Lambert series

Function

S(q)=\sum _{n=1}^{\infty }{\frac {q^{n}}{1-q^{n}}}

, represented as a Matplotlib plot, using a version of the Domain coloring method^[1]

In mathematics, a Lambert series, named for Johann Heinrich Lambert, is a series taking the form

S(q)=\sum_{n=1}^\infty a_n \frac {q^n}{1-q^n}.

It can be resummed formally by expanding the denominator:

S(q)=\sum_{n=1}^\infty a_n \sum_{k=1}^\infty q^{nk} = \sum_{m=1}^\infty b_m q^m

where the coefficients of the new series are given by the Dirichlet convolution of a_n with the constant function 1(n) = 1:

b_m = (a*1)(m) = \sum_{n\mid m} a_n. \,

This series may be inverted by means of the Möbius inversion formula, and is an example of a Möbius transform.

Examples

Since this last sum is a typical number-theoretic sum, almost any natural multiplicative function will be exactly summable when used in a Lambert series. Thus, for example, one has

\sum_{n=1}^\infty q^n \sigma_0(n) = \sum_{n=1}^\infty \frac{q^n}{1-q^n}

where $\sigma_0(n)=d(n)$ is the number of positive divisors of the number n.

For the higher order sigma functions, one has

\sum_{n=1}^\infty q^n \sigma_\alpha(n) = \sum_{n=1}^\infty \frac{n^\alpha q^n}{1-q^n}

where $\alpha$ is any complex number and

\sigma_\alpha(n) = (\textrm{Id}_\alpha*1)(n) = \sum_{d\mid n} d^\alpha \,

is the divisor function.

Lambert series in which the a_n are trigonometric functions, for example, a_n = sin(2n x), can be evaluated by various combinations of the logarithmic derivatives of Jacobi theta functions.

Other Lambert series include those for the Möbius function $\mu (n)$ :

\sum_{n=1}^\infty \mu(n)\,\frac{q^n}{1-q^n} = q.

For Euler's totient function $\varphi (n)$ :

\sum_{n=1}^\infty \varphi(n)\,\frac{q^n}{1-q^n} = \frac{q}{(1-q)^2}.

For Liouville's function $\lambda (n)$ :

\sum_{n=1}^\infty \lambda(n)\,\frac{q^n}{1-q^n} = \sum_{n=1}^\infty q^{n^2}

with the sum on the right similar to the Ramanujan theta function, or Jacobi theta function $\vartheta _{3}(q)$ .

We also have a slightly more generalized Lambert series expansion generating the sum of squares function $r_{2}(n)$ in the form of ^[2]

\sum _{n=1}^{\infty }{\frac {4\cdot (-1)^{n+1}q^{2n+1}}{1-q^{2n+1}}}=\sum _{m=1}^{\infty }r_{2}(m)q^{m}.

In general, if we write the Lambert series over $f(n)$ which generates the arithmetic functions $g(m)=(f\ast 1)(m)$ , the next pairs of functions correspond to other well-known convolutions expressed by their Lambert series generating functions in the forms of

(f,g)=(\mu ,\varepsilon ),(\varphi ,\operatorname {Id} _{1}),(\lambda ,\chi _{\operatorname {sq} }),(\Lambda ,\log ),(|\mu |,2^{\omega }),(J_{t},\operatorname {Id} _{t}),(d^{3},(d\ast 1)^{2}),

where $\varepsilon (n)=\delta _{n,1}$ is the multiplicative identity for Dirichlet convolutions, $\operatorname {Id} _{k}(n)=n^{k}$ is the identity function for $k^{{th}}$ powers, $\chi _{\operatorname {sq} }$ denotes the characteristic function for the squares, $\omega (n)$ which counts the number of distinct prime factors of $n$ (see prime omega function), $J_{t}$ is Jordan's totient function, and $d(n)=\sigma _{0}(n)$ is the divisor function (see Dirichlet convolutions).

Alternate form

Substituting $q=e^{-z}$ one obtains another common form for the series, as

\sum_{n=1}^\infty \frac {a_n}{e^{zn}-1}= \sum_{m=1}^\infty b_m e^{-mz}

where

b_{m}=(a*1)(m)=\sum _{{d\mid m}}a_{d}\,

as before. Examples of Lambert series in this form, with $z=2\pi$ , occur in expressions for the Riemann zeta function for odd integer values; see Zeta constants for details.

