A non-empty topological space without isolated points is called *maximal* if every finer topology on that space has at least an isolated point. The existence of a (Hausdorff) maximal space is a simple consequence of Zorn's Lemma. 


Note that in a maximal space $(X, \tau)$, nowhere dense sets are closed (and discrete). Indeed, if there were a nowhere dense set $N$ that is not closed then $\tau \cup \{X \setminus N \}$ would be a subbase for a finer topology without isolated points on $X$. In particular, a maximal space can't contain any non-trivial convergent sequences, because a discrete set is nowhere dense in a space without isolated points.


Let $\mathfrak{max}$ be the minimal weight of a Hausdorff maximal space. Clearly $\aleph\_1 \leq \mathfrak{max} \leq \mathfrak{c}$, but we can prove a better lower bound, namely: $\mathfrak{d} \leq \mathfrak{max}$. 



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> If $X$ is a countable Hausdorff maximal space then $w(X) \geq \mathfrak{d}$.
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Proof: Fix a point $x \in X$ and let $\{U\_n: n < \omega \}$ be a maximal pairwise disjoint family of non-empty open sets with the property that $x \notin \overline{U\_n}$, for every $n< \omega$: Clearly $x \in \overline{\bigcup \{U\_n: n < \omega \}}$. Let $\{x^n\_k: k < \omega \}$ be an enumeration of $U\_n$.


Suppose by contradiction that $w(X)=\kappa < \mathfrak{d}$ and let $\{B\_\alpha: \alpha < \kappa \}$ enumerate a local base at $x$. For every $\alpha < \kappa$, the set $B\_\alpha$ intersects infinitely many $U\_n$'s, so we can find an integer-valued function $f\_\alpha$ with infinite domain $\subseteq \omega$ such that $x^n\_{f\_\alpha(n)} \in B\_\alpha \cap U\_n$, for every $n \in dom(f\_\alpha)$.


Since $\kappa < \mathfrak{d}$, we can find a function $f: \omega \to \omega$ such that, for every $\alpha < \kappa$, there is $n \in dom(f\_\alpha)$ with $f\_\alpha(n) < f(n)$. Let $D=\{x^n\_k: k \leq f(n), n < \omega \}$. Then $D$ is discrete and $x \in \overline{D} \setminus D$, so $D$ is a nowhere dense set in $X$ which is not closed and that contradicts maximality.



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> QUESTION: Is $\mathfrak{max}=\mathfrak{d}$ in ZFC?
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EDIT(10/05/2019): Will Brian answered the above question in the negative, but the question of the title is still open. What is $\mathfrak{max}$? Is it equal to some product of known cardinal invariants of the continuum?



What's the minimal weight of a maximal space?
No, these cardinals are not provably equal.


This follows from a result of El'kin [1](https://carma.newcastle.edu.au/jon/Preprints/Books/Open%20Probs%20in%20Top/open2.pdf):



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> Let $X$ be a set and let $x \in X$. If $\tau$ is a maximal topology on $X$, then $\{U \setminus \{x\} \,:\, x \in U \in \tau \}$ is a base for a non-principal ultrafilter on $X$.
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If $X$ is countable (as in your question), it follows that any local basis at $x$ has size at least as big as the ultrafilter number $\mathfrak u$, defined as the smallest possible cardinality of a base for a non-principal ultrafilter on a countable set. Hence we have found another lower bound for your cardinal: $\mathfrak u \leq \mathfrak{max}$.


The reason this answers your question is that it is consistent to have $\mathfrak d < \mathfrak u$. (This happens for example in the random real model.) By the previous paragraph, any model in which $\mathfrak d < \mathfrak u$ is also a model in which $\mathfrak d < \mathfrak{max}$.


[1](https://carma.newcastle.edu.au/jon/Preprints/Books/Open%20Probs%20in%20Top/open2.pdf) A. G. El'kin, "Ultrafilters and irresolvable spaces," *Vestnik Moskov. Univ. Ser. I Mat. Mekh.* **24** (1969), no. 5, pp. 51-56. $\ $
I was unable to find an online version of this paper, and in fact I don't even know whether it's been translated from the Russian I presume it was written in. But you can find the result mentioned above quoted in [this book](https://carma.newcastle.edu.au/jon/Preprints/Books/Open%20Probs%20in%20Top/open2.pdf) (page 54), or mentioned (a little vaguely) in [this paper](http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.470.1479&rep=rep1&type=pdf) (page 2).


