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## Hot answers tagged regular-expressions

11

Your answer is $$\frac{22x-7}{20x-10}$$ And the books "correct" answer is $$\frac{7-22x}{10-20x}$$ Yes? Notice what happens when you multiply both the numerator and denominator in your answer by $-1$? You're very welcome.

9

I am going to write $S_i$ for the set of strings that cause the machine to go from the start state into state $i$. From the diagram of the machine, we have: \begin{align} \def\a{\mathtt{a}}\def\b{\mathtt{b}} S_1 & = \epsilon + S_3\a \\ S_2 & = S_1(\a+\b) + S_2\a + S_3\b \\ S_3 & = S_2\b \end{align} (This part is crucial, and if you don't ...

7

You got the correct answer. You just need to multiply both numerator and denominator by -1 $$\frac{22x-7}{20x-10} = \frac{-1}{-1} \times \frac{7-22x}{10-20x}$$

7

Hint: Show that any two regexps over a $1$-letter alphabet commute, i.e. $PQ = QP$ for every two regexps $P,Q$. Hint to the hint: Show that any two words over a $1$-letter alphabet commute, i.e. $ab = ba$ for every two words $a,b$.

7

You want to show that $$\frac{\sqrt{3}+1}{2\sqrt{2}} = \frac{\sqrt{2+\sqrt{3}}} {2}.$$ Rearrange this into $$2(\sqrt{3} + 1) = 2\sqrt{2}\sqrt{2+\sqrt{3}}.$$ Since both sides are evidently positive, this is equivalent to $$(2(\sqrt{3} + 1))^2 = (2\sqrt{2}\sqrt{2+\sqrt{3}})^2,$$ which simplifies to $$4(4 + 2\sqrt{3}) = 8(2+\sqrt{3}),$$ which is of course true. ...

4

$(1+01)^*$ generates all strings of zeroes and ones with the property that every $0$ is immediately followed by a $1$; these are the strings that do not contain $00$ and do not end in $0$. Similarly, $(0+01)^*$ generates all strings of zeroes and ones in which every $1$ is immediately preceded by a $0$; these are the strings that do not contain $11$ and do ...

4

Let $R$ be the set of words described by the regular expression $r$, and let $S$ be the set of words described by the regular expression $s$. Then the regular expression $(r+s)^*$ describes the words in the set $(R\cup S)^*$, and $r^*+s^*$ describes the words in the set $R^*\cup S^*$. Thus, the question boils down to asking whether it’s necessarily true that ...

4

One easy if tedious approach is to design a finite-state automaton that recognizes $L$ and then apply the algorithm that converts such an automaton to a corresponding regular expression. Going about it directly, I observe that if $w\in L$, every $a$ in $w$ must be immediately followed by another $a$ or a $c$ or be at the end of the word; otherwise there are ...

3

Let $w\in S^*$. Since $(S^*)^*$ contains all concatenations of any finite number of words from $S^*$, it certainly contains $w$, which is a concatenation of just one word from $S^*$. So $S^*\subseteq(S^*)^*$. Conversely, let $w\in(S^*)^*$. By definition we have $$w=w_1w_2\cdots w_n$$ for some $n\ge0$, where each $w_j$ is in $S^*$. By definition again, ...

3

(a) The requirement boils down to saying that once you get $xx$, you can never get a $y$. Your regular expression generates only valid strings, but it doesn’t generate all valid strings. For instance, it doesn’t generate $xyxx$. You want something like $y^*(xy^+)^*x^*$. (b) The second part of your regular expression is fine: it generates precisely those ...

