Function OPTpsix transforms vector u as follows $$\psi(x)x =x\rho' (x) = \begin{cases} \frac{2.7692 x^2}{c^2} \qquad |x| \leq \frac{2}{3} c \\ -\frac{5.3834 x^2}{c^2} +\frac{43.0672 x^4}{c^4} -\frac{69.9840 x^6}{c^6} +\frac{32.3 x^8}{c^8} \qquad \frac{2}{3} c < |x| \leq c \\ 0 & \; \vert x \vert > c. \end{cases}$$ Remark: function $\psi(x) *x$ has been precalculated because it is used to find the weights in case of Tau estimators