Eqn (13) from Greenhall2004

def edf_greenhall_simple(alpha, d, m, S, F, N):
    """ Eqn (13) from Greenhall2004 """
    L = m/F+m*d # length of filter applied to phase samples
    M = 1 + np.floor(S*(N-L) / m)
    J = min(M, (d+1)*S)
    inv_edf = (1.0/(pow(greenhall_sz(0, F, alpha, d), 2)*M))* \
               greenhall_BasicSum(J, M, S, F, alpha, d)
    return 1.0/inv_edf