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Specific values of the digamma function

Table of contents: Zeros - Values at integers - Values at simple fractions - Values at general fractions - Values of polygamma functions at integers and simple fractions - Specific complex parts - Limits at singularities - Infinite sums over zeros

Related topic: Digamma function

Zeros

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Table of xnx_{n} to 50 digits for 0≤n≤100 \le n \le 10
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ψ ⁣(xn)=0\psi\!\left(x_{n}\right) = 0

Values at integers

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ψ ⁣(1)=−γ\psi\!\left(1\right) = -\gamma
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ψ ⁣(2)=1−γ\psi\!\left(2\right) = 1 - \gamma
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ψ ⁣(3)=32−γ\psi\!\left(3\right) = \frac{3}{2} - \gamma
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ψ ⁣(n)=Hn−1−γ\psi\!\left(n\right) = H_{n - 1} - \gamma

Values at simple fractions

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ψ ⁣(12)=−2log⁡(2)−γ\psi\!\left(\frac{1}{2}\right) = -2 \log(2) - \gamma
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ψ ⁣(13)=−3π6−γ−3log⁡(3)2\psi\!\left(\frac{1}{3}\right) = -\frac{\sqrt{3} \pi}{6} - \gamma - \frac{3 \log(3)}{2}
45a969
ψ ⁣(23)=3π6−γ−3log⁡(3)2\psi\!\left(\frac{2}{3}\right) = \frac{\sqrt{3} \pi}{6} - \gamma - \frac{3 \log(3)}{2}
7ec4f0
ψ ⁣(14)=−π2−γ−3log⁡(2)\psi\!\left(\frac{1}{4}\right) = -\frac{\pi}{2} - \gamma - 3 \log(2)
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ψ ⁣(34)=π2−γ−3log⁡(2)\psi\!\left(\frac{3}{4}\right) = \frac{\pi}{2} - \gamma - 3 \log(2)
177de7
ψ ⁣(16)=−3π2−γ−2log⁡(2)−3log⁡(3)2\psi\!\left(\frac{1}{6}\right) = -\frac{\sqrt{3} \pi}{2} - \gamma - 2 \log(2) - \frac{3 \log(3)}{2}
967bbb
ψ ⁣(56)=3π2−γ−2log⁡(2)−3log⁡(3)2\psi\!\left(\frac{5}{6}\right) = \frac{\sqrt{3} \pi}{2} - \gamma - 2 \log(2) - \frac{3 \log(3)}{2}
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ψ ⁣(18)=−π2(2+1)−γ−4log⁡(2)−log⁡ ⁣(2+2)−log⁡ ⁣(2−2)2\psi\!\left(\frac{1}{8}\right) = -\frac{\pi}{2} \left(\sqrt{2} + 1\right) - \gamma - 4 \log(2) - \frac{\log\!\left(2 + \sqrt{2}\right) - \log\!\left(2 - \sqrt{2}\right)}{\sqrt{2}}

Values at general fractions

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ψ ⁣(pq)=−γ−log⁡ ⁣(2q)−π2cot⁡ ⁣(πpq)+2∑k=1⌊(q−1)/2⌋cos⁡ ⁣(2πkpq)log⁡ ⁣(sin⁡ ⁣(πkq))\psi\!\left(\frac{p}{q}\right) = -\gamma - \log\!\left(2 q\right) - \frac{\pi}{2} \cot\!\left(\frac{\pi p}{q}\right) + 2 \sum_{k=1}^{\left\lfloor \left( q - 1 \right) / 2 \right\rfloor} \cos\!\left(\frac{2 \pi k p}{q}\right) \log\!\left(\sin\!\left(\frac{\pi k}{q}\right)\right)

