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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Notation of cross entropy

I have a question regarding a notation that seems to be very usual. For starters, cross entropy is defined by: \begin{align}H(X, q) &= H(X) + D(p||q) \\ & =-\sum_x p(x)\log_2 q(x)\end{align} ...
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Is this language context-free or not?

I have a problem to solve, the problem is: Is the language of strings $$L=\{0^x1^y:x\nmid y\}$$ context free? I suspect it isn't, I spent some time trying to make a grammar that could ...
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Pumping Lemma for regular languages proof template

http://www.cs.uiuc.edu/class/fa06/cs273/Lectures/pumping-lemma/pumping-lemma.html So, I went to that site and it says: $w = xyz$ $|xy| \leq p$ $|y| \geq 1$ for all $i$, $xy^iz$ is in ...
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Repeated rules in Chomsky normal form

My question is simple, when you're converting a grammar to CNF, what happens when a rule begins to repeat multiple times? ¿It's good to end with rules like $U_1 \rightarrow SB, U_2 \rightarrow SB, ...
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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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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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39 views

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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371 views

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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25 views

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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17 views

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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Check if language $L^2$ is regular

$L=\{w|w\in\{0,1\}^*\wedge \#_1(w)=\#_2(w)\}$, where $\#_a(w)$ denotes number of symbol $a$ in word $w$. Check if $L^2$ is regular. So idea is: $L^2 \cap 0^*1^*0^*=\{0^n1^{2n}0^n|n\ge 0 \}\notin ...
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$L$ is regular. Prove that $D(L)$ is also regular

I ask you for look at my solution: $L$ is regular. Prove that $D(L)=\{w|ww^R\in L, w\in\Sigma^*\}$ is also regular. Idea I go through states from two places (two fingers). When fingers meet in the ...
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Show that language $L'$ is regular given $L$ is regular

I show you some solution and I ask you for looking at it. $L'=\{y|\exists_{z,x} xyz\in L\wedge |x|=|y|=|z|\}$ Automaton for language $L$: $M=(Q,\Sigma, \delta, q_0, F)$ For language $L':$ $M'=(Q', ...
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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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149 views

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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48 views

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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Minimal DFA for a given regular expression

How can I construct a minimal DFA from the following definition? $L=\{w \in \{a,b,c\}^* $: if the second-to-last letter from w is an $a$, then the number of $c$'s $\le 1\}$ I've already made a ...
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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 ...
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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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48 views

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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Proving that language is regular or not regular

Let $L$ be a regular language. Prove that: $L_{+--}=\left\{w: \exists_u |u|=2|w| \wedge wu\in L\right\}$ $L_{++-}=\left\{w: \exists_u 2|u|=|w| \wedge wu\in L \right\}$ ...
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Isn't the set of all literature a regular language?

Let one piece of literature be one string. Let's define our alphabet to be sufficient to represent all literature (e.g. we may need a page-turn character, etc). So, since the collection of current ...
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Inequality for the set of factors of length $n$ of some regular language

If $W \subseteq X^*$ is some language denote by $$T(W) := \{ u \in X^* : \mbox{there exists }x, y \mbox{ such that } xuy \in W \}$$ the set of factors (infixes) of $W$. If $W$ is regular, then ...
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How can one prove that the Thue-Morse sequence is a morphic sequence?

The Thue-Morse sequence is the infinite sequence $\mathbf{a} = (a_n)_{n \geq 0}$ with $a_n = 0$ if the sum of the digits of the binary expansion of $n$ is even and $a_n = 1$ otherwise. Consider the ...
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Periodic Shifts of finite type

I am learning about periodic shifts and want to look at finite types of periodic shifts. So I have this theorem: A shift space $X$ is an $M$-step shift of finite type iff whenever $uv, \; vw\; \in ...
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Is my LL(1) parse table correct?

I have the following Grammar: ...
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construct automat for language $Cycle(L)$

We have automat $A$ for language $L$. Construct automat for $Cycle(L)$ where $Cycle(L)=\{uv:vu\in L\}$. I have a problem with this exercise. Help me, please. Edit $A = (Q_A, \Sigma, \delta, q_0, ...
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Is the following language $L$ regular?

$L=\{ww^Rv\mid v,w\in \{a,b\}^+\}$ Is $L$ regular ? Edit Is it ok? Let $p$ will be length of pumping lemma. Then, $a^pb(a^pb)^Rv\in L$. When $a^p(a^pb)^Rv = xyz$ $p\ge |y|\ge 1 $ So ...
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How to prove that Pumping lemma can't be used to prove regular languages.

I need a prove that pumping lemma can't be used to prove regular language. Pumping lemma is only used for proving non-regular language, but I need to show that how it can't be used to prove regular ...
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regularity of language $D(L)$, $L$ is regular

Show that $D(L)$ is regular, $L$ is regular. $D(L) = \{w|w\in \Sigma^* \wedge ww^R\in L\}$ Let assume that M has only one state accepting. Therefore, $M'$ has only one state start. ($M'$ recognizes ...
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Conjunction and XOR of Regular Language to DFA

If you could help me with this homework it would be greatly appreciated. If we assume that $M(i)$ is the finite automaton that recognises the regular language $L(i)$ for $i = 1,2,\ldots,n$, how can I ...
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Prove that whenever L is a regular language so is HasPrefix(L)

For a language $L$ over an alphabet $\Sigma$, define the language $\operatorname{HasPrefix}(L)$ as follows: $$\operatorname{HasPrefix}(L) = \{xy\mid x,y \in\Sigma^*, x \in L, xy \in L, |y|> 0\}$$ ...
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Origin of the stable distribution name

Mandelbrot (1963) claimed the name stable distribution comes from Paul Levy work: "...The purpose of this paper will be to present and test such a new model of price behavior in speculative markets. ...
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contradiction of pumping lemma on even palidrome language's completion

I have the language of all the words that are not even length palindromes, or more formally: $L= \{ w: \forall u\in\Sigma^*, w\ne uu^r \}$. And I need to prove that the Pumping Lemma for regular ...
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81 views

Pumping lemma for $a^n b^{2n}$

I have this question and i am not sure and trying to solve in both way that is through diagram and expression: Question : Prove that $a^nb^{2n}$ is not regular. contradiction: ...
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28 views

productions with identical right hand sides for different nonterminals

Can a regular grammar contain two productions with identical right hand sides? I know in context free grammars shift reduce parsers issue reduce reduce conflicts when smth like this happens. What are ...