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# scipy.special.gamma

scipy.special.gamma(z, out=None) = <ufunc 'gamma'>

  gamma function.

  The gamma function is defined as

  \[\Gamma(z) = \int_0^\infty t^{z-1} e^{-t} dt\]

  for \(\Re(z) > 0\) and is extended to the rest of the complex plane by analytic continuation. See [dlmf] for more details.

  Parameters:
    zarray_like

      Real or complex valued argument

    outndarray, optional

      Optional output array for the function values

  Returns:
    scalar or ndarray

      Values of the gamma function

  Notes

  The gamma function is often referred to as the generalized factorial since \(\Gamma(n + 1) = n!\) for natural numbers \(n\). More generally it satisfies the recurrence relation \(\Gamma(z + 1) = z \cdot \Gamma(z)\) for complex \(z\), which, combined with the fact that \(\Gamma(1) = 1\), implies the above identity for \(z = n\).

  The gamma function has poles at non-negative integers and the sign of infinity as z approaches each pole depends upon the direction in which the pole is approached. For this reason, the consistent thing is for gamma(z) to return NaN at negative integers, and to return -inf when x = -0.0 and +inf when x = 0.0, using the signbit of zero to signify the direction in which the origin is being approached. This is for instance what is recommended for the gamma function in annex F entry 9.5.4 of the Iso C 99 standard [isoc99].

  Prior to SciPy version 1.15, `scipy.special.gamma(z)` returned `+inf` at each pole. This was fixed in version 1.15, but with the following consequence. Expressions where gamma appears in the denominator such as

  `gamma(u) * gamma(v) / (gamma(w) * gamma(x))`

  no longer evaluate to 0 if the numerator is well defined but there is a pole in the denominator. Instead such expressions evaluate to NaN. We recommend instead using the function `rgamma` for the reciprocal gamma function in such cases. The above expression could for instance be written as

  `gamma(u) * gamma(v) * (rgamma(w) * rgamma(x))`

  References

  [dlmf]

  NIST Digital Library of Mathematical Functions https://dlmf.nist.gov/5.2#E1

  [isoc99]

  https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf

  Examples

      >>> import numpy as np
      >>> from scipy.special import gamma, factorial

      >>> gamma([0, 0.5, 1, 5])
      array([         inf,   1.77245385,   1.        ,  24.        ])

      >>> z = 2.5 + 1j
      >>> gamma(z)
      (0.77476210455108352+0.70763120437959293j)
      >>> gamma(z+1), z*gamma(z)  # Recurrence property
      ((1.2292740569981171+2.5438401155000685j),
       (1.2292740569981158+2.5438401155000658j))

      >>> gamma(0.5)**2  # gamma(0.5) = sqrt(pi)
      3.1415926535897927

  Plot gamma(x) for real x

      >>> x = np.linspace(-3.5, 5.5, 2251)
      >>> y = gamma(x)

      >>> import matplotlib.pyplot as plt
      >>> plt.plot(x, y, 'b', alpha=0.6, label='gamma(x)')
      >>> k = np.arange(1, 7)
      >>> plt.plot(k, factorial(k-1), 'k*', alpha=0.6,
      ...          label='(x-1)!, x = 1, 2, ...')
      >>> plt.xlim(-3.5, 5.5)
      >>> plt.ylim(-10, 25)
      >>> plt.grid()
      >>> plt.xlabel('x')
      >>> plt.legend(loc='lower right')
      >>> plt.show()