Lambert Series

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|m} a_n
Since this last sum is a typical number-theoretic sum, almost any 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|n} d^\alpha
is the divisor function. Lambert series in which the an are trigonometric functions, for example, an=sin(2n x), can be evaluated by various combinations of the logarithmic derivatives of Jacobi theta functions.

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