Ramanujan's sum

Not to be confused with Ramanujan summation.

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

${\displaystyle c_{q}(n)=\sum _{1\leq a\leq q \atop (a,q)=1}e^{2\pi i{\tfrac {a}{q}}n},}$

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, ${\displaystyle a\mid b}$ is read "a divides b" and means that there is an integer c such that ${\displaystyle {\frac {b}{a}}=c.}$ Similarly, ${\displaystyle a\nmid b}$ is read "a does not divide b". The summation symbol

${\displaystyle \sum _{d\,\mid \,m}f(d)}$

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

${\displaystyle \sum _{d\,\mid \,12}f(d)=f(1)+f(2)+f(3)+f(4)+f(6)+f(12).}$

${\displaystyle (a,\,b)}$ is the greatest common divisor,

${\displaystyle \phi (n)}$ is Euler's totient function,

${\displaystyle \mu (n)}$ is the Möbius function, and

${\displaystyle \zeta (s)}$ is the Riemann zeta function.

Formulas for c_q(n)

Trigonometry

These formulas come from the definition, Euler's formula ${\displaystyle e^{ix}=\cos x+i\sin x,}$ and elementary trigonometric identities.

${\displaystyle {\begin{aligned}c_{1}(n)&=1\\c_{2}(n)&=\cos n\pi \\c_{3}(n)&=2\cos {\tfrac {2}{3}}n\pi \\c_{4}(n)&=2\cos {\tfrac {1}{2}}n\pi \\c_{5}(n)&=2\cos {\tfrac {2}{5}}n\pi +2\cos {\tfrac {4}{5}}n\pi \\c_{6}(n)&=2\cos {\tfrac {1}{3}}n\pi \\c_{7}(n)&=2\cos {\tfrac {2}{7}}n\pi +2\cos {\tfrac {4}{7}}n\pi +2\cos {\tfrac {6}{7}}n\pi \\c_{8}(n)&=2\cos {\tfrac {1}{4}}n\pi +2\cos {\tfrac {3}{4}}n\pi \\c_{9}(n)&=2\cos {\tfrac {2}{9}}n\pi +2\cos {\tfrac {4}{9}}n\pi +2\cos {\tfrac {8}{9}}n\pi \\c_{10}(n)&=2\cos {\tfrac {1}{5}}n\pi +2\cos {\tfrac {3}{5}}n\pi \\\end{aligned}}}$

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

Kluyver

Let ${\displaystyle \zeta _{q}=e^{\frac {2\pi i}{q}}.}$ Then ζ_q is a root of the equation x^q − 1 = 0. Each of its powers,

${\displaystyle \zeta _{q},\zeta _{q}^{2},\ldots ,\zeta _{q}^{q-1},\zeta _{q}^{q}=\zeta _{q}^{0}=1}$

is also a root. Therefore, since there are q of them, they are all of the roots. The numbers ${\displaystyle \zeta _{q}^{n}}$ where 1 ≤ n ≤ q are called the q-th roots of unity. ζ_q is called a primitiveq-th root of unity because the smallest value of n that makes ${\displaystyle \zeta _{q}^{n}=1}$ is q. The other primitive q-th roots of unity are the numbers ${\displaystyle \zeta _{q}^{a}}$ where (a, q) = 1. Therefore, there are φ(q) primitive q-th roots of unity.

Thus, the Ramanujan sum c_q(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 ζ_q are precisely the primitive roots for all the divisors of q.

Example. Let q = 12. Then

${\displaystyle \zeta _{12},\zeta _{12}^{5},\zeta _{12}^{7},}$ and ${\displaystyle \zeta _{12}^{11}}$ are the primitive twelfth roots of unity,

${\displaystyle \zeta _{12}^{2}}$ and ${\displaystyle \zeta _{12}^{10}}$ are the primitive sixth roots of unity,

${\displaystyle \zeta _{12}^{3}=i}$ and ${\displaystyle \zeta _{12}^{9}=-i}$ are the primitive fourth roots of unity,

${\displaystyle \zeta _{12}^{4}}$ and ${\displaystyle \zeta _{12}^{8}}$ are the primitive third roots of unity,

${\displaystyle \zeta _{12}^{6}=-1}$ is the primitive second root of unity, and

${\displaystyle \zeta _{12}^{12}=1}$ is the primitive first root of unity.

