Suppose that I have a set over which I define a collection of -algebra: ; that is, each is a -algebra of . Is the countable union of those also a -algebra? That is, is a -algebra over ? How about countable intersection?

1 - It seems pretty clear that and are contained in .

2 - It is also clear that if , then

So the first two conditions of a -algebra have been fulfilled: contains the full sample space / empty set and it is closed under complements.

3 - But if , is it true that ? I am unable to prove this fact. Yet I cannot find a counterexample.

A counterexample or hint of proof would be very appreciated - thanks.