I'm trying to solve this exercise from Resnick "A Probability Path" (1.41) and found the answer on the topic bellow,
A field being a sigma field if and only if it's a monotone class
Therefore, the answer on this topic is about the third postulate of Sigma-field, that says if A1,A2,... belongs to class A implies that the union (or intersection) also belongs.
My question is: How to prove the postulates one and two?
(i) $\Omega$ belongs to class A
(ii) If An belongs to class A implies An complement belongs to class A.
Thanks for help