\[\widehat{\zeta}=-\left(\frac{3}{2\kappa}\left(-\frac{1}{2}X+2\mu\operatorname{%
arctan}\left(\frac{x\kappa-x\sqrt{\kappa^{2}-\mu^{2}}-2\mu^{2}}{\mu X}\right)+%
\kappa\operatorname{arccos}\left(\frac{x-2\kappa}{2\sqrt{\kappa^{2}-\mu^{2}}}%
\right)\right)\right)^{2/3},\]