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It is just an idea (might be wrong) but, i think that if a Hausdorff space, say $X$, contains too many elements, then a countable subset cannot be dense in it.

Does there exist a cardinality that any space with that (or bigger) cardinality cannot have a countable dense subset?


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If $X$ is a non-empty set equipped with the trivial topology, any singleton will be dense in it, so you should probably require $X$ to be Hausdorff, to make the question more interesting. – Dejan Govc Mar 23 '14 at 22:56
Of course, thanks. I edited the question. – ThePortakal Mar 23 '14 at 22:59
See this answer. – Hanul Jeon Mar 23 '14 at 23:06
up vote 21 down vote accepted

The maximum possible cardinality of a separable Hausdorff space is $2^{2^{\aleph_0}}$.

Let $X$ be a separable Hausdorff space and let $A$ be a countable dense subset of $X$. Define a function $f:X\to\mathcal P(\mathcal P(A))$ by setting $f(x)=\{B\subseteq A:x\in\overline{B}\}$. Using the fact that $X$ is Hausdorff, it's easy to see that $f$ is injective, whence $|X|\le|\mathcal P(\mathcal P(A))|\le2^{2^{\aleph_0}}$. (The Hausdorff separation axiom cannot be replaced by T$_1$ here; an arbitrary set with the minimal T$_1$ topology is a separable T$_1$-space.)

The product of continuum many separable spaces is separable. In particular, then, the product of $2^{\aleph_0}$ copies of the discrete space $\{0,1\}$ is a separable compact Hausdorff space of cardinality $2^{2^{\aleph_0}}$.

The Stone-Čech compactification of $\mathbb N$ is another famous example of a separable compact Hausdorff space of cardinality $2^{2^{\aleph_0}}$.

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Consider a arbitrary set $S$ with the indiscrete topology: the only open sets are $\emptyset$ and $S$. Then even a one-point set is dense in $S$.

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OP edited the question to rule out this example; the space must now be Hausdorff. – MJD Mar 23 '14 at 23:30

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