Solution
= Solution
The cubic terms are the expansion of $(z+\overline z)^3=(2x)^3$. Subtracting $\overline z=x-iy$ therefore gives
$$
\boxed{\operatorname{Re}=8x^3-x,\qquad \operatorname{Im}=y.}
$$
Here the expression is a <polynomial> in $z$ and its <complex conjugate>, rather than a holomorphic polynomial in $z$ alone.