# Lambert summation

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In mathematical analysis, Lambert summation is a summability method for a class of divergent series.

## Definition

A series $\sum a_n$ is Lambert summable to A, written $\sum a_n = A \,(\mathrm{L})$, if

$\lim_{r \rightarrow 1-} (1-r) \sum_{n=1}^\infty \frac{n a_n r^n}{1-r^n} = A . \,$

If a series is convergent to A then it is Lambert summable to A (an Abelian theorem).

## Examples

• $\sum_{n=1}^\infty \frac{\mu(n)}{n} = 0 (\mathrm{L})$, where μ is the Möbius function. Hence if this series converges at all, it converges to zero.