Questions about operations on objects of some kind that result in objects of the same kind.

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0
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2answers
70 views

Intersection of a language with a regular language imply context free

Lets say you have a language $L$ and you want to determine if it is context free. Context free languages intersected with regular languages are context free. Is that enough to prove that $L$ is ...
0
votes
1answer
37 views

Language described by inverting accepting states of NFA

Connecting to When states that are not accepting states become accepting states in NFA, what happens?, what is the formal language described by inverting accepting states of NFA? By inverting, I mean ...
0
votes
1answer
66 views

If L1 ∪ L2 and L1 are regular, is L2 also regular?

This is a problem in a theory of computation book that's stumping me: Suppose that we know that $L_1 ∪ L_2$ and $L_1$ are regular. Can we conclude that $L_2$ is regular? Explain. At first, I ...
2
votes
1answer
94 views

Prove transitivity of big-O notation

I'm doing a practice question (not graded HW) to understand mathematical proofs and their application to Big O proofs. So far, however, the very first problem in my text is stumping me wholly. ...
1
vote
1answer
98 views

If $g ∘ f$ is primitive recursive, are $f$ and $g$, too?

Assuming I have functions $f, g : \mathbb{N} \to \mathbb{N}$ and I know that $g \circ f$ is a primitive recursive function. What can I tell about $f$ and $g$? Are they primitive recursive as well? Or ...
0
votes
1answer
49 views

Union of finite and non-regular language [duplicate]

Question: ($B$ and $C$ are languages) $B$ is finite,$C$ isn't regular: Prove/Disprove: $C\cup B$ isn't regular. Thoughts: My intuition says this is true, but I need an idea to prove it. Since I ...
4
votes
1answer
69 views

Is there $L$ such that $L$ and $\bar L$ are context free, but $L$ is not deterministic context free?

The usual candidates for context free languages whose complement is also context free, but they are not regular are the Deterministic Context Free Languages ($DCFL$). For example, $L=\{a^nb^n\mid ...
0
votes
0answers
45 views

Union, Intersection, Difference, etc. of different types of languages

I am preparing for a competitive exam (GATE) in which questions are asked in Automata about operations among different types of languages. For example, If $L_1$ is recursive & $L_2$ is ...
0
votes
1answer
29 views

If $L$ is a $CFL$, then why isn't $L^*$ also $CFL$

I was studying closure properties of CFLs and I came across this. I want to understand why $L^*$ is not a CFL, can anyone explain me in depth with simple examples?
1
vote
1answer
21 views

Decidability language, intersection

I have two langages $ A, B \in \mathrm{coRE}$. How can I prove that $ A \triangle B= ( A - B) \cup (B - A)$ is also in $\mathrm{coRE}\,$?
2
votes
1answer
116 views

are regular languages closed under division

I am trying to solve this question which appeared in previous exam paper Can someone help me what i am failing to understand For languages $A$ and $B$ define $A \div B = \{x \in \Sigma^{\ast} : xy ...
2
votes
1answer
57 views

How to prove that the Myhill-Nerode equivalence classes for L are the same as for its complement?

Given language $L$, I want to show that its Myhill-Nerode equivalence classes are the same as for its complement $\overline{L}$. I am thinking of constructing a DFA $M$ for the Language $L$ so the ...
0
votes
1answer
31 views

prove decidability and recognizability

I want to prove that for any language $L_1$ described by a Turing machine and any regular language $L_2$, $L_1 \cap L_2$ is described by a Turing machine that its recognizability and decidability is ...
1
vote
1answer
43 views

Is the extension of every undecidable theory undecidable?

While it is not the case that the extension of every decidable theory is decidable, is it true that: the extension of every undecidable theory undecidable? In other words, given an undecidable ...
1
vote
1answer
44 views

If pref(L) is regular, does that imply L is regular?

I have this exercise for homework: Say we have a language L. we know that the language pref(L) (all the prefixes of ...
1
vote
1answer
81 views

Turing-recognizable languages closed under star operation

I'm tasked with demonstrating that the class of Turing-recognizable languages is closed under the operation of star, but I'm confused about how this is true. For example, I have a TM to recognize a ...
1
vote
1answer
162 views

Concatenation of languages in NP

I have a hard time to understand why the concatenation of two languages over an alphabet (concatenation is in NP), doesn't imply that each of the languages for themselves are in NP. I talked with my ...
6
votes
3answers
865 views

How to XOR automata?

Say we have 3 DFAs. We know how to OR, AND, or NOT them. But how does one XOR them? There is not one single mention of this online. $x\; \mathrm{XOR} \;y\; \mathrm{XOR} \;z = ((x|y)(\neg x|y)|z) ...
5
votes
1answer
56 views

Are the complements of $NP$-languages with only $n$ words of length $n$ also in $NP$?