Therefore, if

${\displaystyle \eta _{q}(n)=\sum _{k=1}^{q}\zeta _{q}^{kn}}$

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

${\displaystyle \eta _{q}(n)=\sum _{d\mid q}c_{d}(n),}$

and by Möbius inversion,

${\displaystyle c_{q}(n)=\sum _{d\mid q}\mu \left({\frac {q}{d}}\right)\eta _{d}(n).}$

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

${\displaystyle \eta _{q}(n)={\begin{cases}0&q\nmid n\\q&q\mid n\\\end{cases}}}$

and this leads to the formula

${\displaystyle c_{q}(n)=\sum _{d\mid (q,n)}\mu \left({\frac {q}{d}}\right)d,}$

published by Kluyver in 1906. ^[4]

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

${\displaystyle \phi (q)=\sum _{d\mid q}\mu \left({\frac {q}{d}}\right)d.}$

von Sterneck

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

${\displaystyle {\mbox{If }}\;(q,r)=1\;{\mbox{ then }}\;c_{q}(n)c_{r}(n)=c_{qr}(n).}$

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

${\displaystyle c_{p}(n)={\begin{cases}-1&{\mbox{ if }}p\nmid n\\\phi (p)&{\mbox{ if }}p\mid n\\\end{cases}},}$

and if p^k is a prime power where k > 1,

${\displaystyle c_{p^{k}}(n)={\begin{cases}0&{\mbox{ if }}p^{k-1}\nmid n\\-p^{k-1}&{\mbox{ if }}p^{k-1}\mid n{\mbox{ and }}p^{k}\nmid n\\\phi (p^{k})&{\mbox{ if }}p^{k}\mid n\\\end{cases}}.}$

This result and the multiplicative property can be used to prove

${\displaystyle c_{q}(n)=\mu \left({\frac {q}{(q,n)}}\right){\frac {\phi (q)}{\phi \left({\frac {q}{(q,n)}}\right)}}.}$

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 c_q(n)

For all positive integers q,

${\displaystyle {\begin{aligned}c_{1}(q)&=1\\c_{q}(1)&=\mu (q)\\c_{q}(q)&=\phi (q)\\c_{q}(m)&=c_{q}(n)&&{\text{for }}m\equiv n{\pmod {q}}\\\end{aligned}}}$

For a fixed value of q the absolute value of the sequence ${\displaystyle \{c_{q}(1),c_{q}(2),\ldots \}}$ is bounded by φ(q), and for a fixed value of n the absolute value of the sequence ${\displaystyle \{c_{1}(n),c_{2}(n),\ldots \}}$ is bounded by n.

If q > 1

${\displaystyle \sum _{n=a}^{a+q-1}c_{q}(n)=0.}$

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

${\displaystyle {\frac {1}{m}}\sum _{k=1}^{m}c_{m_{1}}(k)c_{m_{2}}(k)={\begin{cases}\phi (m)&m_{1}=m_{2}=m,\\0&{\text{otherwise}}\end{cases}}}$

Let n, k > 0. Then ^[10]

${\displaystyle \sum _{\stackrel {d\mid n}{\gcd(d,k)=1}}d\;{\frac {\mu ({\tfrac {n}{d}})}{\phi (d)}}={\frac {\mu (n)c_{n}(k)}{\phi (n)}},}$

known as the Brauer - Rademacher identity.

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

${\displaystyle \sum _{\stackrel {1\leq k\leq n}{\gcd(k,n)=1}}c_{n}(k-a)=\mu (n)c_{n}(a),}$

due to Cohen.

