def doseigs(s):
    """
    convert s format for eigenvalues and eigenvectors

    Parameters
    __________
    s=[x11,x22,x33,x12,x23,x13] : the six tensor elements

    Return
    __________
        tau : [t1,t2,t3]
           tau is an list of eigenvalues in decreasing order:
        V : [[V1_dec,V1_inc],[V2_dec,V2_inc],[V3_dec,V3_inc]]
            is an list of the eigenvector directions
    """
#
    A = s2a(s)  # convert s to a (see Tauxe 1998)
    tau, V = tauV(A)  # convert to eigenvalues (t), eigenvectors (V)
    Vdirs = []
    for v in V:  # convert from cartesian to direction
        Vdir = cart2dir(v)
        if Vdir[1] < 0:
            Vdir[1] = -Vdir[1]
            Vdir[0] = (Vdir[0] + 180.) % 360.
        Vdirs.append([Vdir[0], Vdir[1]])
    return tau, Vdirs