# Tagged Questions

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### Formal Notation - A simple example

I'm taking Formal Languages and Automata Theory course and i couldn't understand the notation below. Can someone explain me this please? $\{ d \mid d \in \{ b f \mid b, f \in \{ a, c, e \} \} \}$
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### Sigma hierarchy of logical formulae

In some papers on mathematical logic I've found references to hierarchy like $\Sigma_1^0$-sentence and so on. What does it mean? What is $\Sigma_n^m$, what is $n$ and $m$ here?
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### Converting a DFA to an NFA: Start State

When we try convert some NFA $N = (Q_N,\Sigma,\delta_N,q_0,F_N)$ to a DFA $D = (Q_D,\Sigma,\delta_D,\{q_0\}, F_D)$, why is it that $q_0$ becomes $\{q_0\}$? (Where $q_0$ is a single start state, ...
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### Formal Languages: String Notation

In formal languages, we usually denote an alphabet $\Sigma$ as the set of symbols it contains, e.g. $\Sigma = \{0,1\}$ is the binary alphabet. It can be confusing because we denote $\Sigma^1$, the ...
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### Modern approaches to mathematical notation

I'm interested if there is literature on projects which try to improve formal notation, especially for doing mathematics on an advanced level. For example, I'm thinking along the lines of diagrammatic ...
I'm having trouble with the following exercise: Let $\Sigma = \{a,b,c\}$ and $L$ be a formal language, that consists of all words which contain all three letters at least once. Show that $L$ is ...