Table

Ramanujan sum c_s(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	1	1	1	1	1	1	1	1	1	1	1	1	1	1	1	1	1	1	1	1	1	1	1	1	1	1	1	1	1
	2	−1	1	−1	1	−1	1	−1	1	−1	1	−1	1	−1	1	−1	1	−1	1	−1	1	−1	1	−1	1	−1	1	−1	1	−1	1
	3	−1	−1	2	−1	−1	2	−1	−1	2	−1	−1	2	−1	−1	2	−1	−1	2	−1	−1	2	−1	−1	2	−1	−1	2	−1	−1	2
	4	0	−2	0	2	0	−2	0	2	0	−2	0	2	0	−2	0	2	0	−2	0	2	0	−2	0	2	0	−2	0	2	0	−2
	5	−1	−1	−1	−1	4	−1	−1	−1	−1	4	−1	−1	−1	−1	4	−1	−1	−1	−1	4	−1	−1	−1	−1	4	−1	−1	−1	−1	4
	6	1	−1	−2	−1	1	2	1	−1	−2	−1	1	2	1	−1	−2	−1	1	2	1	−1	−2	−1	1	2	1	−1	−2	−1	1	2
	7	−1	−1	−1	−1	−1	−1	6	−1	−1	−1	−1	−1	−1	6	−1	−1	−1	−1	−1	−1	6	−1	−1	−1	−1	−1	−1	6	−1	−1
	8	0	0	0	−4	0	0	0	4	0	0	0	−4	0	0	0	4	0	0	0	−4	0	0	0	4	0	0	0	−4	0	0
	9	0	0	−3	0	0	−3	0	0	6	0	0	−3	0	0	−3	0	0	6	0	0	−3	0	0	−3	0	0	6	0	0	−3
	10	1	−1	1	−1	−4	−1	1	−1	1	4	1	−1	1	−1	−4	−1	1	−1	1	4	1	−1	1	−1	−4	−1	1	−1	1	4
	11	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	10	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	10	−1	−1	−1	−1	−1	−1	−1	−1
	12	0	2	0	−2	0	−4	0	−2	0	2	0	4	0	2	0	−2	0	−4	0	−2	0	2	0	4	0	2	0	−2	0	−4
	13	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	12	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	12	−1	−1	−1	−1
	14	1	−1	1	−1	1	−1	−6	−1	1	−1	1	−1	1	6	1	−1	1	−1	1	−1	−6	−1	1	−1	1	−1	1	6	1	−1
	15	1	1	−2	1	−4	−2	1	1	−2	−4	1	−2	1	1	8	1	1	−2	1	−4	−2	1	1	−2	−4	1	−2	1	1	8
	16	0	0	0	0	0	0	0	−8	0	0	0	0	0	0	0	8	0	0	0	0	0	0	0	−8	0	0	0	0	0	0
	17	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	16	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1
	18	0	0	3	0	0	−3	0	0	−6	0	0	−3	0	0	3	0	0	6	0	0	3	0	0	−3	0	0	−6	0	0	−3
	19	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	18	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1
	20	0	2	0	−2	0	2	0	−2	0	−8	0	−2	0	2	0	−2	0	2	0	8	0	2	0	−2	0	2	0	−2	0	−8
	21	1	1	−2	1	1	−2	−6	1	−2	1	1	−2	1	−6	−2	1	1	−2	1	1	12	1	1	−2	1	1	−2	−6	1	−2
	22	1	−1	1	−1	1	−1	1	−1	1	−1	−10	−1	1	−1	1	−1	1	−1	1	−1	1	10	1	−1	1	−1	1	−1	1	−1
	23	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	22	−1	−1	−1	−1	−1	−1	−1
	24	0	0	0	4	0	0	0	−4	0	0	0	−8	0	0	0	−4	0	0	0	4	0	0	0	8	0	0	0	4	0	0
	25	0	0	0	0	−5	0	0	0	0	−5	0	0	0	0	−5	0	0	0	0	−5	0	0	0	0	20	0	0	0	0	−5
	26	1	−1	1	−1	1	−1	1	−1	1	−1	1	−1	−12	−1	1	−1	1	−1	1	−1	1	−1	1	−1	1	12	1	−1	1	−1
	27	0	0	0	0	0	0	0	0	−9	0	0	0	0	0	0	0	0	−9	0	0	0	0	0	0	0	0	18	0	0	0
	28	0	2	0	−2	0	2	0	−2	0	2	0	−2	0	−12	0	−2	0	2	0	−2	0	2	0	−2	0	2	0	12	0	2
	29	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	−1	28	−1
	30	−1	1	2	1	4	−2	−1	1	2	−4	−1	−2	−1	1	−8	1	−1	−2	−1	−4	2	1	−1	−2	4	1	2	1	−1	8

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:

${\displaystyle f(n)=\sum _{q=1}^{\infty }a_{q}c_{q}(n)}$

or of the form:

${\displaystyle f(q)=\sum _{n=1}^{\infty }a_{n}c_{q}(n)}$

where the a_k ∈ C, 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

${\displaystyle \sum _{n=1}^{\infty }{\frac {\mu (n)}{n}}}$

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:

${\displaystyle \zeta (s)\sum _{\delta \,\mid \,q}\mu \left({\frac {q}{\delta }}\right)\delta ^{1-s}=\sum _{n=1}^{\infty }{\frac {c_{q}(n)}{n^{s}}}}$

