. [dlmf] Digital Library of Mathematical Functions, 8.19.2
              https://dlmf.nist.gov/8.19#E2

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

    Its domain is nonnegative n and x.

    >>> sc.expn(-1, 1.0), sc.expn(1, -1.0)
    (nan, nan)

    It has a pole at ``x = 0`` for ``n = 1, 2``; for larger ``n`` it
    is equal to ``1 / (n - 1)``.

    >>> sc.expn([0, 1, 2, 3, 4], 0)
    array([       inf,        inf, 1.        , 0.5       , 0.33333333])

    For n equal to 0 it reduces to ``exp(-x) / x``.

    >>> x = np.array([1, 2, 3, 4])
    >>> sc.expn(0, x)
    array([0.36787944, 0.06766764, 0.01659569, 0.00457891])
    >>> np.exp(-x) / x
    array([0.36787944, 0.06766764, 0.01659569, 0.00457891])

    For n equal to 1 it reduces to `exp1`.

    >>> sc.expn(1, x)
    array([0.21938393, 0.04890051, 0.01304838, 0.00377935])
    >>> sc.exp1(x)
    array([0.21938393, 0.04890051, 0.01304838, 0.00377935])

    Ú