
countable union of closed sets
Posted:
Nov 13, 2012 10:29 PM


On Sun, 11 Nov 2012, Daniel J. Greenhoe wrote:
> I have noted that the "countable union of closed sets" is called F_sigma > in the literature while G_delta is the "countable intersection of open > sets":
> Would there be some connection or fundamental result involving F_sigma > sets? Could the Baire Category Theorem be useful here?
Yes, every open subset of a metric space is F_sigma. No, that doesn't need the Baire Category Theorem.

