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Lambert series : ウィキペディア英語版
Lambert series

In mathematics, a Lambert series, named for Johann Heinrich Lambert, is a series taking the form
:S(q)=\sum_^\infty a_n \frac .
It can be resummed formally by expanding the denominator:
:S(q)=\sum_^\infty a_n \sum_^\infty q^ = \sum_^\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_ 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_^\infty q^n \sigma_0(n) = \sum_^\infty \frac
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_^\infty q^n \sigma_\alpha(n) = \sum_^\infty \frac
where \alpha is any complex number and
:\sigma_\alpha(n) = (\textrm_\alpha
*1)(n) = \sum_ d^\alpha \,
is the divisor function.
Lambert series in which the ''a''''n'' are trigonometric functions, for example, ''a''''n'' = sin(2''n'' ''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_^\infty \mu(n)\,\frac = q.
For Euler's totient function \phi(n):
:\sum_^\infty \varphi(n)\,\frac = \frac.
For Liouville's function \lambda(n):
:\sum_^\infty \lambda(n)\,\frac =
\sum_^\infty q^
with the sum on the left similar to the Ramanujan theta function.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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