Logistic function.

    See, e.g `Wikipedia <https://en.wikipedia.org/wiki/Logistic_function>`_.

    :param x: point in x-axis
    :type x: float or numpy.ndarray
    :param float x0: sigmoid's midpoint
    :param float k: steepness of the curve
    :param float L: maximum value of the curve
    :returns: function's value(s) in point **x**
    :rtype: float or numpy.ndarray

def logistic(x, x0, k, L):
    '''Logistic function.

    See, e.g `Wikipedia <https://en.wikipedia.org/wiki/Logistic_function>`_.

    :param x: point in x-axis
    :type x: float or numpy.ndarray
    :param float x0: sigmoid's midpoint
    :param float k: steepness of the curve
    :param float L: maximum value of the curve
    :returns: function's value(s) in point **x**
    :rtype: float or numpy.ndarray
    '''
    return L / (1 + exp(-k * (x - x0)))