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Regular expressions and finite automata, Context-free grammars and push-down automata, Regular and context-free languages, Pumping lemma, Turing machines and undecidability.

$$\scriptsize{\overset{{\large{\textbf{Mark Distribution in Previous GATE}}}}{\begin{array}{|c|c|c|c|c|c|c|c|}\hline
\textbf{Year}&\textbf{2021-1}&\textbf{2021-2}&\textbf{2020}&\textbf{2019}&\textbf{2018}&\textbf{2017-1}&\textbf{2017-2}&\textbf{2016-1}&\textbf{2016-2}&\textbf{Minimum}&\textbf{Average}&\textbf{Maximum}
\\\hline\textbf{1 Mark Count}&2&3&3&2&2&2&3&3&3&2&2.5&3
\\\hline\textbf{2 Marks Count}&3&4&3&3&3&5&3&3&3&3&3.3&5
\\\hline\textbf{Total Marks}&8&11&9&8&8&12&9&9&9&\bf{8}&\bf{9.2}&\bf{12}\\\hline
\end{array}}}$$

Recent questions in Theory of Computation

2 votes
2 answers
1
Let $L=\{0^n1^n|n\ge 0\}$ be a context free language. Which of the following is correct? $\overline L$ is context free and $L^k$ is not context free for any $k\ge1$ $\overline L$ is not context free and $L^k$ is context free for any $k\ge1$ Both $\overline L$ and $L^k$ for any $k\ge1$ are context free Both $\overline L$ and $L^k$ for any $k\ge1$ are not context free
asked May 10 in Theory of Computation soujanyareddy13 174 views
1 vote
1 answer
2
Let $\Sigma=\{a,b\}.$ Given a language $L\underline\subset \Sigma^{\ast}$ and a word $w\in\Sigma^{\ast}$, define the languages: $Extend(L,w) :=\{xw\:|\:x\in L\}$ $Shrink(L,w) :=\{x\:|\:xw\in L\}$Show that if $L$ is regular, both $Extend(L,w)$ and $Shrink(L,w)$ are regular.
asked May 10 in Theory of Computation soujanyareddy13 174 views
7 votes
3 answers
3
Let $L \subseteq \{0,1\}^*$ be an arbitrary regular language accepted by a minimal $\text{DFA}$ with $k$ states. Which one of the following languages must necessarily be accepted by a minimal $\text{DFA}$ with $k$ states? $L-\{01\}$ $L \cup \{01\}$ $\{0,1\}^* – L$ $L \cdot L$
asked Feb 18 in Theory of Computation Arjun 1.6k views
5 votes
3 answers
4
Let $L_1$ be a regular language and $L_2$ be a context-free language. Which of the following languages is/are context-free? $L_1 \cap \overline{L_2} \\$ $\overline{\overline{L_1} \cup \overline{L_2}} \\$ $L_1 \cup (L_2 \cup \overline{L_2}) \\$ $(L_1 \cap L_2) \cup (\overline{L_1} \cap L_2)$
asked Feb 18 in Theory of Computation Arjun 1.1k views
4 votes
3 answers
5
6 votes
2 answers
6
Suppose we want to design a synchronous circuit that processes a string of $0$'s and $1$'s. Given a string, it produces another string by replacing the first $1$ in any subsequence of consecutive $1$'s by a $0$ ... $\begin{array}{l} t=s+b \\ y=s \overline{b} \end{array}$
asked Feb 18 in Theory of Computation Arjun 891 views
5 votes
1 answer
7
​​​​​​Consider the following two statements about regular languages: $S_1$: Every infinite regular language contains an undecidable language as a subset. $S_2$: Every finite language is regular. Which one of the following choices is correct? Only $S_1$ is true Only $S_2$ is true Both $S_1$ and $S_2$ are true Neither $S_1$ nor $S_2$ is true
asked Feb 18 in Theory of Computation Arjun 1.2k views
6 votes
2 answers
8
For a string $w$, we define $w^R$ to be the reverse of $w$. For example, if $w=01101$ then $w^R=10110$. Which of the following languages is/are context-free? $\{ wxw^Rx^R \mid w,x \in \{0,1\} ^* \}$ $\{ ww^Rxx^R \mid w,x \in \{0,1\} ^* \}$ $\{ wxw^R \mid w,x \in \{0,1\} ^* \}$ $\{ wxx^Rw^R \mid w,x \in \{0,1\} ^* \}$
asked Feb 18 in Theory of Computation Arjun 997 views
4 votes
1 answer
9
​​​​​​​Which of the following regular expressions represent(s) the set of all binary numbers that are divisible by three? Assume that the string $\epsilon$ is divisible by three. $(0+1(01^*0)^*1)^*$ $(0+11+10(1+00)^*01)^*$ $(0^*(1(01^*0)^*1)^*)^*$ $(0+11+11(1+00)^*00)^*$
asked Feb 18 in Theory of Computation Arjun 1.1k views
2 votes
3 answers
10
Suppose that $L_1$ is a regular language and $L_2$ is a context-free language. Which one of the following languages is $\text{NOT}$ necessarily context-free? $L_1 \cap L_2$ $L_1 \cdot L_2$ $L_1- L_2$ $L_1\cup L_2$
