Grammar: Decidable and Undecidable Problems Revision as of 20:33, 27 February 2014 by Arjun Suresh (talk | contribs) (diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)

Grammar: Decidable and Undecidable Problems

Grammar	<math>w \in L(G)</math>	<math>L(G) = \phi</math>	<math>L(G) = \Sigma^*</math>	<math>L(G_1) \subseteq L(G_2)</math>	<math>L(G_1) = L(G_2)</math>	<math>L(G_1) \cap L(G_2) = \phi</math>	<math>L(G)</math> is finite
Regular Grammar	D	D	D	D	D	D	D
Det. Context Free	D	D	D	UD	?	UD	D
Context Free	D	D	UD	UD	UD	UD	D
Context Sensitive	D	UD	UD	UD	UD	UD	UD
Recursive	D	UD	UD	UD	UD	UD	UD
Recursively Enumerable	UD	UD	UD	UD	UD	UD	UD

Checking if <math>L(CFG)</math> is finite is decidable because we just need to see if <math>L(CFG)</math> contains any string with length between <math>n</math> and <math>2n-1</math>, where <math>n</math> is the pumping lemma constant. If so, <math>L(CFG)</math> is infinite otherwise its finite.

Other Undecidable Problems

For arbitrary CFGs G, G1 and G2 and an arbitrary regular set R

The following problems are undecidable:

1. Whether <math>(L(G1))^\complement</math> is a CFL?
2. Whether <math>L(G1) \cap L(G2)</math> is a CFL?
3. Whether <math>L(G1) \cap L(G2)</math> is empty?
4. Whether <math>L(G) = R</math>?
5. Whether <math>R \subseteq L(G)</math>?
6. Whether <math>G</math> is ambiguous?
7. Whether <math>L(G)</math> is a DCFL?

But whether <math>L(G) \subseteq R</math> is decidable. (We can test if <math>L(G) \cap compl(R)</math> is <math>\phi</math>)

For arbitrary DCFGs G, G1 and G2 and an arbitrary regular set R

The following problems are decidable:

1. Whether <math>(L(G1))^\complement</math> is a DCFL? (trivial)
2. Whether <math>L(G) = R</math>?
3. Whether <math>L(G) \subseteq R</math>?
4. Whether <math>R \subseteq L(G)</math>?
5. Whether <math>L(G)</math> is a CFL? (trivial)

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