# Open and Closed Sets

1. Oct 10, 2013

### mathanon

Prove that for any collection {Oα} of open subsets of ℝ, $\bigcap$ Oα is open.

I did the following for the union, but I don't see where to go with the intersection of a set.

Here's what I have so far:

Suppose Oα is an open set for each x $\ni$ A. Let O= $\bigcap$ Oα. Consider an arbitrary x in O. By definition of O, x is in O, and O is open by hypothesis. So x is an interior point of Oα

Last edited: Oct 10, 2013
2. Oct 10, 2013

### LCKurtz

What if $O_n = (-\frac 1 n, \frac 1 n)$?