Assuming $\Sigma = \{ 0, 1\}$.. Given a language $L$, such that for each $n\in \mathbb{N}$ we have $n$ words of length $n$ in $L$ and assuming $L\in NP$, can we prove also that $L\in Co-NP$? So it ...
1
vote
1answer
103 views

Find a CFG for the set of prefixes of a CFL [duplicate]

How do i generate grammar for Prefix of Langauge L, SupposeG=(V,􏰀,P,S)is a context-free grammar generating a CFL L then pref(L) is defined as pref(L)={x∈􏰀∗ : ∃ y such that xy∈L}. I understand for ...
6
votes
1answer
128 views

If L is context-free, must FH(L) be context-free?

Define $FH(L) = \{x \in \Sigma^* : \exists y \in \Sigma^* \text{ with } |x| = |y| \text{ such that } xy \in L\}$. In other words, $FH(L)$ is the set of first halves of even length strings in $L$. ...
0
votes
1answer
207 views

Is Context Free Language closed under perfect shuffle?

Note that this is not shuffle but perfect shuffle, defined as follows: Let $w = a_{1}a_{2} \ldots a_{n}$ and $x = b_{1}b_{2} \ldots b_{n}$ be two strings of the same length. Then the perfect shuffle ...
0
votes
1answer
61 views

Kleene positive closure - help in proofing this claim

I just started a course called 'Automata and Formal Languages'. I'm having difficulty in proofing\disproofing this equality. $ (L_{1} \circ L_{2})^{+} = L_{1}^{+} \circ L_{2}^{+} $ Where: $ ...
3
votes
2answers
492 views

If both the concatenation of two languages and the second “half” are regular, is the first too?

Given that $L_2$ is regular and infinite and $L_1 \cdot L_2$ is regular, then $L_1$ is also regular. I need some help on getting started on proving this is the case. My intuition is that if $L_1 ...
2
votes
3answers
132 views

Prove that the equal-length concatenation of regular languages is context free

If A and B are regular, then prove that $A@B = \{xy \mid x \in A \text{ and } y \in B \text{ and } |x|=|y|\}$ is always context free. So I'm trying to come up with the proof that looks something like ...
2
votes
1answer
276 views

Are DCFLs closed under reversal?

According to this chart, DCFLs are closed under reversal. However, I am not convinced as the intuitive proof (reversing the arrows of the controlling finite state machine and switching the pushes and ...
8
votes
3answers
1k views

Union of regular languages that is not regular

I've come across that question : "Give examples of two regular languages which their union doesn't output a regular language. " This is pretty shocking to me because I believe that regular languages ...
2
votes
2answers
98 views

Prove that REG is closed against removing all but lexicographicaly largest words (per length)

Let $\Sigma_n = \{0, 1, ... , n-1\}$. Suppose $L \subseteq$ $\Sigma^*_n$, and let $\qquad\displaystyle\mathcal{B}(L) = \{ x \in L : x = \textrm{lex}\max L_m, m \in \mathbb{N}_0 \}$, ...
9
votes
1answer
268 views

Constructing all context-free languages from a set of base languages and closure properties?

One way of looking at regular expressions is as a constructive proof of the following fact: it's possible to construct the regular languages by starting with a small set of languages and combining ...
1
vote
1answer
69 views

Don't understand closure under string reversal

I am trying to learn from http://www.cs.uiuc.edu/class/su08/cs273/lectures/lect_06.pdf #2 and I understand everything except for the 2nd line of delta prime prime function, I having breaking down ...
1
vote
0answers
87 views

Given a non-deterministic Mealy machine $M$, if $L$ is regular, is $M(L)$ regular?

Consider a nondeterministic Mealy machine, $M$, defined as follows: $M = (Q, \Sigma, \Delta, \delta, \tau, q_0)$ where $Q$ is a finite set of states $\Sigma$ is an input alphabet $\Delta$ is an ...
0
votes
1answer
74 views

For two regular languages, why is the set of words from one that don't have a subsequence in the other also regular?

