Sequential Continuity is Equivalent to Continuity in the Reals
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Theorem
Let $A \subseteq \R$ be a subset of the real numbers.
Let $c \in A$.
Let $f : A \to \R$ be a real function.
Then $f$ is continuous at $c$ if and only if:
- for each sequence $\sequence {x_n}$ in $A$ that converges to $c$, the sequence $\sequence {\map f {x_n} }$ converges to $\map f c$.
Corollary
Let $I$ be a real interval.
Let $x \in I$.
Let $f : I \to \R$ be a real function.
Then $f$ is continuous at $x$ if and only if:
- for all monotone sequences $\sequence {x_n}_{n \mathop \in \N}$ converging to $x$ we have:
- $\map f {x_n} \to \map f x$
Proof
Sufficient Condition
It suffices to show that if $f$ is discontinuous at $c$:
- there exists a real sequence $\sequence {x_n}$ in $A$ such that $\sequence {x_n}$ converges to $c$ but $\sequence {\map f {x_n} }$ does not converge to $\map f c$.
As $f$ is discontinuous, there exists some $\varepsilon > 0$ such that for all $\delta > 0$:
- there exists $x \in A$ with $\size {x - c} < \delta$ such that $\size {\map f x - \map f c} \ge \varepsilon$.
Using this property, we can construct a sequence $\sequence {x_n}$ as follows:
- for each $n \in \N$, pick $x_n \in A$ such that $\size {x_n - c} \le \dfrac 1 n$ and $\size {\map f {x_n} - \map f c} \ge \varepsilon$
Note that since:
- $\ds \lim_{n \mathop \to \infty} \frac 1 n = 0$
We have by the Squeeze Theorem for Real Sequences:
- $\ds \lim_{n \mathop \to \infty} \size {x_n - c} = 0$
so $\sequence {x_n}$ converges to $c$.
However $\sequence {\map f {x_n} }$ cannot converge to $\map f c$ since:
- $\size {\map f {x_n} - \map f c} \ge \varepsilon > 0$
for all $n \in \N$.
Therefore, our $\sequence {x_n}$ satisfies our original demand.
$\Box$
Necessary Condition
Let $c \in \R$.
Let $\sequence {x_n}$ be a sequence in $A$ that converges to $c$.
Let $\varepsilon \in \R_{> 0}$.
Since $f$ is continuous at $c$, there exists $\delta > 0$ such that:
- for all $x \in A$ with $\size {x - c} < \delta$, we have $\size {\map f x - \map f c} < \varepsilon$.
Additionally, since $\sequence {x_n}$ converges to $c$, there exists $N \in \N$ such that:
- for all $n > N$ we have $\size {x_n - c} < \delta$.
Therefore, since $x_n \in A$ for all $n \in \N$:
- for all $n > N$, we have $\size {\map f {x_n} - \map f c} < \varepsilon$.
Since $\varepsilon$ was arbitrary, we have:
- $\sequence {\map f {x_n} }$ converges to $\map f c$.
$\blacksquare$
Sources
- 2011: Robert G. Bartle and Donald R. Sherbert: Introduction to Real Analysis (4th ed.): $\S 5.1$: Theorem $2$