Current usage

In the literature we find Lambert series applied to a wide variety of sums. For example, since $q^n/(1 - q^n ) = \mathrm{Li}_0(q^{n})$ is a polylogarithm function, we may refer to any sum of the form

\sum_{n=1}^{\infty} \frac{\xi^n \,\mathrm{Li}_u (\alpha q^n)}{n^s} = \sum_{n=1}^{\infty} \frac{\alpha^n \,\mathrm{Li}_s(\xi q^n)}{n^u}

as a Lambert series, assuming that the parameters are suitably restricted. Thus

12\left(\sum_{n=1}^{\infty} n^2 \, \mathrm{Li}_{-1}(q^n)\right)^{\!2} = \sum_{n=1}^{\infty} n^2 \,\mathrm{Li}_{-5}(q^n) - \sum_{n=1}^{\infty} n^4 \, \mathrm{Li}_{-3}(q^n),

which holds for all complex q not on the unit circle, would be considered a Lambert series identity. This identity follows in a straightforward fashion from some identities published by the Indian mathematician S. Ramanujan. A very thorough exploration of Ramanujan's works can be found in the works by Bruce Berndt.

Factorization theorems

A somewhat newer construction recently published over 2017–2018 relates to so-termed Lambert series factorization theorems of the form ^[3]

\sum _{n\geq 1}{\frac {a_{n}q^{n}}{1\pm q^{n}}}={\frac {1}{(\mp q;q)_{\infty }}}\sum _{n\geq 1}\left((s_{o}(n,k)\pm s_{e}(n,k))a_{k}\right)q^{n},

where $s_{o}(n,k)\pm s_{e}(n,k)=[q^{n}](\mp q;q)_{\infty }{\frac {q^{k}}{1\pm q^{k}}}$ is the respective sum or difference of the restricted partition functions $s_{e/o}(n,k)$ which denote the number of $k$ 's in all partitions of $n$ into an even (respectively, odd) number of distinct parts. Let $s_{n,k}:=s_{e}(n,k)-s_{o}(n,k)=[q^{n}](q;q)_{\infty }{\frac {q^{k}}{1-q^{k}}}$ denote the invertible lower triangular sequence whose first few values are shown in the table below.

n \ k	1	2	3	4	5	6	7	8
1	1	0	0	0	0	0	0	0
2	0	1	0	0	0	0	0	0
3	-1	-1	1	0	0	0	0	0
4	-1	0	-1	1	0	0	0	0
5	-1	-1	-1	-1	1	0	0	0
6	0	0	1	-1	-1	1	0	0
7	0	0	-1	0	-1	-1	1	0
8	1	0	0	1	0	-1	-1	1

Another characteristic form of the Lambert series factorization theorem expansions is given by ^[4]

L_{f}(q):=\sum _{n\geq 1}{\frac {f(n)q^{n}}{1-q^{n}}}={\frac {1}{(q;q)_{\infty }}}\sum _{n\geq 1}\left(s_{n,k}f(k)\right)q^{n},

where $(q;q)_{\infty }$ is the (infinite) q-Pochhammer symbol. The invertible matrix products on the right-hand-side of the previous equation correspond to inverse matrix products whose lower triangular entries are given in terms of the partition function and the Möbius function by the divisor sums

s_{n,k}^{(-1)}=\sum _{d|n}p(d-k)\mu \left({\frac {n}{d}}\right)

The next table lists the first several rows of these corresponding inverse matrices.^[5]

n \ k	1	2	3	4	5	6	7	8
1	1	0	0	0	0	0	0	0
2	0	1	0	0	0	0	0	0
3	1	1	1	0	0	0	0	0
4	2	1	1	1	0	0	0	0
5	4	3	2	1	1	0	0	0
6	5	3	2	2	1	1	0	0
7	10	7	5	3	2	1	1	0
8	12	9	6	4	3	2	1	1

We let $G_{j}:={\frac {1}{2}}\left\lceil {\frac {j}{2}}\right\rceil \left\lceil {\frac {3j+1}{2}}\right\rceil$ denote the sequence of interleaved pentagonal numbers, i.e., so that the pentagonal number theorem is expanded in the form of

(q;q)_{\infty }=\sum _{n\geq 0}(-1)^{\left\lceil {\frac {n}{2}}\right\rceil }q^{G_{n}}.

