quare probability density function with `v` degrees of
    freedom:

    .. math::

        \frac{1}{2^{v/2} \Gamma(v/2)} \int_x^\infty t^{v/2 - 1} e^{-t/2} dt

    Here :math:`\Gamma` is the Gamma function; see `gamma`. This
    integral can be expressed in terms of the regularized upper
    incomplete gamma function `gammaincc` as
    ``gammaincc(v / 2, x / 2)``. [1]_

    Parameters
    ----------
    v : array_like
        Degrees of freedom.
    x : array_like
        Lower bound of the integral.
    out : ndarray, optional
        Optional output array for the function results.

    Returns
    -------
    scalar or ndarray
        Values of the survival function.

    See Also
    --------
    chdtr, chdtri, chdtriv, gammaincc

    References
    ----------
    .. [1] Chi-Square distribution,
        https://www.itl.nist.gov/div898/handbook/eda/section3/eda3666.htm

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

    It can be expressed in terms of the regularized upper incomplete
    gamma function.

    >>> v = 1
    >>> x = np.arange(4)
    >>> sc.chdtrc(v, x)
    array([1.        , 0.31731051, 0.15729921, 0.08326452])
    >>> sc.gammaincc(v / 2, x / 2)
    array([1.        , 0.31731051, 0.15729921, 0.08326452])

    