def pdf(self, x, e=0., w=1., a=0.):
        """
        probability density function
        see: https://en.wikipedia.org/wiki/Skew_normal_distribution
        :param x: input value
        :param e:
        :param w:
        :param a:
        :return:
        """
        t = (x-e) / w
        return 2. / w * stats.norm.pdf(t) * stats.norm.cdf(a*t)