z8a−4−2z6a−2 + 6z6a−4−z6a−6−10z4a−2 + 13z4a−4−4z4a−6 + z4−15z2a−2 + 13z2a−4−4z2a−6 + 4z2−7a−2 + 5a−4−a−6 + 4