Combination Theorem for Limits of Functions/Complex/Product Rule

Theorem
Let $\C$ denote the complex numbers.

Let $f$ and $g$ be complex functions defined on an open subset $S \subseteq \C$, except possibly at the point $c \in S$.

Let $f$ and $g$ tend to the following limits:


 * $\ds \lim_{z \mathop \to c} \map f z = l$
 * $\ds \lim_{z \mathop \to c} \map g z = m$

Then:
 * $\ds \lim_{z \mathop \to c} \ \paren {\map f z \map g z} = l m$

Proof
Let $\sequence {z_n}$ be a sequence of elements of $S$ such that:
 * $\forall n \in \N: z_n \ne c$
 * $\ds \lim_{n \mathop \to \infty} z_n = c$

By Limit of Complex Function by Convergent Sequences:
 * $\ds \lim_{n \mathop \to \infty} \map f {z_n} = l$
 * $\ds \lim_{n \mathop \to \infty} \map g {z_n} = m$

By the Product Rule for Complex Sequences:
 * $\ds \lim_{n \mathop \to \infty} \paren {\map f {z_n} \map g {z_n} } = l m$

Applying Limit of Complex Function by Convergent Sequences again, we get:
 * $\ds \lim_{z \mathop \to c} \paren {\map f z \map g z} = l m$