Then for any Lambert series $L_{f}(q)$ generating the sequence of $g(n)=(f\ast 1)(n)$ , we have the corresponding inversion relation of the factorization theorem expanded above given by ^[6]

f(n)=\sum _{k=1}^{n}\sum _{d|n}p(d-k)\mu (n/d)\times \sum _{j:k-G_{j}>0}(-1)^{\left\lceil {\frac {j}{2}}\right\rceil }b(k-G_{j}).

This work on Lambert series factorization theorems is extended in ^[7] to more general expansions of the form

\sum _{n\geq 1}{\frac {a_{n}q^{n}}{1-q^{n}}}={\frac {1}{C(q)}}\sum _{n\geq 1}\left(\sum _{k=1}^{n}s_{n,k}(\gamma ){\widetilde {a}}_{k}(\gamma )\right)q^{n},

where $C(q)$ is any (partition-related) reciprocal generating function, $\gamma (n)$ is any arithmetic function, and where the modified coefficients are expanded by

{\widetilde {a}}_{k}(\gamma )=\sum _{d|k}\sum _{r|{\frac {k}{d}}}a_{d}\gamma (r).

The corresponding inverse matrices in the above expansion satisfy

s_{n,k}^{(-1)}(\gamma )=\sum _{d|n}[q^{d-k}]{\frac {1}{C(q)}}\gamma \left({\frac {n}{d}}\right),

so that as in the first variant of the Lambert factorization theorem above we obtain an inversion relation for the right-hand-side coefficients of the form

{\widetilde {a}}_{k}(\gamma )=\sum _{k=1}^{n}s_{n,k}^{(-1)}(\gamma )\times [q^{k}]\left(\sum _{d=1}^{k}{\frac {a_{d}q^{d}}{1-q^{d}}}C(q)\right).

Recurrence relations

Within this section we define the following functions for natural numbers $n,x\geq 1$ :

g_{f}(n):=(f\ast 1)(n),

\Sigma _{f}(x):=\sum _{1\leq n\leq x}g_{f}(n).

We also adopt the notation from the previous section that

s_{n,k}=[q^{n}](q;q)_{\infty }{\frac {q^{k}}{1-q^{k}}},

where $(q;q)_{\infty }$ is the infinite q-Pochhammer symbol. Then we have the following recurrence relations for involving these functions and the pentagonal numbers proved in:^[6]

g_{f}(n+1)=\sum _{b=\pm 1}\sum _{k=1}^{\left\lfloor {\frac {{\sqrt {24n+1}}-b}{6}}\right\rfloor }(-1)^{k+1}g_{f}\left(n+1-{\frac {k(3k+b)}{2}}\right)+\sum _{k=1}^{n+1}s_{n+1,k}f(k),

\Sigma _{f}(x+1)=\sum _{b=\pm 1}\sum _{k=1}^{\left\lfloor {\frac {{\sqrt {24n+1}}-b}{6}}\right\rfloor }(-1)^{k+1}\Sigma _{f}\left(n+1-{\frac {k(3k+b)}{2}}\right)+\sum _{n=0}^{x}\sum _{k=1}^{n+1}s_{n+1,k}f(k).

Derivatives

Derivatives of a Lambert series can be obtained by differentiation of the series termwise with respect to $q$ . We have the following identities for the termwise $s^{th}$ derivatives of a Lambert series for any $s\geq 1$ ^[8]^[9]

q^{s}\cdot D^{(s)}\left[{\frac {q^{i}}{1-q^{i}}}\right]=\sum _{m=0}^{s}\sum _{k=0}^{m}\left[{\begin{matrix}s\\m\end{matrix}}\right]\left\{{\begin{matrix}m\\k\end{matrix}}\right\}{\frac {(-1)^{s-k}k!i^{m}}{(1-q^{i})^{k+1}}}

