# How are values of the Dirichlet Beta function derivative derived?

Wolfram Mathworld gives the following values for the beta function derivative.

$$\beta'(-1) = \frac{2K}{\pi},\quad \beta'(0) = \ln \left[\frac{\Gamma^{2}(\frac{1}{4})}{2\pi\sqrt{2}} \right],\quad \beta'(1) = \frac{\pi}{4}\left(\gamma +2\ln 2+3\ln\pi-4\ln \left(\Gamma \left(\frac{1}{4}\right) \right)\right)$$

I can see how $\beta'(1)$ could be derived from $\beta'(0)$ using the functional equation (or vice versa) but how was one of them found originally. The same for $\beta'(-1)$ and $\beta'(2)$. Is there any other known values of the derivative? Also can I have links to any papers on the subject, I cannot seem to find any?

• The Wikipedia page en.wikipedia.org/wiki/Dirichlet_beta_function reads "Also it was derived by Malmsten in 1842 that" $\beta'(1)=\cdots$. That might be a place to start. Jul 14, 2018 at 16:09
• Can't find it... Jul 19, 2018 at 21:55

## 2 Answers

Another way to find $$\beta'(1)$$ is from Kummer's Fourier series for the logarithm of the gamma function: $$\log \Gamma(x) = \left(\tfrac{1}{2}-x \right)(\gamma+\log2)+(1-x)\log \pi - \frac{1}{2} \log \sin(\pi x) + \frac{1}{\pi} \sum_{n=1}^{\infty} \frac{\log n}{n} \sin(2\pi nx)$$ valid for $$0. Setting $$x=\frac{1}{4}$$ gives $$\log \Gamma \left(\tfrac{1}{4} \right) = \frac{1}{4} (\gamma+\log 2) +\frac{3}{4}\log \pi -\frac{1}{2} \log \left(\tfrac{1}{\sqrt{2}} \right) +\frac{1}{\pi} \sum_{n=0}^{\infty} \frac{(-1)^n \log(2n+1)}{2n+1}$$ The sum appearing on the right is $$-\beta'(1)$$.

$$\beta'(-1)$$ can actually be found directly from the functional equation. We have $$\beta'(-1) = - \frac{\mathrm{d}}{\mathrm{d}s} \left[\left(\frac{2}{\pi}\right)^s \sin \left(\frac{\pi}{2} s\right) \Gamma(s) \beta(s)\right] ~ \Bigg\rvert_{s=2} \, .$$ Now note that the only non-vanishing term on the right-hand side is the one containing the derivative of the sine. Therefore $$\beta'(-1) = \left(\frac{2}{\pi}\right)^2 \frac{\pi}{2} [-\cos(\pi)] \Gamma(2) \beta(2) = \frac{2 \mathrm{K}}{\pi} \, .$$ However, this also means that $$\beta'(2)$$ cannot be calculated from $$\beta'(-1)$$.

$$\beta'(1)$$ is not as easy to find. We can start from the integral \begin{align} \frac{1}{2} \int \limits_0^\infty \frac{-\ln(x)}{\cosh(x)} \, \mathrm{d} x &= - \frac{1}{2} \frac{\mathrm{d}}{\mathrm{d}s} \int \limits_0^\infty \frac{x^{s-1}}{\cosh(x)} \, \mathrm{d} x ~ \Bigg\rvert_{s=1} = - \frac{1}{2} \frac{\mathrm{d}}{\mathrm{d}s} [2 \Gamma(s) \beta(s)] ~ \Bigg\rvert_{s=1} \\ &= \frac{\pi \gamma}{4} - \beta'(1) \, . \end{align} Since the left-hand side is a representation of Vardi's integral $$\int \limits_0^1 \frac{-\ln(-\ln(x))}{1+x^2} \, \mathrm{d} x = \frac{\pi}{4} \ln\left(\frac{\operatorname{\Gamma}^4\left(\frac{1}{4}\right)}{4\pi^3}\right) \, ,$$ we obtain the desired result, from which the value of $$\beta'(0)$$ follows by the functional equation.

Various ways to evaluate the integral are discussed in this paper by Blagouchine. By the way, the author convincingly argues that the integral should actually be called Malmsten's integral, since Malmsten first computed it in 1842, which is in line with the claim from the Wikipedia page.

