GATE CS Theory of Computation questions

Let $N$ be an NFA with $n$ states. Let $k$ be the number of states of a minimal DFA which is equivalent to $N$. Which one of the following is necessarily true?

The set of all recursively enumerable languages is:

Let $A$ and $B$ be finite alphabets and let $\#$ be a symbol outside both $A$ and $B$. Let $f$ be a total function from $A^{*}$ to $B^{*}$. We say $f$ is computable if there exists a Turing machine $M$ which given an input $x \in A^{*}$, always halts with $f(x)$ on its tape. Let $L_{f}$ denote the language …

Consider the following languages over the alphabet $\Sigma = \left \{ a, b, c \right \}$. Let $L_{1} = \left \{ a^{n}b^{n}c^{m}\mid m,n \geq 0 \right \}$ and $L_{2} = \left \{ a^{m}b^{n}c^{n}\mid m,n \geq 0 \right \}$. Which of the following are context-free languages? $L_{1} \cup L_{2}$ $L_{1} \cap L_{2}$

Consider the context-free grammars over the alphabet $\left \{ a, b, c \right \}$ given below. $S$ and $T$ are non-terminals. $G_{1}:S\rightarrow aSb \mid T, T \rightarrow cT \mid \epsilon$ $G_{2}:S\rightarrow bSa \mid T, T \rightarrow cT \mid \epsilon$ The language $L\left ( G_{1} \right )\cap L(G_{2})$ is

If $G$ is a grammar with productions $S\rightarrow SaS\mid aSb\mid bSa\mid SS\mid\epsilon$ where $S$ is the start variable, then which one of the following strings is not generated by $G$?

Consider the language $L$ given by the regular expression $(a+b)^{*} b (a+b)$ over the alphabet $\{a,b\}$. The smallest number of states needed in a deterministic finite-state automaton (DFA) accepting $L$ is ___________ .

Consider the following context-free grammar over the alphabet $\Sigma = \{a,b,c\}$ with $S$ as the start symbol:$$S \rightarrow abScT \mid abcT$$$$T \rightarrow bT \mid b$$Which one of the following represents the language generated by the above grammar?

Let $L(R)$ be the language represented by regular expression $R$. Let $L(G)$ be the language generated by a context free grammar $G$. Let $L(M)$ be the language accepted by a Turing machine $M$. Which of the following decision problems are undecidable? Given a regular expression $R$ and a string $w$, is $w \in L(R)$? Given a context-free …

Consider the following languages. $L_1 = \{a^p \mid p \text{ is a prime number} \}$ $L_2 = \{ a^nb^mc^{2m} \mid n \geq 0, m \geq 0 \}$ $L_3 = \{a^n b^n c^{2n} \mid n \geq 0 \}$ $L_4 = \{ a^n b^n \mid n \geq 1\}$ Which of the following are CORRECT? $L_1$ is context free but not regular $L_2$ is not context free $L_3$ is not context free but …

Identify the language generated by the following grammar, where $S$ is the start variable. $S \rightarrow XY$ $X \rightarrow aX \mid a$ $Y \rightarrow aYb \mid \epsilon$

The minimum possible number of states of a deterministic finite automaton that accepts the regular language $L$ = {$w_{1}aw_{2}$ | $w_{1},w_{2}$ $\in$ $\left \{ a,b \right \}^{*}$ , $\left | w_{1} \right | = 2, \left | w_{2} \right |\geq 3$} is ______________ .

Let $L_1, L_2$ be any two context-free languages and $R$ be any regular language. Then which of the following is/are CORRECT? $L_1 \cup L_2$ is context-free $\overline{L_1}$ is context-free $L_1 - R$ is context-free $L_1 \cap L_2$ is context-free

Consider the transition diagram of a PDA given below with input alphabet $\Sigma=\{a,b\}$ and stack alphabet $\Gamma = \{X,Z\}$. $Z$ is the initial stack symbol. Let $L$ denote the language accepted by the PDA Which one of the following is TRUE?

Let $X$ be a recursive language and $Y$ be a recursively enumerable but not recursive language. Let $W$ and $Z$ be two languages such that $\overline{Y}$ reduces to $W$, and $Z$ reduces to $\overline{X}$ (reduction means the standard many-one reduction). Which one of the following statements is TRUE?

Consider the following context-free grammars; $G_1 : S \to aS \mid B, B \to b \mid bB$ $G_2 : S \to aA \mid bB, A \to aA \mid B \mid \varepsilon,B \to bB \mid \varepsilon$ Which one of the following pairs of languages is generated by $G_1$ and $G_2$ ,respectively?

Which of the following decision problems are undecidable? Given NFAs $N_1$ and $N_2$ , is $L(N_1) \cap L(N_2) = \Phi$ Given a CFG $G = (N,\Sigma,P,S)$ and a string $x \in \Sigma^{*}$, does $x \in L(G)$} ? Given CFGs $G_1$ and $G_2$, is $L (G_1) = L(G_2)$? Given a TM $M$, is $L(M)=\Phi$ ?

Which one of the following regular expressions represents the language: the set of all binary strings having two consecutive $0$'s and two consecutive $1$'s?

Which of the following languages is generated by the given grammar? $$S \rightarrow aS \mid bS \mid \varepsilon$$

Consider the following languages: $L_{1}=\left\{a^{n}b^{m}c^{n+m}:m, n\geq 1\right\}$ $L_{2}=\left\{a^{n}b^{n}c^{2n} :n\geq 1\right\}$ Which one of the following is TRUE?