# Tagged Questions

Regular languages are formal languages which are recognized by a finite automaton. It is equivalently the languages which are expressible as a regular expression. In addition to these two, there are several other equivalent definitions.

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### This proof in my textbook involving the pumping lemma appears incorrect - is it?

It states Let $B$ be the language $\{0^n1^n2^n | n \geq 0\}$. We use the pumping lemma to prove that $B$ is not regular. The proof is by contradiction. Assume to the contrary that $B$ is regular. ...
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### Applying the Myhill-Nerode Theorem

Consider the language $$L=\{x y^{(n)} z y^{(n)} w: x,z,w \in \Sigma^*, y \in \Sigma, z\text{ does not contain }y, n \geq 0 \}.$$ To show that the language is not regular using the Myhill-Nerode ...
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### Is regular following language?

I try to prove that the language $L$ is not regular: $$L = \{w\in(a+b)^*:\#_a(u)>2009\#_b(u)\ \text{for every nonempty prefix u of word w} \}$$ Note: $\#_a(u)$ means the number of symbols $a$ ...
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### Pumping lemma for $a^nb^{2n+1}$

I know how to solve pumping lemma for $a^nb^n:n\geq 0$. But I don't understand how can I solve this example : $a^nb^{2n+1}:n\geq 0$. I tried to solve it but I am not sure that I have solved it ...
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### Regular Expressions with Repetition

I'm learning about regular expressions and how they represent regular languages of an alphabet. Conceptually, I'm having trouble imagining what a regular expression would look like, representing a ...
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### General question about pumping lemma statement for regular languages

According to the formal statement of the lemma here: https://en.wikipedia.org/wiki/Pumping_lemma_for_regular_languages It is written at (3) that for all $i≥0, xy^iz∈L$. Until this ...
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### Is $L = \{a^n b^m b^m \mid n,m \ge 0 \}$ regular language or not regular language?

Is $L = \{a^n b^m b^m \mid n,m \ge 0 \}$ regular language or not regular language? I think that $L$ is regular because the regular expression a*(bb)* describes ...
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### Question about periodic points in shift spaces

Let $A$ be a finite set endowed with the discrete topology. Then, the pair $(A^{\mathbb{Z}}, \sigma)$ is said to be the full shift over the alphabet $A$ where $A^{\mathbb{Z}}$ is endowed with the ...
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### Trouble with induction on the length of a word

In the accepted solution of the question If L is regular, prove that $\sqrt{L}=\{w:ww\in L\}$ is regular the answerer made the claim that "What's left is to show that $δ ′ (q_{0}' ,w)=h$ , which can ...
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### show that language is regular

Let $B_n = \{a^k\ |\text{ where } k\text{ is a multiple of } n\}$. Show that for each $n\ge 1$ the $B_n$ language is regular. My proposition of solution: What about it ?
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### Expressiveness of finite memory programs

Assume we have a simple programming language with while, if, := (assignment), ...
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### Regular, Context-free, Recursive, Recursively Enumerable Language Relationships

I am currently under the believe that all regular languages are context free, thus all regular languages are recursive, and therefore all regular languages are recursively enumerable. If I am given ...
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### Elementary proof that a particular language is not regular

I want to show that given an alphabet $A = \{ L, R \}$, the language $$\mathcal{L} = \{ x_{1} \ldots x_{n} \in A^{*} : \# \{ j \leq n : x_{j} = L \} = \# \{ j \leq n : x_{j} = R \} \}$$ cannot be ...
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### Is $r(^∗)=r^∗$ valid regular expression?

Which of the following regular expression identities is/are TRUE? $r(^∗)=r^∗$ $(r^∗s^∗)=(r+s)^∗$ $(r+s)^∗=r^∗+s^∗$ $r^∗s^∗=r^∗+s^∗$ My attempt : I can't say anything, but it should be invalid....
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### Which automata recognizes the language defined by the regular expression

I am trying to understand why the following NFA is the one which recognizes the language defined by the regular expression: $(00+10)0^*((1100+1110)0^*)^*$ As far as I can see the NFA will get stuck ...
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### Why does a certain turing machine only accept regular languages?

why does a single tape turing machine, which can not write on the part of its tape containing the input, only accept regular languages? I am asking this because this question is being opposed to me ...
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### If $L$ is context-free, then $L$ is regular

Let $L \subset \{a\}^*$ and assume that $L$ is context-free. Prove that $L$ is regular. Please hint me.
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### How many states (minimally) for an automaton recognizing the set of words without palindromic subwords on a $k$-letter alphabet?

Let $\Sigma_k = \{0,1,...,k\}$ and let $NPA_{k}$ be the set of words that do not contain a palindrom (of length $\ge 2$) as a subword. How many states (minimally) must have an automaton that ...
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### Are all finite languages regular?

I've been thinking about this for a while and still cannot come up with a way to show that all finite languages are regular. I know that all finite languages consist of finite number of strings that ...
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### prove that language is regular (other language is regular)

Let $L$ is regular. Prove that $L'$ is regular. $L'=\{uv: u\cdot rev(v)\in L\}$ $rev(v)=v^{-1}$ Idea: $L^{-1}$ is regular and recognized by $R$, and $L$ by $M$. Let's assume that $M$ is NFA such ...
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### Prove that language is regular given other regular language

Let $L\subseteq A^*$ be regular. Prove that $L'$ is regular where $L' = \{vw \mid \exists u\in A^*\ vuw \in L\wedge |u|=|w|+|v|\}$ Help me please. It is very hard for me, I don't know how to start. ...
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### Why is $a^nb^n|n\geq1$ not regular and $a^nb^n|n\leq {10^{10}}$ regular?

