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$$
f_{Q}({\mathcal  {K}})\equiv y_{1}{\biggl [}{\frac  {2}{a_{1}^{2}}}{\biggr ]}^{{1/3}}={\biggl \{}1+{\biggl [}1+{\mathcal  {K}}{\biggr ]}^{{1/2}}{\biggr \}}^{{1/3}}+{\biggl \{}1-{\biggl [}1+{\mathcal  {K}}{\biggr ]}^{{1/2}}{\biggr \}}^{{1/3}},
$$
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