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

${\displaystyle {\frac {\sigma _{r-1}(n)}{n^{r-1}\zeta (r)}}=\sum _{q=1}^{\infty }{\frac {c_{q}(n)}{q^{r}}}}$

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

There is also the double Dirichlet series

${\displaystyle {\frac {\zeta (s)\zeta (r+s-1)}{\zeta (r)}}=\sum _{q=1}^{\infty }\sum _{n=1}^{\infty }{\frac {c_{q}(n)}{q^{r}n^{s}}}.}$

σ_k(n)

σ_k(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,

${\displaystyle {\begin{aligned}\sigma _{s}(n)&=n^{s}\zeta (s+1)\left({\frac {c_{1}(n)}{1^{s+1}}}+{\frac {c_{2}(n)}{2^{s+1}}}+{\frac {c_{3}(n)}{3^{s+1}}}+\cdots \right)\\\sigma _{-s}(n)&=\zeta (s+1)\left({\frac {c_{1}(n)}{1^{s+1}}}+{\frac {c_{2}(n)}{2^{s+1}}}+{\frac {c_{3}(n)}{3^{s+1}}}+\cdots \right)\end{aligned}}}$

Setting s = 1 gives

${\displaystyle \sigma (n)={\frac {\pi ^{2}}{6}}n\left({\frac {c_{1}(n)}{1}}+{\frac {c_{2}(n)}{4}}+{\frac {c_{3}(n)}{9}}+\cdots \right).}$

If the Riemann hypothesis is true, and ${\displaystyle -{\tfrac {1}{2}}<s<{\tfrac {1}{2}},}$

${\displaystyle \sigma _{s}(n)=\zeta (1-s)\left({\frac {c_{1}(n)}{1^{1-s}}}+{\frac {c_{2}(n)}{2^{1-s}}}+{\frac {c_{3}(n)}{3^{1-s}}}+\cdots \right)=n^{s}\zeta (1+s)\left({\frac {c_{1}(n)}{1^{1+s}}}+{\frac {c_{2}(n)}{2^{1+s}}}+{\frac {c_{3}(n)}{3^{1+s}}}+\cdots \right).}$

d(n)

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

${\displaystyle {\begin{aligned}-d(n)&={\frac {\log 1}{1}}c_{1}(n)+{\frac {\log 2}{2}}c_{2}(n)+{\frac {\log 3}{3}}c_{3}(n)+\cdots \\-d(n)(2\gamma +\log n)&={\frac {\log ^{2}1}{1}}c_{1}(n)+{\frac {\log ^{2}2}{2}}c_{2}(n)+{\frac {\log ^{2}3}{3}}c_{3}(n)+\cdots \end{aligned}}}$

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

${\displaystyle n=p_{1}^{a_{1}}p_{2}^{a_{2}}p_{3}^{a_{3}}\cdots }$

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

${\displaystyle \varphi _{s}(n)=n^{s}(1-p_{1}^{-s})(1-p_{2}^{-s})(1-p_{3}^{-s})\cdots ,}$

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

He proves that

${\displaystyle {\frac {\mu (n)n^{s}}{\varphi _{s}(n)\zeta (s)}}=\sum _{\nu =1}^{\infty }{\frac {\mu (n\nu )}{\nu ^{s}}}}$

and uses this to show that

${\displaystyle {\frac {\varphi _{s}(n)\zeta (s+1)}{n^{s}}}={\frac {\mu (1)c_{1}(n)}{\varphi _{s+1}(1)}}+{\frac {\mu (2)c_{2}(n)}{\varphi _{s+1}(2)}}+{\frac {\mu (3)c_{3}(n)}{\varphi _{s+1}(3)}}+\cdots .}$

Letting s = 1,

${\displaystyle \varphi (n)={\frac {6}{\pi ^{2}}}n\left(c_{1}(n)-{\frac {c_{2}(n)}{2^{2}-1}}-{\frac {c_{3}(n)}{3^{2}-1}}-{\frac {c_{5}(n)}{5^{2}-1}}+{\frac {c_{6}(n)}{(2^{2}-1)(3^{2}-1)}}-{\frac {c_{7}(n)}{7^{2}-1}}+{\frac {c_{10}(n)}{(2^{2}-1)(5^{2}-1)}}-\cdots \right).}$

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 = p^k is a power of a prime number, in which case it is the natural logarithm log p.

