Psi (Digamma) Function - GNU Scientific Library -- Reference Manual
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### 7.28 Psi (Digamma) Function

The polygamma functions of order n are defined by

         \psi^{(n)}(x) = (d/dx)^n \psi(x) = (d/dx)^{n+1} \log(\Gamma(x))

where \psi(x) = \Gamma'(x)/\Gamma(x) is known as the digamma function. These functions are declared in the header file `gsl_sf_psi.h`.

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