def calc_a1(s_star):
        """Вычисляется а1"""
        j = 0
        t = (4*j + 1)**2 / (16 * s_star)

        inf_sum = (scsp.gamma(j + 0.5) * np.sqrt(4*j + 1) *\
            np.exp(-t)) * i_func(s_star) /\
            (scsp.gamma(0.5) * scsp.gamma(j + 1))

        return inf_sum / np.sqrt(2 * s_star)

def i_func(s_star):
        v = 1/4
        z = 1/(16*s_star)
        sum1 = scsp.iv(-v,z)
        sum2 = scsp.iv(v,z)
        return sum1 - sum2