Let X be a separable metric space, possibly assumed to be complete, and $B\_i, i \in J$ an infinite collection of open balls. Is it true that there always exists a countable subset K of J such that the intersection of the $B\_i, i \in J$ is equal to the intersection of the $B\_i, i \in K$ ? If not, what kind of additional condition would be needed on X ? (the spaces X in which I am interested are not locally compact.)



Uncountable intersections of open balls in a separable metric space
I don't think so. Here is a counterexample on the circle ${\bf R}/{\bf Z}$.
The subset ${\bf Q}/{\bf Z}$ is an (uncountable) intersection of open balls:
$${{\bf Q}/ {\bf Z}}= \bigcap\_{x\notin {\bf Q}/{\bf Z}} \{x\}^c$$ 
It is not a $G\_\delta$-set (a countable intersection of open sets) because of the Baire theorem.


As a side-note, all subsets of the circle are intersections of balls, but not all subsets of the circle are $G\_\delta$ sets.


