Let \(L_1 = \{ 0^n 1^n 0^m | n>=1, m>=1 \} \\L_2 = \{ 0^n 1^m 0^m | n>=1, m>=1…
2021
Let\(L_1 = \{ 0^n 1^n 0^m | n>=1, m>=1 \} \\L_2 = \{ 0^n 1^m 0^m | n>=1, m>=1 \} \\L_3 = \{ 0^n 1^n 0^n | n>=1\} \)
Which of the following are correct statements?
A. \( L_3 =L_1 \cap L_2\)
B. \(L_1\) and \(L_2\) are context free languages but L is not a context free language
C. \(L_1\) and \(L_2\) are not context free languages but L is a context free language
D. \(L_1\) is a subset of \(L_3\)
Choose the correct answer from the options given below:
- A.
A and B only
- B.
A and C only
- C.
A and D only
- D.
A only
Attempted by 24 students.
Show answer & explanation
Correct answer: A
Final answer: A and B only
Reasoning summary:
Equality of sets: The intersection of L1 and L2 forces all three block lengths to match. Any string in the intersection has form 0^a 1^b 0^c with a=b (from L1) and b=c (from L2), hence a=b=c. Thus the intersection is exactly {0^n 1^n 0^n}, which is L3.
Context-freeness of L1 and L2, and non-context-freeness of L3:
L1 is context-free because it is the concatenation of {0^n1^n | n>=1} (a standard CFL) with {0^m | m>=1} (a regular language). For example, a grammar: S -> A B, A -> 0 A 1 | 0 1, B -> 0 B | 0, generates L1.
L2 is context-free because it is the concatenation of {0^n | n>=1} (regular) with {1^m 0^m | m>=1} (a standard CFL). Thus L2 is CFL by closure under concatenation with a regular language.
L3 = {0^n 1^n 0^n | n>=1} is a standard example of a non-context-free language (can be shown using the pumping lemma for context-free languages or Ogden's lemma).
Why the other combined statements are false:
The claim that L1 and L2 are not context-free but L3 is context-free is false because the truth values are the opposite: L1 and L2 are context-free, L3 is not.
The claim that L1 is a subset of L3 is false. Counterexample: the string 0^1 1^1 0^2 (i.e., 0 1 0 0) belongs to L1 (take n=1, m=2) but not to L3 because the three block lengths are not all equal.
A video solution is available for this question — log in and enroll to watch it.