(i) 𝒮 ∈ 𝒜 ; (ii) if E ∈ 𝒜 then E ‾ ∈ 𝒜 ; (iii) if E 1 , E 2 ∈ 𝒜 then E 1 ∪ E 2 ∈ 𝒜 .
( A . 1 ) If A ∈ ℱ then 0 ≤ P { A } ≤ 1 . ( 1 ) ( A . 2 ) P { 𝒮 } = 1 . ( 2 ) ( A . 3 ) If { E n , n ≥ 1 } ∈ ℱ is a sequence of disjoint events ( 3 ) then P { E 1 ∪ … ∪ E n } = P { E 1 } + … + P { E n }