Hi,

I was thinking about this problem:

We have for example $\displaystyle X=\{ A,B,C \} $and the question is: how many diffrent (sigma) algebras we may construct?

In this case we can have {empty set, X}, {empty set, X, {A}, {B,C}}, {empty set, X, {B}, {A,C}}, {empty set, X, {C}, {B,A}} and power set. There are 5 possibilities, but what in general case?