(a) Note that (3.4) assumes P(A)=0 since PA(B) is meaningless if P(A)=0. Assuming both P(A)=0 and P(B)=0, show that if (3.4) is true, then P(A)=PB(A); that is, if B is independent of A, then A is independent of B. If either P(A) or P(B) is zero, then we use (3.5) to define independence.
(b) When is an event E independent of itself? When is E independent of ``not E''?