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# scipy.special.expi

scipy.special.expi(x, out=None) = <ufunc 'expi'>

  Exponential integral Ei.

  For real \(x\), the exponential integral is defined as [1]

  \[Ei(x) = \int_{-\infty}^x \frac{e^t}{t} dt.\]

  For \(x > 0\) the integral is understood as a Cauchy principal value.

  It is extended to the complex plane by analytic continuation of the function on the interval \((0, \infty)\). The complex variant has a branch cut on the negative real axis.

  Parameters:
    xarray_like

      Real or complex valued argument

    outndarray, optional

      Optional output array for the function results

  Returns:
    scalar or ndarray

      Values of the exponential integral

  See also

  `exp1`

    Exponential integral \(E_1\)

  `expn`

    Generalized exponential integral \(E_n\)

  Notes

  The exponential integrals \(E_1\) and \(Ei\) satisfy the relation

  \[E_1(x) = -Ei(-x)\]

  for \(x > 0\).

  References

  [1]

  Digital Library of Mathematical Functions, 6.2.5 https://dlmf.nist.gov/6.2#E5

  Examples

      >>> import numpy as np
      >>> import scipy.special as sc

  It is related to `exp1`.

      >>> x = np.array([1, 2, 3, 4])
      >>> -sc.expi(-x)
      array([0.21938393, 0.04890051, 0.01304838, 0.00377935])
      >>> sc.exp1(x)
      array([0.21938393, 0.04890051, 0.01304838, 0.00377935])

  The complex variant has a branch cut on the negative real axis.

      >>> sc.expi(-1 + 1e-12j)
      (-0.21938393439552062+3.1415926535894254j)
      >>> sc.expi(-1 - 1e-12j)
      (-0.21938393439552062-3.1415926535894254j)

  As the complex variant approaches the branch cut, the real parts approach the value of the real variant.

      >>> sc.expi(-1)
      -0.21938393439552062

  The SciPy implementation returns the real variant for complex values on the branch cut.

      >>> sc.expi(complex(-1, 0.0))
      (-0.21938393439552062-0j)
      >>> sc.expi(complex(-1, -0.0))
      (-0.21938393439552062-0j)