chore: import upstream snapshot with attribution

docs/functions/gamma.rst

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+Factorials and gamma functions
+------------------------------
+
+Factorials and factorial-like sums and products are basic tools of
+combinatorics and number theory.  Much like the exponential function is
+fundamental to differential equations and analysis in general, the factorial
+function (and its extension to complex numbers, the gamma function) is
+fundamental to difference equations and functional equations.
+
+A large selection of factorial-like functions is implemented in mpmath.  All
+functions support complex arguments, and arguments may be arbitrarily large.
+Results are numerical approximations, so to compute *exact* values a high
+enough precision must be set manually::
+
+    >>> from mpmath import mp, fac
+    >>> mp.dps = 15
+    >>> mp.pretty = True
+    >>> fac(100)
+    9.33262154439442e+157
+    >>> print(int(_))    # most digits are wrong
+    93326215443944150965646704795953882578400970373184098831012889540582227238570431295066113089288327277825849664006524270554535976289719382852181865895959724032
+    >>> mp.dps = 160
+    >>> fac(100)
+    93326215443944152681699238856266700490715968264381621468592963895217599993229915608941463976156518286253697920827223758251185210916864000000000000000000000000.0
+
+The gamma and polygamma functions are closely related to :doc:`zeta`.  See also
+:doc:`qfunctions` for q-analogs of factorial-like functions.
+
+
+Factorials
+..........
+
+.. autofunction:: mpmath.factorial
+.. autofunction:: mpmath.fac2
+
+
+Binomial coefficients
+.....................
+
+.. autofunction:: mpmath.binomial
+
+
+Gamma function
+..............
+
+.. autofunction:: mpmath.gamma
+.. autofunction:: mpmath.rgamma
+.. autofunction:: mpmath.gammaprod
+.. autofunction:: mpmath.loggamma
+
+
+Rising and falling factorials
+.............................
+
+.. autofunction:: mpmath.rf
+.. autofunction:: mpmath.ff
+
+
+Beta function
+.............
+
+.. autofunction:: mpmath.beta
+.. autofunction:: mpmath.betainc
+
+
+Super- and hyperfactorials
+..........................
+
+.. autofunction:: mpmath.superfac
+.. autofunction:: mpmath.hyperfac
+.. autofunction:: mpmath.barnesg
+
+
+Polygamma functions and harmonic numbers
+........................................
+
+.. autofunction:: mpmath.psi
+.. autofunction:: mpmath.digamma
+.. autofunction:: mpmath.harmonic