Lambert series
= Lambert series
{c}
A Lambert series has the form $\sum_{n\ge1}a_nq^n/(1-q^n)$. Expanding each denominator as a <geometric series> shows that its coefficient of $q^m$ is $\sum_{d\mid m}a_d$.
= Lambert series
{c}
A Lambert series has the form $\sum_{n\ge1}a_nq^n/(1-q^n)$. Expanding each denominator as a <geometric series> shows that its coefficient of $q^m$ is $\sum_{d\mid m}a_d$.