设/:E→R记EA={x∈E:f()>a,则 [f≥a] n=1[f> (=∩E n=1 [+∞)=(a-1,+∞)(a-n,+∞) a-n [([ 1/na-1/+1a
例 [ ] 1 [ ] 1 n f a n E f a E − = = 设f : E → R,记E[ f a] ={x E : f (x) a},则 ( [ a-1/n a [ , ) ( , ) 1 1 + = − + = n n a a ( ) [ ] 1 1 n f a n E − = = ( [ , )) 1 1 − + = n n a ( [ ( [ [ a-1/n-1 a-1/n a-1/n+1 a
例设/:E→R记EA={x∈E:f(x)>a则 Lf>al ∪E n=1 ∫≥a+ (=∪E n=11/>0 n (,+∞)=Ua+n,+) (=∪(a+1,+∞)) a a+1
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