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For the metric space defined by distance formula,

$d(p,q) = \begin{cases}1,&\text{if }p \ne q\\ 0,&\text{if }p = q\;,\end{cases}$ does the notion of number of dimensions exist?

If yes, then is dimension of the above metric space uncountable?

Can a discrete metric space exist in finite dimensions?

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    @Ilya: Not really, since you’ve specified the sense in which you’re using the term *manifold*.2012-03-02

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There are several different notions of dimension for topological spaces in general and hence metric spaces in particular, including small and large inductive dimension and covering dimension; all discrete metric spaces have dimension $0$ in all of these senses. There is also a notion of Hausdorff dimension, that applies specifically to metric spaces. If your discrete metric space is countable, its Hausdorff dimension is also $0$; if it’s uncountable, its Hausdorff dimension is $\infty$. There are other notions of dimension for metric spaces, but these are the most familiar.

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    then everything is cle$a$r - I thought I should have missed some point.2012-03-02
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As a manifold, this object is zero dimensional, as every point is open, the set of all points is an open cover, and the unique map $\{p\}\to\mathbb{R}^0=\{0\}$ is a homeomorphism for each $p$.

(Edited after Ilya's comment: This assumes the metric space is countable, otherwise it is not second countable and thus not a manifold.)

There are also other notions of dimension for metric spaces though, such as Hausdorff dimension, and they may not agree with this one. For a start many metric spaces are not manifolds, so this approach won't give you an answer at all.