${\displaystyle -\Lambda (m)=c_{m}(1)+{\frac {1}{2}}c_{m}(2)+{\frac {1}{3}}c_{m}(3)+\cdots }$

Zero

For all n > 0,

${\displaystyle 0=c_{1}(n)+{\frac {1}{2}}c_{2}(n)+{\frac {1}{3}}c_{3}(n)+\cdots .}$

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

r_2s(n) (sums of squares)

r_2s(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 δ_2s(n) and references a paper ^[21] in which he proved that r_2s(n) = δ_2s(n) for s = 1, 2, 3, and 4. For s > 4 he shows that δ_2s(n) is a good approximation to r_2s(n).

s = 1 has a special formula:

${\displaystyle \delta _{2}(n)=\pi \left({\frac {c_{1}(n)}{1}}-{\frac {c_{3}(n)}{3}}+{\frac {c_{5}(n)}{5}}-\cdots \right).}$

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

${\displaystyle {\begin{aligned}\delta _{2s}(n)&={\frac {\pi ^{s}n^{s-1}}{(s-1)!}}\left({\frac {c_{1}(n)}{1^{s}}}+{\frac {c_{4}(n)}{2^{s}}}+{\frac {c_{3}(n)}{3^{s}}}+{\frac {c_{8}(n)}{4^{s}}}+{\frac {c_{5}(n)}{5^{s}}}+{\frac {c_{12}(n)}{6^{s}}}+{\frac {c_{7}(n)}{7^{s}}}+{\frac {c_{16}(n)}{8^{s}}}+\cdots \right)&&s\equiv 0{\pmod {4}}\\[6pt]\delta _{2s}(n)&={\frac {\pi ^{s}n^{s-1}}{(s-1)!}}\left({\frac {c_{1}(n)}{1^{s}}}-{\frac {c_{4}(n)}{2^{s}}}+{\frac {c_{3}(n)}{3^{s}}}-{\frac {c_{8}(n)}{4^{s}}}+{\frac {c_{5}(n)}{5^{s}}}-{\frac {c_{12}(n)}{6^{s}}}+{\frac {c_{7}(n)}{7^{s}}}-{\frac {c_{16}(n)}{8^{s}}}+\cdots \right)&&s\equiv 2{\pmod {4}}\\[6pt]\delta _{2s}(n)&={\frac {\pi ^{s}n^{s-1}}{(s-1)!}}\left({\frac {c_{1}(n)}{1^{s}}}+{\frac {c_{4}(n)}{2^{s}}}-{\frac {c_{3}(n)}{3^{s}}}+{\frac {c_{8}(n)}{4^{s}}}+{\frac {c_{5}(n)}{5^{s}}}+{\frac {c_{12}(n)}{6^{s}}}-{\frac {c_{7}(n)}{7^{s}}}+{\frac {c_{16}(n)}{8^{s}}}+\cdots \right)&&s\equiv 1{\pmod {4}}{\text{ and }}s>1\\[6pt]\delta _{2s}(n)&={\frac {\pi ^{s}n^{s-1}}{(s-1)!}}\left({\frac {c_{1}(n)}{1^{s}}}-{\frac {c_{4}(n)}{2^{s}}}-{\frac {c_{3}(n)}{3^{s}}}-{\frac {c_{8}(n)}{4^{s}}}+{\frac {c_{5}(n)}{5^{s}}}-{\frac {c_{12}(n)}{6^{s}}}-{\frac {c_{7}(n)}{7^{s}}}-{\frac {c_{16}(n)}{8^{s}}}+\cdots \right)&&s\equiv 3{\pmod {4}}\\\end{aligned}}}$