q^{s}\cdot D^{(s)}\left[{\frac {q^{i}}{1-q^{i}}}\right]=\sum _{r=0}^{s}\left[\sum _{m=0}^{s}\sum _{k=0}^{m}\left[{\begin{matrix}s\\m\end{matrix}}\right]\left\{{\begin{matrix}m\\k\end{matrix}}\right\}{\binom {s-k}{r}}{\frac {(-1)^{s-k-r}k!i^{m}}{(1-q^{i})^{k+1}}}\right]q^{(r+1)i},

where the bracketed triangular coefficients in the previous equations denote the Stirling numbers of the first and second kinds. We also have the next identity for extracting the individual coefficients of the terms implicit to the previous expansions given in the form of

[q^{n}]\left(\sum _{i\geq t}{\frac {a_{i}q^{mi}}{(1-q^{i})^{k+1}}}\right)=\sum _{\begin{matrix}d|n\\t\leq d\leq \left\lfloor {\frac {n}{m}}\right\rfloor \end{matrix}}{\binom {{\frac {n}{d}}-m+k}{k}}a_{d}.

Now if we define the functions $A_{t}(n)$ for any $n,t\geq 1$ by

A_{t}(n):=\sum _{\begin{matrix}0\leq k\leq m\leq t\\0\leq r\leq t\end{matrix}}\sum _{d|n}\left[{\begin{matrix}t\\m\end{matrix}}\right]\left\{{\begin{matrix}m\\k\end{matrix}}\right\}{\binom {t-k}{r}}{\binom {{\frac {n}{d}}-1-r+k}{k}}(-1)^{t-k-r}k!d^{m}\cdot a_{d}\cdot \left[t\leq d\leq \left\lfloor {\frac {n}{r+1}}\right\rfloor \right]_{\delta },

where $[\cdot ]_{\delta }$ denotes Iverson's convention, then we have the coefficients for the $t^{th}$ derivatives of a Lambert series given by

{\begin{aligned}A_{t}(n)&=[q^{n}]\left(q^{t}\cdot D^{(t)}\left[\sum _{i\geq t}{\frac {a_{i}q^{i}}{1-q^{i}}}\right]\right)\\&=[q^{n}]\left(\sum _{n\geq 1}{\frac {(A_{t}\ast \mu )(n)q^{n}}{1-q^{n}}}\right).\end{aligned}}

Of course, by a typical argument purely by operations on formal power series we also have that

[q^{n}]\left(q^{t}\cdot D^{(t)}\left[\sum _{i\geq 1}{\frac {f(i)q^{i}}{1-q^{i}}}\right]\right)={\frac {n!}{(n-t)!}}\cdot (f\ast 1)(n).

References

↑ http://nbviewer.ipython.org/github/empet/Math/blob/master/DomainColoring.ipynb
↑ Weisstein, Eric W. "Lambert Series". MathWorld. Retrieved 22 April 2018.
↑ Merca, Mircea (13 January 2017). "The Lambert series factorization theorem". The Ramanujan Journal. 44 (2): 417–435.
↑ Merca, M. and Schmidt, M. D. (2018). "Generating Special Arithmetic Functions by Lambert Series Factorizations". Contributions to Discrete Mathematics. to appear. arXiv:1706.00393. Bibcode:2017arXiv170600393M.
↑ "A133732". Online Encylopedia of Integer Sequences. Retrieved 22 April 2018.
1 2 Schmidt, Maxie D. (8 December 2017). "New Recurrence Relations and Matrix Equations for Arithmetic Functions Generated by Lambert Series". Acta Arithmetica. 181: 355–367. arXiv:1701.06257. Bibcode:2017arXiv170106257S.
↑ M. Merca and Schmidt, M. D. "New Factor Pairs for Factorizations of Lambert Series Generating Functions". arXiv:1706.02359.
↑ Schmidt, Maxie D. (2017). "Combinatorial Sums and Identities Involving Generalized Divisor Functions with Bounded Divisors". arXiv:1704.05595.
↑ Schmidt, Maxie D. (2017). "Factorization Theorems for Hadamard Products and Higher-Order Derivatives of Lambert Series Generating Functions". arXiv:1712.00608.

