divisors of n, is usually written σ(n).

If s > 0,

Setting s = 1 gives

If the Riemann hypothesis is true, and

d(n)

d(n) = σ0(n) is the number of divisors of n, including 1 and n itself.

where γ = 0.5772... is the Euler–Mascheroni constant.

φ(n)

Euler's totient function φ(n) is the number of positive integers less than n and coprime to n. Ramanujan defines a generalization of it, if

is the prime factorization of n, and s is a complex number, let

so that φ1(n) = φ(n) is Euler's function.[17]

He proves that

and uses this to show that

Letting s = 1,

Note that the constant is the inverse[18] of the one in the formula for σ(n).

Λ(n)

Von Mangoldt's function Λ(n) = 0 unless n = pk is a power of a prime number, in which case it is the natural logarithm log p.

Zero

For all n > 0,

This is equivalent to the prime number theorem.[19][20]

r2s(n) (sums of squares)

r2s(n) is the number of way of representing n as the sum of 2s squares, counting different orders and signs as different (e.g., r2(13) = 8, as 13 = (±2)2 + (±3)2 = (±3)2 + (±2)2.)

Ramanujan defines a function δ2s(n) and references a paper[21] in which he proved that r2s(n) = δ2s(n) for s = 1, 2, 3, and 4. For s > 4 he shows that δ2s(n) is a good approximation to r2s(n).

s = 1 has a special formula:

In the following formulas the signs repeat with a period of 4.

and therefore,

r2s(n) (sums of triangles)

is the number of ways n can be represented as the sum of 2s triangular numbers (i.e. the numbers 1, 3 = 1 + 2, 6 = 1 + 2 + 3, 10 = 1 + 2 + 3 + 4, 15, ...; the n-th triangular number is given by the formula n(n + 1)/2.)

The analysis here is similar to that for squares. Ramanujan refers to the same paper as he did for the squares, where he showed that there is a function such that for s = 1, 2, 3, and 4, and that for s > 4, is a good approximation to

Again, s = 1 requires a special formula:

If s is a multiple of 4,

Therefore,

Sums

Let

Then for s > 1,

See also

Notes

  1. Ramanujan, On Certain Trigonometric Sums ...
    These sums are obviously of great interest, and a few of their properties have been discussed already. But, so far as I know, they have never been considered from the point of view which I adopt in this paper; and I believe that all the results which it contains are new.
    (Papers, p. 179). In a footnote cites pp. 360370 of the Dirichlet–Dedekind Vorlesungen über Zahlentheorie, 4th ed.
  2. Nathanson, ch. 8.
  3. Hardy & Wright, Thms 65, 66
  4. G. H. Hardy, P. V. Seshu Aiyar, & B. M. Wilson, notes to On certain trigonometrical sums ..., Ramanujan, Papers, p. 343
  5. Schwarz & Spilken (1994) p.16
  6. B. Berndt, commentary to On certain trigonometrical sums..., Ramanujan, Papers, p. 371
  7. Knopfmacher, p. 196
  8. Hardy & Wright, p. 243
  9. Tóth, external links, eq. 6
  10. Tóth, external links, eq. 17.
  11. Tóth, external links, eq. 8.
  12. B. Berndt, commentary to On certain trigonometrical sums..., Ramanujan, Papers, pp. 369371
  13. Ramanujan, On certain trigonometrical sums...
    The majority of my formulae are "elementary" in the technical sense of the word they can (that is to say) be proved by a combination of processes involving only finite algebra and simple general theorems concerning infinite series
    (Papers, p. 179)
  14. The theory of formal Dirichlet series is discussed in Hardy & Wright, § 17.6 and in Knopfmacher.
  15. Knopfmacher, ch. 7, discusses Ramanujan expansions as a type of Fourier expansion in an inner product space which has the cq as an orthogonal basis.
  16. Ramanujan, On Certain Arithmetical Functions
  17. This is Jordan's totient function, Js(n).
  18. Cf. Hardy & Wright, Thm. 329, which states that
  19. Hardy, Ramanujan, p. 141
  20. B. Berndt, commentary to On certain trigonometrical sums..., Ramanujan, Papers, p. 371
  21. Ramanujan, On Certain Arithmetical Functions

References

  • Hardy, G. H. (1999), Ramanujan: Twelve Lectures on Subjects Suggested by his Life and Work, Providence RI: AMS / Chelsea, ISBN 978-0-8218-2023-0
  • Ramanujan, Srinivasa (1918), "On Certain Trigonometric Sums and their Applications in the Theory of Numbers", Transactions of the Cambridge Philosophical Society, 22 (15): 259–276 (pp. 179199 of his Collected Papers)
  • Ramanujan, Srinivasa (1916), "On Certain Arithmetical Functions", Transactions of the Cambridge Philosophical Society, 22 (9): 159–184 (pp. 136163 of his Collected Papers)
  • Ramanujan, Srinivasa (2000), Collected Papers, Providence RI: AMS / Chelsea, ISBN 978-0-8218-2076-6
  • Schwarz, Wolfgang; Spilker, Jürgen (1994), Arithmetical Functions. An introduction to elementary and analytic properties of arithmetic functions and to some of their almost-periodic properties, London Mathematical Society Lecture Note Series, vol. 184, Cambridge University Press, ISBN 0-521-42725-8, Zbl 0807.11001
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