In general, a string $x$ is a subsequence of $w = w_1\dots w_n$ if there are integers $i_1<\dots< i_k$ such that $x = w_{i_1}\dots w_{i_k}$. The subsequence is proper if $k < n$ and $k > ...
3
votes
3answers
263 views

Clearing a Confusion regarding the Proof of equal no of a's and b's not being a regular language

I was wondering about its proof. The direct use of pumping lemma here is not a viability. So a certain teacher of mine proved this with the notion that $a^{n}b^{n}$ being a subset of this language ...
0
votes
1answer
171 views

Show that the regular languages are closed against taking “the second half” [duplicate]

Given $L$ is regular, the proof that $\mathrm{HALF}(L)$ is regular is pretty straightforward to me (e.g., #11 in this link): simply making a NFA and meeting in the middle with 2 original DFAs, the ...
1
vote
2answers
106 views

Proving Regularity of Languages that are 1/k of an already known regular language

There is this question in Kozen, that states if a language is regular then the first half would also be regular. Also I found a material on the internet that extends the thinking saying a language ...
-1
votes
1answer
161 views

How to construct a DFA for this?

Let $C = shuffle(A, B)$ denote the shuffle $C$ of two languages $A$ and $B$, it consists of all strings $w$ of the form $w = a_1b_1a_2b_2....a_kb_k$, for $k > 0$, with $a_1a_2 ··· a_k \in A$ and ...
0
votes
2answers
111 views

The image of a recursive language under a computable function

Let $f:\Sigma^{*}\to\Sigma^{*}$ be a computable function and let $L$ be a recursive language. Is $f(L):=\left \{{f(w)|w\in L} \right\}$ recursive? Here, I see clearly, that $f^{-1}(L)$ is recursive ...
-2
votes
1answer
129 views

Is a language closed under string concatenation, repetition, and/or taking substring regular?

Is a language $L$ regular, context-free, context-sensitive, recursively enumerable, or ..., if $L$ is closed under string concatenation, and/or string repetition, and/or taking substring? ...
0
votes
1answer
57 views

Proof that finite automata is closed under intersection

I'm looking at a proof that says that: If $M_1=(Q_1, \Sigma , q_1, A_1, \delta)$ and $M_2=(Q_2, \Sigma , q_2, A_2, \delta)$ are two finite automata(FA) then $M=M_1 \cup M_2$ is also an FA. We define ...
3
votes
1answer
125 views

Generators of families of langauges?

From Wikipedia's definition of regular langauges The collection of regular languages over an alphabet $Σ$ is defined recursively as follows: The empty language $Ø$ is a regular language. ...
1
vote
3answers
134 views

What happens with trios, full trio, (full) semi-AFL, (full) AFL if we require closure under intersection?

Wikipedia says: A trio is a family of languages closed under e-free homomorphism, inverse homomorphism, and intersection with regular language. A full trio, also called a cone, is a trio ...
1
vote
1answer
74 views

Set of $\mathsf{NP}$-hard languages closed under set inclusion?

As the title says, my question is whether the set of $\mathsf{NP}$-hard languages is closed under set inclusion, i.e. whether for any $\mathsf{NP}$-hard language $L$, all subsets of $L$ are also ...
3
votes
1answer
51 views

Situations where Kleene star of A is context-free, but A is not

This question appeared on my Theory of Computer Science final: True | False: $A^*$ is context-free $\implies$ $A$ is context-free. My professor says the answer is false, and I believe him, but am ...
3
votes
2answers
1k views

are NP Complete languages closed under any regular operations?

I have tried looking online, but I couldn't find any definitive statements. It would make sense to me that Union and Intersection of two NPC languages would produce a language not necessarily in NPC. ...
1
vote
3answers
876 views

Is the class NP closed under complement?

Is the class $\sf NP$ closed under complement or is it unknown? I have looked online, but I couldn't find anything.
6
votes
1answer
855 views

how to draw a complement of a Turing Machine?

I am now pretty confident on how I would turn something into a Turing Machine. Now my question is how do you convert TM into a complement of a Turing Machine. From what I can remember in Finite ...
2
votes
3answers
248 views

Does there exist a proof of closure of regular languages under regular substitution by giving the corresponding DFA?

Every proof I can find of this result is by way of regular expressions. Is there any "constructive" proof that defines the corresponding DFA (probably NFA)? For instance the proof of concatenation ...
5
votes
2answers
62 views

Intersection of two NPDAs

Is there a way to take the interection of two NPDAs? I can't seem to find anything that can make that happen, but it seems like the type of thing that is should be relatively trival.
2
votes
0answers
78 views

If $L_1$ is regular and $L_1 \cap L_2$ context-free, is $L_2$ always context-free? [closed]

If $L_1$ is a regular language and $L_1 \cap L_2$ is a context-free language, does it mean that $L_2$ is a context-free language too? I attempted to prove that $L_2$ was not required to be ...
3
votes
1answer
127 views

Prove that context free languages aren't closed under DropMiddle

The question is simple: $\qquad \operatorname{DropMiddle}(L)=\{xy\in\Sigma^* \mid |x|=|y| \land \exists a\in\Sigma\colon xay\in L\}$. Prove that CFL's aren't closed under ...