I've heard somewhere that since the latter is bounded, it is regular. Can anyone explain me what a bound actually means? And if the latter is regular, then how would you write the regular expression ...
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### Help with proving a language is regular (Sipser problem 1.49a)

I am working through Sipser's Introduction to the theory of computation on my own, so I don't have access to a teacher. Hopefully you can help me! Giving hints is highly appreciated. This question is ...
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### Is this correct way to prove that { $uv$ $\mid$ $u$ and $v$ are strings over $(0,1)$ & $|u| = |v|$ } language is not regular?

I proved this using Pumping Lemma in the following way, Taking $p$ as pumping length then let us take a string $S$ where, $|u| = |v| = p$ Now according to pumping lemma $S$ can be split into three ...
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### Nonregular languages that satisfy the pumping lemma at different strengths

There are three versions of the pumping lemma that I've seen, each one stronger than the last (as in it fails on some non-regular languages that pass the weaker ones) The three versions are as ...
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### Proving irregularity using Myhill-Nerode theorem

I'm trying to prove that the following language is irregular using the Myhill-Nerode theorem $$L = \{ w\space\epsilon \{a,b,c\}^* | \#_b(w) > (\#_a(w) + \#_c(w))! \}$$ While it's completely ...
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### Why every regular language is in $\text{TIME}(n)$?

How can I prove that every regular language $R$ has linear time complexity, i.e. every regular language satisfies $$R \in \text{TIME}(n)$$
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### $DFA/NFA$ for $L(OPPOSITE)=\{uv:vu\in L\}$

I'm trying to prove that: $L(OPPOSITE)=\{uv:vu\in L\} \in L_{FA}$ given that: $L \in L_{FA}$ . I'm trying to construct a finite automata that accepts $L(OPPOSITE)$ in order to prove it but I got stuck....
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### Constructing a Turing decidable machine from DFA

I am a trying to prove that every regular language is decidable. So in order to prove that I am trying to show that I can move from deterministic finite automaton (DFA) to a Turing decidable machine. ...
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### Systematic way of creating the complement of a regular grammar?

Regular languages are closed under complement. And any regular language can be generated using a regular grammar. Is there a systematic way to create the rewrite rules for the complement of a regular ...
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### Regular Functional Algorithms

A language is regular if it is accepted by a read-only Turing machine. I am curious about applying this model to functional problems rather than decision problems. Definition: A functional read-only ...
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### First and Follow for the Context Free Grammar

I am trying to understand how to calculate first and follow for given rules Let's say here are two grammars. They are quite unusual so I am not sure if I made any mistakes. ...
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### Converting Automata To Regular Expression Using State Removal Method

From the following automaton this solution is given: $$(a\mid b)^*aa(ba)^*a(a\mid b)^*$$ But when I try to convert this automaton into a regular expression I always end up with the wrong ...
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### How to prove that a simple NFA is minimal, without any algorithm?

First, I will present the question I was doing: We will define $p(L)$ to be the minimal natural number so that a language L fulfill the pumping lemma. We will also define $n(L)$ to be the minimal NFA ...
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### Is the resulting language regular?

If $L$ is a regular language then is $L'=\{w \mid wx \in L \text{ for some string }x\}$ regular? First step is understand $L'$. So it is a subset of $L$ that contains strings with a certain prefix?
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### Prove that the set of palindromes are not regular languages

Let L = {w| w ∈ {a,b,c} * is palindrome} Could someone explain me how to prove that L is not regular, because all answers I've found are done with 2 symbols(a,b), and I'd need to prove it with 3.
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### Regular Expression for a Set of Strings of Even Length

Can the language for the set of even strings be represented by L={ε,aa,ab,ba,bb.....} Isnt Epsilon Odd?
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### Prove that $L = \{0^n1^m \mid n ≥ 10, m ≤ 50\}$ is regular and that any subset of it is regular

Question: Let L = {0n1m, n ≥ 10 m ≤ 50}. Prove that this is a regular language and that any subset of it is also regular. Answer or approach: 0 is regular, 1 is regular since any symbol in ∑ is ...
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### Closure property of regular languages

I am trying to understand the closure property of regular languages. I am defining two functions func1(w) and func2(w) over alphabet Σ for any string w ∈ Σ∗. func1(w) gets the string of symbols of w ...
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### What does it mean “the transition function of FA M is derivable in CFG G”?

I don't understand this question. Let $M$ be a finite automaton. If every production of G is accepted by M and the transition function of $m$ is derivable in G, then $L(G)$ = $L(M) ?$ What does ...
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### Topology on a free monoid using regular languages.

A free monoid together with arbitrary unions of regular language subsets forms a topological free monoid. Every free monoid homomorphism is continuous with respect to the topology described in 1. ...
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### Using Pumping Lemma to show a language is not regular

Let $$L=\left\{0^n 1^{2n} \mid n > 1\right\}.$$ Show that $L$ is not regular. Attempt: If the language is regular, it must satisfy the Pumping lemma. $P$ will be our pumping length and our string ...