9
$\begingroup$

It is proven that any metric $d$ is continuous. Consider the metric space $(\mathbb{R}, d)$ where:

$$d:\mathbb{R}\times\mathbb{R}\rightarrow \mathbb{R}$$

$$d(x, y) = \begin{cases} 0 & x=y \\ 1 & x\neq y \end{cases} $$

Let $x_n \rightarrow x=0$ and $y_n \rightarrow y=0$. If you take $d(x_n, y_n)\rightarrow 1 \neq 0=d(x, y)$. This shows that this metric is discontinuous. What is wrong with my reasoning?

$\endgroup$
  • $\begingroup$ $x_n=0=y_n$ for all $n$ large. $\endgroup$ – Jo' Oct 7 '18 at 15:25
  • $\begingroup$ You write $\mathbb{R}$ but it has two different topologies for this statement. See my answer. $\endgroup$ – Henno Brandsma Oct 7 '18 at 21:30
8
$\begingroup$

The fact that $d(x_{n},x)\to 0$ implies that eventually $d(x_{n},x)<1/2$. Thus, eventually the $x_{n}$'s must all be $0$. The same goes for the $y_{n}$'s. Therefore, $d(x_{n},y_{n})$ is eventually $0$ for such sequences.

$\endgroup$
5
$\begingroup$

You misrepresent the statement.

Correct is: let $(X,d)$ be a metric space. This induces a topology $\mathcal{T}_d$ on $X$ induced by the metric (the minimal topology where all balls $B(x,r)$ are open). Then the map $d: (X,\mathcal{T}_d) \times (X,\mathcal{T}_d) \to \mathbb{R}$ is continuous where the left hand side has the product topology of the metric topology with itself and the reals have the usual Euclidean topology.

If we use the discrete metric, which induces the discrete topology, then this product topology is also discrete and $d$ is indeed continuous (as is any map on a discrete space, so it's not very informative).

$\endgroup$
2
$\begingroup$

A sequence $x_n$ has limit $0$ in the discrete metric if and only if eventually $x_n = 0$. Does this help?

$\endgroup$
  • $\begingroup$ Why is that so? Isn't $x_n$ a normal sequence such as $\frac{1}{n}$? In which case $x_n\neq 0 \forall n$? $\endgroup$ – Dylan Zammit Oct 7 '18 at 15:26
  • $\begingroup$ Ok, understood what you mean with the help of ervx's answer :) thanks $\endgroup$ – Dylan Zammit Oct 7 '18 at 15:29

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.