# Definition:Uniform Convergence

## Contents

## Definition

### Metric Space

Let $S$ be a set.

Let $M = \left({A, d}\right)$ be a metric space.

Let $\left \langle {f_n} \right \rangle$ be a sequence of mappings $f_n: S \to A$.

Let:

- $\forall \epsilon \in \R_{>0}: \exists N \in \R: \forall n \ge N, \forall x \in S: d \left({f_n \left({x}\right), f \left({x}\right)}\right) < \epsilon$

Then **$\left \langle {f_n} \right \rangle$ converges to $f$ uniformly on $S$ as $n \to \infty$**.

### Real Numbers

The above definition can be applied directly to the real numbers treated as a metric space:

Let $\sequence {f_n}$ be a sequence of real functions defined on $D \subseteq \R$.

Let:

- $\forall \epsilon \in \R_{>0}: \exists N \in \R: \forall n \ge N, \forall x \in D: \size {\map {f_n} x - \map f x} < \epsilon$

That is:

- $\displaystyle \forall \epsilon \in \R_{>0}: \exists N \in \R: \forall n \ge N: \sup_{x \in D} \size {\map {f_n} x - \map f x} < \varepsilon$

Then **$\sequence {f_n}$ converges to $f$ uniformly on $D$ as $n \to \infty$**.

### Infinite Series

Let $S \subseteq \R$.

Let $\sequence {f_n}$ be a sequence of real functions $S \to \R$.

Let $\sequence {s_n}$ be sequence of real functions $S \to \R$ with:

- $\displaystyle \map {s_n} x = \sum_{k \mathop = 1}^n \map {f_n} x$

for each $n \in \N$ and $x \in S$.

We say that:

- $\displaystyle \sum_{n \mathop = 1}^\infty f_n$

**converges uniformly** to a real function $f : S \to \R$ on $S$ if $\sequence {s_n}$ converges uniformly to $f$ on $S$.

## Also defined as

Some sources insist that $N \in \N$ but this is unnecessary and makes proofs more cumbersome.

## Also see

## Comment

Note that this definition of convergence of a function is stronger than that for pointwise convergence, in which it is necessary to specify a value of $N$ given $\epsilon$ for each individual point.

In uniform convergence, given $\epsilon$ you need to specify a value of $N$ which holds for *all* points in the domain of the function.

## Historical Note

The concept of uniform convergence was created by Karl Weierstrass during his investigation of power series.

## Sources

- 2014: Christopher Clapham and James Nicholson:
*The Concise Oxford Dictionary of Mathematics*(5th ed.) ... (previous) ... (next): Entry:**convergence of functions**