and therefore,

${\displaystyle {\begin{aligned}r_{2}(n)&=\pi \left({\frac {c_{1}(n)}{1}}-{\frac {c_{3}(n)}{3}}+{\frac {c_{5}(n)}{5}}-{\frac {c_{7}(n)}{7}}+{\frac {c_{11}(n)}{11}}-{\frac {c_{13}(n)}{13}}+{\frac {c_{15}(n)}{15}}-{\frac {c_{17}(n)}{17}}+\cdots \right)\\[6pt]r_{4}(n)&=\pi ^{2}n\left({\frac {c_{1}(n)}{1}}-{\frac {c_{4}(n)}{4}}+{\frac {c_{3}(n)}{9}}-{\frac {c_{8}(n)}{16}}+{\frac {c_{5}(n)}{25}}-{\frac {c_{12}(n)}{36}}+{\frac {c_{7}(n)}{49}}-{\frac {c_{16}(n)}{64}}+\cdots \right)\\[6pt]r_{6}(n)&={\frac {\pi ^{3}n^{2}}{2}}\left({\frac {c_{1}(n)}{1}}-{\frac {c_{4}(n)}{8}}-{\frac {c_{3}(n)}{27}}-{\frac {c_{8}(n)}{64}}+{\frac {c_{5}(n)}{125}}-{\frac {c_{12}(n)}{216}}-{\frac {c_{7}(n)}{343}}-{\frac {c_{16}(n)}{512}}+\cdots \right)\\[6pt]r_{8}(n)&={\frac {\pi ^{4}n^{3}}{6}}\left({\frac {c_{1}(n)}{1}}+{\frac {c_{4}(n)}{16}}+{\frac {c_{3}(n)}{81}}+{\frac {c_{8}(n)}{256}}+{\frac {c_{5}(n)}{625}}+{\frac {c_{12}(n)}{1296}}+{\frac {c_{7}(n)}{2401}}+{\frac {c_{16}(n)}{4096}}+\cdots \right)\end{aligned}}}$

r′_2s(n) (sums of triangles)

${\displaystyle r'_{2s}(n)}$ 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 ${\displaystyle \delta '_{2s}(n)}$ such that ${\displaystyle r'_{2s}(n)=\delta '_{2s}(n)}$ for s = 1, 2, 3, and 4, and that for s > 4, ${\displaystyle \delta '_{2s}(n)}$ is a good approximation to ${\displaystyle r'_{2s}(n).}$

Again, s = 1 requires a special formula:

${\displaystyle \delta '_{2}(n)={\frac {\pi }{4}}\left({\frac {c_{1}(4n+1)}{1}}-{\frac {c_{3}(4n+1)}{3}}+{\frac {c_{5}(4n+1)}{5}}-{\frac {c_{7}(4n+1)}{7}}+\cdots \right).}$

If s is a multiple of 4,

${\displaystyle {\begin{aligned}\delta '_{2s}(n)&={\frac {({\frac {\pi }{2}})^{s}}{(s-1)!}}\left(n+{\frac {s}{4}}\right)^{s-1}\left({\frac {c_{1}(n+{\frac {s}{4}})}{1^{s}}}+{\frac {c_{3}(n+{\frac {s}{4}})}{3^{s}}}+{\frac {c_{5}(n+{\frac {s}{4}})}{5^{s}}}+\cdots \right)&&s\equiv 0{\pmod {4}}\\[6pt]\delta '_{2s}(n)&={\frac {({\frac {\pi }{2}})^{s}}{(s-1)!}}\left(n+{\frac {s}{4}}\right)^{s-1}\left({\frac {c_{1}(2n+{\frac {s}{2}})}{1^{s}}}+{\frac {c_{3}(2n+{\frac {s}{2}})}{3^{s}}}+{\frac {c_{5}(2n+{\frac {s}{2}})}{5^{s}}}+\cdots \right)&&s\equiv 2{\pmod {4}}\\[6pt]\delta '_{2s}(n)&={\frac {({\frac {\pi }{2}})^{s}}{(s-1)!}}\left(n+{\frac {s}{4}}\right)^{s-1}\left({\frac {c_{1}(4n+s)}{1^{s}}}-{\frac {c_{3}(4n+s)}{3^{s}}}+{\frac {c_{5}(4n+s)}{5^{s}}}-\cdots \right)&&s\equiv 1{\pmod {2}}{\text{ and }}s>1\end{aligned}}}$

