Combination Theorem for Continuous Functions/Complex/Combined Sum Rule
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Theorem
Let $\C$ denote the complex numbers.
Let $f$ and $g$ be complex functions which are continuous on an open subset $S \subseteq \C$.
Let $\lambda, \mu \in \C$ be arbitrary complex numbers.
Then:
- $\lambda f + \mu g$ is continuous on $S$.
Proof
By definition of continuous, we have that
- $\forall c \in S: \ds \lim_{z \mathop \to c} \map f z = \map f c$
- $\forall c \in S: \ds \lim_{z \mathop \to c} \map g z = \map g c$
Let $f$ and $g$ tend to the following limits:
- $\ds \lim_{z \mathop \to c} \map f x = l$
- $\ds \lim_{z \mathop \to c} \map g x = m$
From the Combined Sum Rule for Limits of Complex Functions, we have that:
- $\ds \lim_{z \mathop \to c} \paren {\lambda \map f z + \mu \map g z} = \lambda l + \mu m$
So, by definition of continuous again, we have that $\lambda f + \mu g$ is continuous on $S$.
$\blacksquare$