Berry, Michael V. (2010). Functions of Number Theory. CAMBRIDGE UNIVERSITY PRESS. pp. 637–641. ISBN 978-0-521-19225-5.
Lambert, Preston A. (1904). "Expansions of algebraic functions at singular points". Proc. Am. Philos. Soc. 43 (176): 164–172. JSTOR 983503.
Apostol, Tom M. (1976), Introduction to analytic number theory, Undergraduate Texts in Mathematics, New York-Heidelberg: Springer-Verlag, ISBN 978-0-387-90163-3, MR 0434929, Zbl 0335.10001
Hazewinkel, Michiel, ed. (2001) [1994], "Lambert series", Encyclopedia of Mathematics, Springer Science+Business Media B.V. / Kluwer Academic Publishers, ISBN 978-1-55608-010-4
Weisstein, Eric W. "Lambert Series". MathWorld.

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[1] ttp://nbviewer.ipython.org/github/empet/Math/blob/master/DomainColoring.ipynb

[2] Weisstein, Eric W. "Lambert Series". MathWorld. Retrieved 22 April 2018.

[3] Merca, Mircea (13 January 2017). "The Lambert series factorization theorem". The Ramanujan Journal. 44 (2): 417–435.

[4] Merca, M. and Schmidt, M. D. (2018). "Generating Special Arithmetic Functions by Lambert Series Factorizations". Contributions to Discrete Mathematics. to appear. arXiv:1706.00393. Bibcode:2017arXiv170600393M.

[5] "A133732". Online Encylopedia of Integer Sequences. Retrieved 22 April 2018.

[SCHMIDT_ACTA-6] 1 2 Schmidt, Maxie D. (8 December 2017). "New Recurrence Relations and Matrix Equations for Arithmetic Functions Generated by Lambert Series". Acta Arithmetica. 181: 355–367. arXiv:1701.06257. Bibcode:2017arXiv170106257S.

[7] M. Merca and Schmidt, M. D. "New Factor Pairs for Factorizations of Lambert Series Generating Functions". arXiv:1706.02359.

[8] Schmidt, Maxie D. (2017). "Combinatorial Sums and Identities Involving Generalized Divisor Functions with Bounded Divisors". arXiv:1704.05595.

[9] Schmidt, Maxie D. (2017). "Factorization Theorems for Hadamard Products and Higher-Order Derivatives of Lambert Series Generating Functions". arXiv:1712.00608.

n \ k	1	2	3	4	5	6	7	8
1	1	0	0	0	0	0	0	0
2	0	1	0	0	0	0	0	0
3	-1	-1	1	0	0	0	0	0
4	-1	0	-1	1	0	0	0	0
5	-1	-1	-1	-1	1	0	0	0
6	0	0	1	-1	-1	1	0	0
7	0	0	-1	0	-1	-1	1	0
8	1	0	0	1	0	-1	-1	1

n \ k	1	2	3	4	5	6	7	8
1	1	0	0	0	0	0	0	0
2	0	1	0	0	0	0	0	0
3	1	1	1	0	0	0	0	0
4	2	1	1	1	0	0	0	0
5	4	3	2	1	1	0	0	0
6	5	3	2	2	1	1	0	0
7	10	7	5	3	2	1	1	0
8	12	9	6	4	3	2	1	1

n \ k	1	2	3	4	5	6	7	8
1	1	0	0	0	0	0	0	0
2	0	1	0	0	0	0	0	0
3	-1	-1	1	0	0	0	0	0
4	-1	0	-1	1	0	0	0	0
5	-1	-1	-1	-1	1	0	0	0
6	0	0	1	-1	-1	1	0	0
7	0	0	-1	0	-1	-1	1	0
8	1	0	0	1	0	-1	-1	1

n \ k	1	2	3	4	5	6	7	8
1	1	0	0	0	0	0	0	0
2	0	1	0	0	0	0	0	0
3	1	1	1	0	0	0	0	0
4	2	1	1	1	0	0	0	0
5	4	3	2	1	1	0	0	0
6	5	3	2	2	1	1	0	0
7	10	7	5	3	2	1	1	0
8	12	9	6	4	3	2	1	1