# Combination Theorem for Continuous Functions

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## Contents

## Theorem

Let $X$ be one of the standard number fields $\Q, \R, \C$.

Let $f$ and $g$ be functions which are continuous on an open subset $S \subseteq X$.

Let $\lambda, \mu \in X$ be arbitrary numbers in $X$.

Then the following results hold:

### Sum Rule

- $f + g$ is continuous on $S$.

### Multiple Rule

- $\lambda f$ is continuous on $S$.

### Combined Sum Rule

- $\lambda f + \mu g$ is continuous on $S$.

### Product Rule

- $f g$ is continuous on $S$

### Quotient Rule

- $\dfrac f g$ is continuous on $S \setminus \set {x \in S: \map g x = 0}$

that is, on all the points $x$ of $S$ where $\map g x \ne 0$.

## Also see

## Sources

- 1953: Walter Rudin:
*Principles of Mathematical Analysis*... (previous): $4.9$