Therefore,

${\displaystyle {\begin{aligned}r'_{2}(n)&={\frac {\pi }{4}}\left({\frac {c_{1}(4n+1)}{1}}-{\frac {c_{3}(4n+1)}{3}}+{\frac {c_{5}(4n+1)}{5}}-{\frac {c_{7}(4n+1)}{7}}+\cdots \right)\\[6pt]r'_{4}(n)&=\left({\frac {\pi }{2}}\right)^{2}\left(n+{\frac {1}{2}}\right)\left({\frac {c_{1}(2n+1)}{1}}+{\frac {c_{3}(2n+1)}{9}}+{\frac {c_{5}(2n+1)}{25}}+\cdots \right)\\[6pt]r'_{6}(n)&={\frac {({\frac {\pi }{2}})^{3}}{2}}\left(n+{\frac {3}{4}}\right)^{2}\left({\frac {c_{1}(4n+3)}{1}}-{\frac {c_{3}(4n+3)}{27}}+{\frac {c_{5}(4n+3)}{125}}-\cdots \right)\\[6pt]r'_{8}(n)&={\frac {({\frac {\pi }{2}})^{4}}{6}}(n+1)^{3}\left({\frac {c_{1}(n+1)}{1}}+{\frac {c_{3}(n+1)}{81}}+{\frac {c_{5}(n+1)}{625}}+\cdots \right)\end{aligned}}}$

Sums

Let

${\displaystyle {\begin{aligned}T_{q}(n)&=c_{q}(1)+c_{q}(2)+\cdots +c_{q}(n)\\U_{q}(n)&=T_{q}(n)+{\tfrac {1}{2}}\phi (q)\end{aligned}}}$

Then for s > 1,

${\displaystyle {\begin{aligned}\sigma _{-s}(1)+\cdots +\sigma _{-s}(n)&=\zeta (s+1)\left(n+{\frac {T_{2}(n)}{2^{s+1}}}+{\frac {T_{3}(n)}{3^{s+1}}}+{\frac {T_{4}(n)}{4^{s+1}}}+\cdots \right)\\&=\zeta (s+1)\left(n+{\tfrac {1}{2}}+{\frac {U_{2}(n)}{2^{s+1}}}+{\frac {U_{3}(n)}{3^{s+1}}}+{\frac {U_{4}(n)}{4^{s+1}}}+\cdots \right)-{\tfrac {1}{2}}\zeta (s)\\d(1)+\cdots +d(n)&=-{\frac {T_{2}(n)\log 2}{2}}-{\frac {T_{3}(n)\log 3}{3}}-{\frac {T_{4}(n)\log 4}{4}}-\cdots \\d(1)\log 1+\cdots +d(n)\log n&=-{\frac {T_{2}(n)(2\gamma \log 2-\log ^{2}2)}{2}}-{\frac {T_{3}(n)(2\gamma \log 3-\log ^{2}3)}{3}}-{\frac {T_{4}(n)(2\gamma \log 4-\log ^{2}4)}{4}}-\cdots \\r_{2}(1)+\cdots +r_{2}(n)&=\pi \left(n-{\frac {T_{3}(n)}{3}}+{\frac {T_{5}(n)}{5}}-{\frac {T_{7}(n)}{7}}+\cdots \right)\end{aligned}}}$

See also

• Gaussian period
• Kloosterman sum

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. 360–370 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. 369–371
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 c_q as an orthogonal basis.
16. Ramanujan, On Certain Arithmetical Functions
17. This is Jordan's totient function, J_s(n).
18. Cf. Hardy & Wright, Thm. 329, which states that ${\displaystyle \;{\frac {6}{\pi ^{2}}}<{\frac {\sigma (n)\phi (n)}{n^{2}}}<1.}$
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

• Hardy, G. H.; Wright, E. M. (1979) [1938]. An Introduction to the Theory of Numbers (5th ed.). Oxford: Clarendon Press. ISBN   0-19-853171-0. MR   0568909. Zbl   0423.10001.
• Knopfmacher, John (1990) [1975], Abstract Analytic Number Theory (2nd ed.), New York: Dover, ISBN   0-486-66344-2, Zbl   0743.11002

• Nathanson, Melvyn B. (1996), Additive Number Theory: the Classical Bases, Graduate Texts in Mathematics, vol. 164, Springer-Verlag, Section A.7, ISBN   0-387-94656-X, Zbl   0859.11002 .
• Nicol, C. A. (1962). "Some formulas involving Ramanujan sums". Can. J. Math. 14: 284–286. doi: 10.4153/CJM-1962-019-8 .

• 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. 179–199 of his Collected Papers)

• Ramanujan, Srinivasa (1916), "On Certain Arithmetical Functions", Transactions of the Cambridge Philosophical Society, 22 (9): 159–184 (pp. 136–163 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

External links

• Tóth, László (2011). "Sums of products of Ramanujan sums". Annali dell'universita' di Ferrara. 58: 183–197. arXiv: 1104.1906 . doi:10.1007/s11565-011-0143-3. S2CID   119134250.