PolyGamma[z] gives the digamma function .
PolyGamma[n, z] gives the derivative of the digamma function .
PolyGamma[z] is the logarithmic derivative of the gamma function, given by .
PolyGamma[n, z] is given by .
The digamma function is ; is the logarithmic derivative of the gamma function.
PolyGamma[z] and PolyGamma[n, z] are meromorphic functions of z with no branch cut discontinuities.
FullSimplify and FunctionExpand include transformation rules for PolyGamma.
See The Mathematica Book: Section 3.2.10.
Implementation Notes: see section A.9.4.
See also: Gamma, LogGamma, EulerGamma.
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