Values of polygamma functions at integers and simple fractions

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ψ′ ⁣(1)=π26\psi'\!\left(1\right) = \frac{{\pi}^{2}}{6}
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ψ′′ ⁣(1)=−2ζ ⁣(3)\psi''\!\left(1\right) = -2 \zeta\!\left(3\right)
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ψ(m) ⁣(1)=(−1)m+1m!ζ ⁣(m+1)\psi^{(m)}\!\left(1\right) = {\left(-1\right)}^{m + 1} m ! \zeta\!\left(m + 1\right)
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ψ′ ⁣(2)=π26−1\psi'\!\left(2\right) = \frac{{\pi}^{2}}{6} - 1
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ψ(m) ⁣(n)=(−1)m+1m!ζ ⁣(m+1,n)\psi^{(m)}\!\left(n\right) = {\left(-1\right)}^{m + 1} m ! \zeta\!\left(m + 1, n\right)
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ψ′ ⁣(12)=π22\psi'\!\left(\frac{1}{2}\right) = \frac{{\pi}^{2}}{2}
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ψ′′ ⁣(12)=−14ζ ⁣(3)\psi''\!\left(\frac{1}{2}\right) = -14 \zeta\!\left(3\right)
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ψ′′′ ⁣(12)=π4\psi'''\!\left(\frac{1}{2}\right) = {\pi}^{4}
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ψ(m) ⁣(12)=(−1)m+1(2m+1−1)m!ζ ⁣(m+1)\psi^{(m)}\!\left(\frac{1}{2}\right) = {\left(-1\right)}^{m + 1} \left({2}^{m + 1} - 1\right) m ! \zeta\!\left(m + 1\right)
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ψ′ ⁣(14)=π2+8G\psi'\!\left(\frac{1}{4}\right) = {\pi}^{2} + 8 G
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ψ′ ⁣(34)=π2−8G\psi'\!\left(\frac{3}{4}\right) = {\pi}^{2} - 8 G
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ψ′′ ⁣(14)=−2π3−56ζ ⁣(3)\psi''\!\left(\frac{1}{4}\right) = -2 {\pi}^{3} - 56 \zeta\!\left(3\right)
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ψ′′ ⁣(34)=2π3−56ζ ⁣(3)\psi''\!\left(\frac{3}{4}\right) = 2 {\pi}^{3} - 56 \zeta\!\left(3\right)
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ψ′′ ⁣(16)=−182ζ ⁣(3)−43π3\psi''\!\left(\frac{1}{6}\right) = -182 \zeta\!\left(3\right) - 4 \sqrt{3} {\pi}^{3}
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ψ′′ ⁣(56)=−182ζ ⁣(3)+43π3\psi''\!\left(\frac{5}{6}\right) = -182 \zeta\!\left(3\right) + 4 \sqrt{3} {\pi}^{3}

Specific complex parts

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Im⁡ ⁣(ψ ⁣(iy))=π2coth⁡ ⁣(πy)+12y\operatorname{Im}\!\left(\psi\!\left(i y\right)\right) = \frac{\pi}{2} \coth\!\left(\pi y\right) + \frac{1}{2 y}
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Im⁡ ⁣(ψ ⁣(1+iy))=π2coth⁡ ⁣(πy)−12y\operatorname{Im}\!\left(\psi\!\left(1 + i y\right)\right) = \frac{\pi}{2} \coth\!\left(\pi y\right) - \frac{1}{2 y}
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Im⁡ ⁣(ψ ⁣(12+iy))=π2tanh⁡ ⁣(πy)\operatorname{Im}\!\left(\psi\!\left(\frac{1}{2} + i y\right)\right) = \frac{\pi}{2} \tanh\!\left(\pi y\right)

Limits at singularities

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ψ ⁣(−n)=∞~\psi\!\left(-n\right) = {\tilde \infty}
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ψ(m) ⁣(−n)=∞~\psi^{(m)}\!\left(-n\right) = {\tilde \infty}
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ψ(m) ⁣(∞)=lim⁡x→∞ψ ⁣(x)={∞,m=00,m>0\psi^{(m)}\!\left(\infty\right) = \lim_{x \to \infty} \psi\!\left(x\right) = \begin{cases} \infty, & m = 0\\0, & m > 0\\ \end{cases}
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lim⁡x→0+ψ ⁣(x)=(−1)m+1∞\lim_{x \to {0}^{+}} \psi\!\left(x\right) = {\left(-1\right)}^{m + 1} \infty

Infinite sums over zeros

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∑n=0∞1xn2=γ2+π22\sum_{n=0}^{\infty} \frac{1}{x_{n}^{2}} = {\gamma}^{2} + \frac{{\pi}^{2}}{2}
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∑n=0∞1xn3=−γ3−γπ22−4ζ ⁣(3)\sum_{n=0}^{\infty} \frac{1}{x_{n}^{3}} = -{\gamma}^{3} - \frac{\gamma {\pi}^{2}}{2} - 4 \zeta\!\left(3\right)
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∑n=0∞1xn4=γ4+π49+2γ2π23+4γζ ⁣(3)\sum_{n=0}^{\infty} \frac{1}{x_{n}^{4}} = {\gamma}^{4} + \frac{{\pi}^{4}}{9} + \frac{2 {\gamma}^{2} {\pi}^{2}}{3} + 4 \gamma \zeta\!\left(3\right)
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∑n=0∞1xnr+1=f(r)(0)r!   where f(z)=lim⁡t→z(ψ ⁣(t)−ψ′ ⁣(t)ψ ⁣(t))\sum_{n=0}^{\infty} \frac{1}{x_{n}^{r + 1}} = \frac{{f}^{(r)}(0)}{r !}\; \text{ where } f(z) = \lim_{t \to z} \left(\psi\!\left(t\right) - \frac{\psi'\!\left(t\right)}{\psi\!\left(t\right)}\right)

Copyright (C) Fredrik Johansson and contributors. Fungrim is provided under the MIT license. The source code is on GitHub.

2021-03-15 19:12:00.328586 UTC