“…Therefore, the distance from u 0 to the Hopf quadric equals √ z * ≈ 2.4898. To find the nearest point on the quadric, we first evaluate the multiple zero for the polynomial 1 (μ, z * ) (defined by (26)) via formula (21) to get μ * ≈ −0.1508. Then, we apply formula (24): Check u 0 − u * = √ z * , L 1 (u * ) = 0 and u 0 − u * is normal to the Hopf quadric at u = u * , i.e., it is parallel to the gradient to this quadric at u * :…”