Function OPTpsix transforms vector u as follows
\[
OPTpsix(u,c) = \left\{
\begin{array}{cc}
\frac{1}{3.25*c^2} u^2 & |u| \leq 2c \\
(1/3.25) * (-1.944 * \frac{u^2}{c^2} + 1.728 \frac{u^4}{c^4} - 0.312 \frac{x^6}{c^6} + 0.016 \frac{x^8}{c^8} & \qquad 2c \leq |u| \leq 3c \\
0 & |u|>3c \\
\end{array}
\right.
\]
Remark: function $\psi(x) *x$ has been precalculated because it is used to find
the weights in case of Tau estimators