asked Feb 18 in Theory of Computation Arjun 943 views
4 votes
3 answers
11
Let $\langle M \rangle$ denote an encoding of an automaton $M$. Suppose that $\Sigma = \{0,1\}$. Which of the following languages is/are $\text{NOT}$ recursive? $L= \{ \langle M \rangle \mid M$ is a $\text{DFA}$ such that $L(M)=\emptyset \}$ $L= \{ \langle M \rangle \mid M$ is ... $L(M)=\emptyset \}$ $L= \{ \langle M \rangle \mid M$ is a $\text{PDA}$ such that $L(M)=\Sigma ^* \}$
asked Feb 18 in Theory of Computation Arjun 986 views
1 vote
1 answer
12
Consider the following language: $L= \{ w \in \{0,1\}^* \mid w \text{ ends with the substring } 011 \}$ Which one of the following deterministic finite automata accepts $L?$
asked Feb 18 in Theory of Computation Arjun 604 views
3 votes
5 answers
13
For a Turing machine $M$, $\langle M \rangle$ denotes an encoding of $M$ ... and $L_2$ are decidable $L_1$ is decidable and $L_2$ is undecidable $L_1$ is undecidable and $L_2$ is decidable Both $L_1$ and $L_2$ are undecidable
asked Feb 18 in Theory of Computation Arjun 938 views
6 votes
2 answers
14
In a pushdown automaton $P=(Q, \Sigma, \Gamma, \delta, q_0, F)$, a transition of the form, where $p,q \in Q$, $a \in \Sigma \cup \{ \epsilon \}$, and $X,Y \in \Gamma \cup \{ \epsilon \}$, represents $(q,Y) \in \delta(p,a,X). $Consider the ... $\Gamma = \{ \#, A\}$. The number of strings of length $100$ accepted by the above pushdown automaton is ___________
asked Feb 18 in Theory of Computation Arjun 1.2k views
1 vote
2 answers
15
Consider $L=L_1 \cap L_2$ where $L_1 = \{ 0^m 1^m 20^n 1^n \mid m,n \geq 0 \}$ $L_2 = \{0^m1^n2^k \mid m,n,k \geq 0 \}$ Then, the language $L$ is Recursively enumerable but not context free Regular Context free but not regular Not recursive
asked Nov 20, 2020 in Theory of Computation jothee 428 views
1 vote
4 answers
16
Let $L_1$ and $L_2$ be languages over $\Sigma = \{a,b\}$ represented by the regular expressions $(a^* +b)^*$ and $(a+b)^*$ respectively. Which of the following is true with respect to the two languages? $L_1 \subset L_2$ $L_2 \subset L_1$ $L_1 = L_2$ $L_1 \cap L_2 = \phi$
asked Nov 20, 2020 in Theory of Computation jothee 445 views
1 vote
2 answers
17
Which of the following statements is true? The union of two context free languages is context free The intersection of two context free languages is context free The complement of a context free language is context free If a language is context free, it can always be accepted by a deterministic pushdown automaton
asked Nov 20, 2020 in Theory of Computation jothee 746 views
2 votes
2 answers
18
Let $G_1$ and $G_2$ be arbitrary context free languages and $R$ an arbitrary regular language. Consider the following problems: Is $L(G_1)=L(G_2)$? Is $L(G_2) \leq L(G_1)$? Is $L(G_1)=R$? Which of the problems are undecidable? Choose the correct answer from the options given below: $(a)$ only $(b)$ only $(a)$ and $(b)$ only $(a)$, $(b)$ and $(c)$
asked Nov 20, 2020 in Theory of Computation jothee 272 views
0 votes
2 answers
19
Consider the following languages: $L_1=\{a^{\grave{z}^z} \mid \grave{Z} \text{ is an integer} \}$ $L_2=\{a^{z\grave{z}} \mid \grave{Z} \geq 0\}$ $L_3=\{ \omega \omega \mid \omega \epsilon \{a,b\}^*\}$ Which of the languages is(are) regular? Choose the correct answer from the options given below: $L_1$ and $L_2$ only $L_1$ and $L_3$ only $L_1$ only $L_2$ only
asked Nov 20, 2020 in Theory of Computation jothee 342 views
0 votes
2 answers
20
Which of the following grammars is(are) ambiguous? $s \rightarrow ss \mid asb \mid bsa \mid \lambda$ $s \rightarrow asbs \mid bsas \mid \lambda$ $s \rightarrow aAB \\ A \rightarrow bBb \\ B \rightarrow A \mid \lambda \text{ where } \lambda \text{ denotes empty string}$ Choose the correct answer from the options given below: $(a)$ and $(c)$ only $(b)$ only $(b)$ and $(c)$ only $(a)$ and $(b)$ only
asked Nov 20, 2020 in Theory of Computation jothee 459 views
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