例设f:E→R记E1={x∈E:f(x)>a,则 ∪E (= n=1 a+ LEL >a+ (a,+∞)=[a+1,+∞) (=∪(a+1,+∞) a a+1/n例 设f : E → R,记E[ f a] ={x E : f (x)  a},则 [ ] 1 [ ] 1 n f a n E f a E  +  =  =  ( [ a a+1/n ( ( , ) ) 1 1 =  + +  = n n a ( ) [ ] 1 1 n f a n E  +  = =  ( , ) [ , ) 1 1 + =  + +  = n n a a