让 L1 成为递归语言。令 L2 和 L3 是可递归枚举但不可递归的语言。以下哪项陈述不一定正确?
(A) L2 – L1 是可递归枚举的。
(B) L1 – L3 是可递归枚举的
(C) L2 ∩ L1 是递归可枚举的
(D) L2 ∪ L1 是递归可枚举的
(一) A
(乙)乙
(C)丙
(四)丁答案：(乙)
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