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

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.