function OPTpsi transforms vector u as follows $$\psi(x) =\rho' (x) = \begin{cases} \frac{2.7692 x}{c^2} \qquad |x| \leq \frac{2}{3} c \\ -\frac{5.3834 x}{c^2} +\frac{43.0672 x^3}{c^4} -\frac{69.9840 x^5}{c^6} +\frac{32.3 x^7}{c^8} \qquad \frac{2}{3} c < |x| \leq c \\ 0 & \; \vert x \vert > c. \end{cases}$$

Remark: Optimal psi-function is almost linear around u = 0 in accordance with Winsor's principle that all distributions are normal in the middle.

This means that $\psi(u)/u$ is approximately constant over the linear region of $\psi$, so the points in that region tend to get equal weight.