# Thread: Algebras of sets

1. ## Algebras of sets

When operating with sigma algebras, what does it mean when we talk about a countably infinite set? And, what are then closed countably infinite interesections?

2. Originally Posted by seigna
When operating with sigma algebras, what does it mean when we talk about a countably infinite set? And, what are then closed countably infinite interesections?

Countable infinite set = a set equipolent with the set of all natural numbers. How come you don't know this if you're dealing

with $\sigma -$algebras??

Tonio

3. Well, ask my prof... he never introduced the topic, just went of with it during the lecture... maybe the others know it from high school... dunno.

Thanks for the answer.