मान लें कि $$z = {{1 - i\sqrt 3 } \over 2}$$, $$i = \sqrt { - 1} $$. तब $$21 + {\left( {z + {1 \over z}} \right)^3} + {\left( {{z^2} + {1 \over {{z^2}}}} \right)^3} + {\left( {{z^3} + {1 \over {{z^3}}}} \right)^3} + .... + {\left( {{z^{21}} + {1 \over {{z^{21}}}}} \right)^3}$$ का मान ______________ है।