# Convergent Real Sequence is Bounded

## Theorem

Let $\sequence {x_n}$ be a sequence in $\R$.

Let $l \in A$ such that $\ds \lim_{n \mathop \to \infty} x_n = l$.

Then $\sequence {x_n}$ is bounded.

That is, all convergent real sequences are bounded.

## Proof 1

From Real Number Line is Metric Space, the set $\R$ under the usual metric is a metric space.

By Convergent Sequence in Metric Space is Bounded it follows that:

- $\exists M > 0: \forall n, m \in \N: \size {x_n - x_m} \le M$

Then for $n \in \N$, by the Triangle Inequality for Real Numbers:

\(\ds \size {x_n}\) | \(=\) | \(\ds \size {x_n - x_1 + x_1}\) | ||||||||||||

\(\ds \) | \(\le\) | \(\ds \size {x_n - x_1} + \size {x_1}\) | ||||||||||||

\(\ds \) | \(\le\) | \(\ds M + \size {x_1}\) |

Hence $\sequence {x_n}$ is bounded.

$\blacksquare$

## Proof 2

Let $\sequence {x_n}$ be a sequence in $\R$.

Let $x_n \to l$ as $n \to \infty$.

To show that $\sequence {x_n}$ is bounded sequence, we need to find $K$ such that:

- $\forall n \in \N: \size {x_n} \le K$

Because $\sequence {x_n}$ converges:

- $\forall \epsilon > 0: \exists N: n > N \implies \size {x_n - l} < \epsilon$

In particular, this is true when $\epsilon = 1$.

That is:

- $\exists N_1: \forall n > N_1: \size {x_n - l} < 1$

By Backwards Form of Triangle Inequality:

- $\forall n > N_1: \size {x_n} - \size l \le \size {x_n - l} < 1$

That is:

- $\size {x_n} < \size l + 1$

So we set:

- $K = \max \set {\size {x_1}, \size {x_2}, \ldots, \size {x_{N_1} }, \size l + 1}$

and the result follows.

$\blacksquare$

## Sources

- 1975: Bert Mendelson:
*Introduction to Topology*(3rd ed.) ... (previous) ... (next): Chapter $2$: Metric Spaces: $\S 5$: Limits: Exercise $4$

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- 1953: Walter Rudin:
*Principles of Mathematical Analysis*: $3.2 \, \text{c}$