# Is known the function $\sum_{n=1}^\infty\frac{(-1)^n\mu(n)}{n^s}$, where $s$ is the complex variable and $\mu(n)$ the Möbius function?

Let $\mu(n)$ the Möbius function, see its definition for example from this MathWorld, and we denote with $s$ the complex variable.

I'm curious to know if some case of the series $$\sum_{n=1}^\infty\frac{(-1)^n\mu(n)}{n^s}$$ for $\Re s\geq \frac{1}{2}$, were in the literature.

I did simple experiments with Wolfram Alpha and from those my belief is that one can calculate a closed-form for the case $s=2+0\cdot i=2$, and write an identity in terms of the constant $\frac{1}{\zeta(3)}$ for the case $s=3$.

Question. Was in the literature the formal series (or complex function defined on a domain of the complex plane) $$\sum_{n=1}^\infty\frac{(-1)^n\mu(n)}{n^s}\,?\tag{1}$$ Then refer the literature and I try to find and read those known facts about the complex function $(1)$. Many thanks.

• I doubt it's in the literature, it's simple, but useless. – Professor Vector Dec 27 '17 at 10:34
• Do you know how calculate $\sum_{n=1}^{\infty}\frac{(-1)^n\mu(n)}{n^2}$? Or properties of our function $(1)$ as the convergence abscissa @ProfessorVector ? – user243301 Dec 27 '17 at 10:35
• It's trivial to derive from the known result without the $(-1)^n$. Give us a few own thoughts for a change, please! – Professor Vector Dec 27 '17 at 10:39
• Many thanks Professor Vector. – user243301 Dec 27 '17 at 11:38
• $(-1)^{n+1}$ is multiplicative not $(-1)^n$. Dirichlet series with multiplicative coefficients $\implies$ Euler product, this is the 1st page of every book on analytic number theory. Can you write this Euler product ? – reuns Dec 27 '17 at 23:27

Assume for a moment that $\operatorname{Re}(s) > 1$. Then using the fact that $\mu$ is multiplicative, we have

$$\sum_{n\text{ even}} \frac{\mu(n)}{n^s} = \sum_{k=1}^{\infty} \frac{\mu(2k)}{(2k)^s} = - \frac{1}{2^s} \sum_{k\text{ odd}} \frac{\mu(k)}{k^s}.$$

So if we write $D(s) = \sum_{k\text{ odd}} \frac{\mu(k)}{k^s}$, then

$$\frac{1}{\zeta(s)} = \sum_{n=1}^{\infty} \frac{\mu(n)}{n^s} = (1 - 2^{-s})D(s)$$

and hence

\begin{align*} \sum_{n=1}^{\infty} \frac{(-1)^n \mu(n)}{n^s} &= \sum_{n\text{ even}} \frac{\mu(n)}{n^s} - \sum_{n\text{ odd}} \frac{\mu(n)}{n^s} \\ &= -(1+2^{-s})D(s) = - \frac{2^s+1}{2^s-1} \cdot \frac{1}{\zeta(s)}. \end{align*}

• Many thanks I would like to study more your answer, but it seems that agree with my examples. – user243301 Dec 27 '17 at 10:56