function OPTpsi transforms vector u as follows
\[
OPTpsi(u,c) = \left\{
\begin{array}{cc}
\frac{u}{3.25*c^2} & |u| \leq 2c \\
= (1/3.25) \left( -1.944 \frac{u}{c^2} + 1.728 \frac{u^3}{c^4} - 0.312 \frac{u^5}{c^6} + 0.016 \frac{u^7}{c^8} \right) & \qquad 2c \leq |u| \leq 3c \\
0 & |u|>3c \\
\end{array}
\right.
\]

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.