令L1为递归语言。令L2和L3为可递归枚举但不可递归的语言。下列哪个陈述不一定正确?
(A)L2 – L1可递归枚举。
(B)L1 – L3可递归枚举
(C)L2∩L1可递归枚举
(D)L2∪L1可递归枚举
(A) A
(B) B
(C) C
(D) D答案： (B)
解释：
A) Always True
(Recursively enumerable - Recursive ) is
Recursively enumerable
B) Not always true
L1 - L3 = L1 intersection ( Complement L3 )
L1 is recursive , L3 is recursively enumerable
but not recursive Recursively enumerable languages
are NOT closed under complement.
C) and D) Always true Recursively enumerable languages
are closed under intersection and union. 这个问题的测验