As an alternative route, here is a brief sketch of how I have calculated $$\beta'(1)$$ :

Plugging $$z=\frac{1}{4}$$ into Binet's first log-gamma formula, we obtain (after some algebra and integration by parts/substitutions) \begin{align} \ln\left(\frac{\operatorname{\Gamma}^4\left(\frac{1}{4}\right)}{4\pi^2}\right) &= 2 \ln(2) - 1 + \int \limits_0^\infty \frac{\tanh(x)}{x} \mathrm{e}^{-x} \, \mathrm{d} x \\ &\phantom{=} + \int \limits_0^\infty \left[\frac{\coth(x)}{x} -\frac{1}{\sinh^2 (x)}\right] \mathrm{e}^{-x} \, \mathrm{d} x - \int \limits_0^\infty \left[\coth(x)-\frac{1}{x}\right] \mathrm{e}^{-x} \, \mathrm{d} x \\ &\equiv 2\ln(2) - 1 + I_1 + I_2 - I_3 \, . \end{align}

We can then compute \begin{align} I_1 &= \gamma + \ln \left(\frac{\pi}{2}\right) - \frac{4}{\pi} \beta'(1) \, ,\\ I_2 &= \gamma \, ,\\ I_3 &= \gamma + \ln(2) - 1 \, \end{align} and solve for $$\beta'(1)$$ .

In order to derive the functional equation for $$\beta$$ we consider $$s \in \mathbb{C}$$ with $$0 < \operatorname{Re}(s) < 1$$ first. Using the geometric series we obtain $$\operatorname{\beta} (1 - s) = \frac{1}{2 \operatorname{\Gamma} (1-s)} \int \limits_0^\infty \frac{t^{-s}}{\cosh(t)} \, \mathrm{d} t = \frac{1}{4 \operatorname{\Gamma} (1-s)} \int \limits_{-\infty}^\infty \lvert t \rvert^{-s}\operatorname{sech}(t) \, \mathrm{d} t \, .$$ We have $$\mathcal{F} (\lvert \cdot \rvert^{-s}) (k) = 2 \operatorname{\Gamma}(1-s)\sin \left(\frac{\pi}{2} s\right) \lvert k \rvert^{s-1}$$ from this question and $$\mathcal{F} (\operatorname{sech}) (k) = \pi \operatorname{sech} \left(\frac{\pi}{2} k\right)$$ from here, so we can use Parseval's theorem (we are cheating a bit here, since the first function is not in $$L^2 (\mathbb{R})$$, but this is easily fixed by considering $$\lvert t \rvert^{-s} \mathrm{e}^{-\varepsilon \lvert t \rvert}$$ instead and taking the limit $$\varepsilon \to 0^+$$) to conclude that \begin{align} \beta(1 - s) &= \frac{1}{8 \pi \operatorname{\Gamma} (1-s)} \int \limits_{-\infty}^\infty 2 \operatorname{\Gamma}(1-s)\sin \left(\frac{\pi}{2} s\right) \lvert k \rvert^{s-1} \pi \operatorname{sech}\left(\frac{\pi}{2} k\right) \, \mathrm{d} k \\ &= \left(\frac{2}{\pi}\right)^s \sin \left(\frac{\pi}{2} s\right) \frac{1}{2} \int \limits_0^\infty \frac{u^{s-1}}{\cosh(u)} \, \mathrm{d} u = \left(\frac{2}{\pi}\right)^s \sin \left(\frac{\pi}{2} s\right) \operatorname{\Gamma} (s) \operatorname{\beta} (s) \end{align} holds for $$0 < \operatorname{Re} (s) < 1$$. Since the right-hand side is holomorphic on the right half-plane we can use this equation to extend $$\beta$$ to an entire function. Then the functional equation holds on all of $$\mathbb{C}$$ by the identity theorem.

• Are any other values known? Aug 23, 2018 at 9:02
• @JoshuaFarrell Other values seem to have expressions in terms of derivatives of other special functions only. Mathematica gives $$\beta'(2) = \frac{\partial_1 \zeta\left(2,\frac{1}{4}\right)-\partial_1 \zeta\left(2,\frac{3}{4}\right)}{16} - 2 \mathrm{K} \ln(2)$$ for example, which can be derived from the series definition of $\beta$ . Apparently there is no nicer form. Aug 23, 2018 at 10:03
• How to prove the functional equation, where does it come frm and how is beta function related to dirichlet l beta function?? Mar 25, 2021 at 11:52
• @VukStojiljkovic I have added a derivation of the functional equation. The Dirichlet beta function is a special case of the Dirichlet L-functions, who satisfy similar functional equations. Mar 27, 2021 at 14:55