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.