3

Another approach: identities in Kleene algebra... (a) $M \subseteq M^* \subseteq M^*N^* \text{ and } N \subseteq N^* \subseteq M^*N^* \\\text{ so } (M+N) \subseteq M^*N^* \\\text{ so } (M+N)^* \subseteq (M^*N^*)^* \\\text{QED one way}$ (b) $M^* \subseteq (M+N)^* \text{ and } N^* \subseteq (M+N)^* \\\text { so } M^*N^* \subseteq (M+N)^*(M+N)^* ... 3$r$and$s$are regular expressions, which represent regular sets. If$r$and$s$are regular expressions that represent sets$R$and$S$, then the regular expression$r+s$denotes the set$R\cup S$. If$r$is a regular expression denoting the set$R$, then $$r^*$$ is a shorthand for $$\epsilon + r + rr + rrr + \cdots .$$ 3 If something is divisible by 5 then it ends in zero or five. So (1|2|4|5|7|9)*5 would characterize your language. 3 For any fixed value of$n$, the language $$L_{n}=\{w:|w|_a=2^n+273\}$$ is certainly regular, and the regular expression you described will recognize it. But the question here is whether $$L=\bigcup_{n\in\mathbb{N}}L_n = \{w:|w|_a=2^n+273 \text{ for some } n\in\mathbb{N}\}$$ is regular. It's not; and the easiest way to prove it is to use the so-called ... 3 The basic idea for an automated proof is: convert both languages to their deterministic finite state machines. Calculate the cross product of the finite state machines with the transitions$F: ([x,y],\sigma) => [F_x(x,\sigma), F_y(y,\sigma)]$Collect the list of reachable states, and their acceptivity status from both languages. If there are any ... 3 Suppose that I’ve a one-state machine$E_a$that recognizes$a^*$and another,$E_b$, that recognizes$b^*$; combining them in the second way would result in a one-state machine that recognized$(a\lor b)^*$, not$a^*\lor b^*$. This violates both (2) and (3), but you can use similar ideas to show that violating either of them individually can produce ... 3 Try this: $$\frac{(-1)^{n}n(n+1)}{2}+(-1)^{n+1}(n+1)^2 = (-1)^n\bigg(\frac{n(n+1)}{2}-\frac{2(n+1)^2}{2}\bigg)\\ =\frac{(-1)^n(n+1)}{2}(n-2(n+1))\\ =\frac{(-1)^{n+1}(n+1)(n+2)}{2}$$ 3 A comment before I get to your (1)-(3):$p$and$P$are not the same symbol, so either the pumping length is$p$, or it’s$P$, but you shouldn’t keep jumping back and forth between the two. I’ll use$p$. This is correct: the pumping lemma is strictly a tool for showing that a language is not regular. This objection is not really well-taken. It’s true that ... 3 It has problems in both directions: it doesn’t accept the empty word, which is in the language, and it does accept a lot of words, like$aba$and$abb$, that aren’t in the language. The first problem is easily fixed: just make$q_0$an acceptor state. The second requires some more significant changes. The$b$-transition from$q_3$should go to$q_1$, the ... 3 Usually, in a monoid, we define$x^0$to be the identity element of the monoid, that is, the element$e$such that$ex = x = xe$for all$x$. This is so that the standard "exponent rules" such as$x^{a+b} = x^a \cdot x^b$hold even when$a$or$b=0$. In your case, since the operation is concatenation, it makes sense for$x^0$to mean the empty string, since ... 3 Let$H$denote the unit step function.$xH(x)$is zero when$x$is negative and equal to$x$when$x$is positive.$xH(x-1)$is zero when$x$is less than 1 and equal to$x$otherwise. If I'm not mistaken, $$xH(x) - (x-1)H(x-1)$$ is the function you want. 3 $$f(x) = \frac{|x|+x}{2} - \frac{(x-1)+|x-1|}{2}.$$ (This can be simplified a bit.) 2 A language is just a set of strings. A language$L$over$\Sigma$is a subset of$\Sigma^\ast$, which is the set of all strings made from letters in$\Sigma$.$L$contains some of the strings in$\Sigma^\ast$, but not necessarily all; it might contain, or not contain, any string of letters from$\Sigma$.$\Sigma^\ast$itself is a countable set, and a ... 2 $$\frac{\sqrt{2+\sqrt{3}}}{2}=\frac{\sqrt{4+2\sqrt{3}}}{2\sqrt{2}}=\frac{\sqrt{(\sqrt{3})^2+2\sqrt{3}+1}}{2\sqrt{2}}=\frac{\sqrt{(\sqrt{3}+1)^2}}{2\sqrt{2}}=\frac{\sqrt{3}+1}{2\sqrt{2}}$$ 2 Hint: Let$A = (a^*b)^*$and$A' = \Sigma^* \setminus A$(i.e. the complement of$A$). Observe that any word$w$that ends with the letter$b$belongs to$A$. Observe that$A' = \Sigma^*a$. I hope this helps$\ddot\smile$2 The definition of$A$is misleading. There is no reason to ever have$k>1$. Think of it as $$A=\{1y|y\in\{0,1\}^\star\textrm{ and }y\textrm{ contains at least one }1\}$$ The question from the commments is why does the regular expression equal this. There are many ways to express a string that contains at least one 1.$(0\cup 1)^\star10^\star$is one ... 2 Convert the regular expression to a finite automaton accepting$L$. Then interchange accepting and non-accepting states, which produces an automaton accepting$\Sigma^*\setminus L$. Finally, convert the automaton to a regular expression. 2 In regular expression the symbol$\cup$means essentially or: the regular expression$01\cup 10$, for instance, is$01$or$10$and is matched by both$01$and$10$. If$S$is the set of strings matching a regular expression$\sigma$, the strings that match$\sigma^*$are the ones that can be formed by concatenating any finite number of strings in$S$. If ... 2 HINTS: Look at the grammar, find some stuff that you can produce, find some stuff that you can't produce. For instance, I can produce the strings$a, aa, aaa, aaaa, ...$. I can also produce the strings$bb, bab, baab, ...$. I cannot produce the strings$ab, aba, abaa, abaaa...$Now, this is written in a way that makes it really easy to convert it into a ... 2 This solution is due to @prateek. We will inductively decide whether a given regex$\alpha$is ambiguous. If$\alpha = \epsilon$,$\alpha = a \in \Sigma$, or$\alpha = \varnothing$, it is unambiguous. If$\alpha = \beta+\gamma$, then$L(\alpha) = L(\beta) \cup L(\gamma)$, so,$\alpha$is unambiguous if and only if both$\beta$and$\gamma\$ are ...

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