易见 (1)0≤fn(4)≤l (2)fn(2)=1 (3若4…4两两互不相容,则/(4)=∑f(4)(1)0  f (A) 1; n 易见： (2) () =1; n f   k i k i n n i n Ai A A f A f 1 1 1 (3) , , ( ) ( ). = = 若 两两互不相容，则 = 