Solution
= Solution
<Liouville's theorem> states that every <bounded> <entire function> is constant:
$$
\boxed{f\text{ holomorphic on }\mathbb C,\quad\sup_{z\in\mathbb C}|f(z)|<\infty\quad\Longrightarrow\quad f\text{ constant}.}
$$
For example, the Cauchy <derivative> estimate on a <circle> of radius $R$ gives $|f'(z)|\leq M/R$. Letting $R\to\infty$ gives $f'=0$ everywhere, proving the conclusion.