# proof of the fundamental theorem of algebra (Liouville’s theorem)

Let $f\colon\mathbb{C}\rightarrow\mathbb{C}$ be a polynomial, and suppose $f$ has no root in $\mathbb{C}$. We will show $f$ is constant.

Let $g=\frac{1}{f}$. Since $f$ is never zero, $g$ is defined and holomorphic on $\mathbb{C}$ (ie. it is entire). Moreover, since $f$ is a polynomial, $|f(z)|\rightarrow\infty$ as $|z|\rightarrow\infty$, and so $|g(z)|\rightarrow 0$ as $|z|\rightarrow\infty$. Then there is some $M>0$ such that $|g(z)|<1$ whenever $|z|>M$, and $g$ is continuous and so bounded on the compact set $\{z\in\mathbb{C}:|z|\leq M\}$.

So $g$ is bounded and entire, and therefore by Liouville’s theorem $g$ is constant. So $f$ is constant as required.$\square$

Title proof of the fundamental theorem of algebra (Liouville’s theorem) ProofOfTheFundamentalTheoremOfAlgebraLiouvillesTheorem 2013-03-22 12:18:59 2013-03-22 12:18:59 Evandar (27) Evandar (27) 6 Evandar (27) Proof msc 12D99 msc 30A99