Limit of Function by Convergent Sequences

Theorem

Let $M_1 = \left({A_1, d_1}\right)$ and $M_2 = \left({A_2, d_2}\right)$ be metric spaces.

Let $S \subseteq A_1$ be an open set of $M_1$.

Let $f$ be a mapping defined on $S$, except possibly at the point $c \in S$.

Then $\displaystyle \lim_{x \to c} f \left({x}\right) = l$ iff, for each sequence $\left \langle {x_n} \right \rangle$ of points of $S$ such that $\forall n \in \N^*: x_n \ne c$ and $\displaystyle \lim_{n \to \infty} x_n = c$, it is true that $\displaystyle \lim_{n \to \infty} f \left({x_n}\right) = l$.

Proof

• Suppose that $\displaystyle \lim_{x \to c} f \left({x}\right) = l$.

Let $\epsilon > 0$.

Then by the definition of the limit of a function, $\exists \delta > 0: d_2 \left({f \left({x}\right), l}\right) < \epsilon$ provided $0 < d_1 \left({x, c}\right) < \delta$.

Now suppose that $\left \langle {x_n} \right \rangle$ is a sequence of points of $S$ such that such that $\forall n \in \N^*: x_n \ne c$ and $\displaystyle \lim_{n \to \infty} x_n = c$.

Since $\delta > 0$, from the definition of the limit of a sequence, $\exists N: \forall n > N: d_1 \left({x_n, c}\right) < \delta$.

But $\forall n \in \N^*: x_n \ne c$.

That means $0 < d_1 \left({x_n, c}\right) < \delta$.

But that implies that $d_2 \left({f \left({x_n}\right), l}\right) < \epsilon$.

That is, given a value of $\epsilon > 0$, we have found a value of $N$ such that $\forall n > N: d_2 \left({f \left({x_n}\right), l}\right) < \epsilon$.

Thus $\displaystyle \lim_{n \to \infty} f \left({x_n}\right) = l$.

• Now suppose that for each sequence $\left \langle {x_n} \right \rangle$ of points of $S$ such that $\displaystyle \forall n \in \N^*: x_n \ne c$ and $\displaystyle \lim_{n \to \infty} x_n = c$, it is true that $\displaystyle \lim_{n \to \infty} f \left({x_n}\right) = l$.

What we will try to do is assume that it is not true that $\displaystyle \lim_{x \to c} f \left({x}\right) = l$, and try to find a contradiction.

So, if it not true that $\displaystyle \lim_{x \to c} f \left({x}\right) = l$, then:

$\exists \epsilon > 0: \forall \delta > 0: \exists x: 0 < d_1 \left({x, c}\right) < \delta: d_2 \left({f \left({x}\right), l}\right) \ge \epsilon$

In particular, if $\delta = \dfrac 1 n$, we can find an $x_n$ where $0 < d_2 \left({x_n, c}\right) < \dfrac 1 n$ such that $d_2 \left({f \left({x_n}\right), l}\right) \ge \epsilon$.

But then $\left \langle {x_n} \right \rangle$ is a sequence of points of $S$ such that $\forall n \in \N^*: x_n \ne c$ and $\displaystyle \lim_{n \to \infty} x_n = c$, but for which it is not true that $\displaystyle \lim_{n \to \infty} f \left({x_n}\right) = l$.

So there is our contradiction, and so the result follows.

$\blacksquare$

Corollary

Let $f: \R \to \R$ be a real function.

The above result holds for $f$ tending to a limit both from the right and from the left:

• $\displaystyle \lim_{x \to b^-} f \left({x}\right) = l \iff \forall \left \langle {x_n} \right \rangle: \lim_{n \to \infty} f \left({x_n}\right) = l$
• $\displaystyle \lim_{x \to a^+} f \left({x}\right) = l \iff \forall \left \langle {x_n} \right \rangle: \lim_{n \to \infty} f \left({x_n}\right) = l$

where $f$ is defined on the open interval $\left({a .. b}\right)$.

$\blacksquare$

Sources

• W.A. Sutherland: Introduction to Metric and Topological Spaces (1975): Lemma $1.3.5$
• K.G. Binmore: Mathematical Analysis: A Straightforward Approach (1977)... (previous)... (next): $\S 8.9$