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My question is: what are the minimal conditions on a topological space for it have the following property?

$$x\in \bar{A}\iff \exists (x_n)\subset A | x_n \to x$$

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I assume you require $x\notin \{x_n\}$? –  Alex Becker Mar 28 '12 at 1:12
Not necessarily! I am not considering only accumulation points! –  André Lima Mar 28 '12 at 1:13
en.wikipedia.org/wiki/Sequential_space –  t.b. Mar 28 '12 at 1:16
@AndréLima A space with that property is called a Frechet space. (@t.b.) there are sequential spaces that are not Frechet a classical example is the sequential fan or Aren's space. –  azarel Mar 28 '12 at 2:40
@azarel: thanks for the clarification, I thought the page I linked to was enough (there's a section on Fréchet-Urysohn spaces there). –  t.b. Mar 28 '12 at 3:00

1 Answer 1

In this paper there is the answer (section 2, on Fréchet spaces, also known as Fréchet-Urysohn spaces): Your property defines the notion of a Fréchet space and he shows that these spaces are the pseudo-open images of metric spaces. He also defines the weaker concept of a sequential space and in the follow up paper he shows that a sequential space is Fréchet iff all of its subspaces are sequential (hereditarily sequential).

A sequential space has the cleaner characterization: a space $X$ is sequential iff there is a metric space $M$ and a surjective quotient map $f: M \rightarrow X$ ($X$ is a quotient image of a metric space).

As said, Fréchet spaces can be similarly characterized, using not quotient maps but pseudo-open maps: $f: X \rightarrow Y$ is pseudo-open iff for every $y \in Y$ and every open neighborhood $U$ of $f^{-1}[\{y\}]$ we have that $y \in \operatorname{int}(f[U])$. Every open or closed surjective map is pseudo-open and all pseudo-open maps are quotient.

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