4. Let $z=1-2 i$. Find $\frac{1}{z}+\frac{2}{z^{2}}+\frac{3}{z^{3}}+\cdots$.
Solution:
$$
(2 i-1) / 4
$$

Let $x=\frac{1}{z}+\frac{2}{z^{2}}+\frac{3}{z^{3}}+\cdots$, so $z \cdot x=\left(1+\frac{2}{z}+\frac{3}{z^{2}}+\frac{4}{z^{3}}+\cdots\right)$. Then $z \cdot x-x=$ $1+\frac{1}{z}+\frac{1}{z^{2}}+\frac{1}{z^{3}}+\cdots=\frac{1}{1-1 / z}=\frac{z}{z-1}$. Solving for $x$ in terms of $z$, we obtain $x=\frac{z}{(z-1)^{2}}$. Plugging in the original value of $z$ produces $x=(2 i-1) / 4$.