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Lambert SeriesIn mathematics, a Lambert series, named for Johann Heinrich Lambert, is a series taking the form -
It can be resummed formally by expanding the denominator: -
where the coefficients of the new series are given by the Dirichlet convolution of with the constant function : -
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 -
where is the number of positive divisors of the number . For the higher order sigma functions, one has -
where is any complex number and -
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. Related topics
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