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Revision difference : standard enumeration
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The standard enumeration of \lbrace 0,1\rbrace ^{*} is the sequence of strings $s_0 =\lamda$, $s_1 = 0$, $s_2 = 1$, $s_3 = 00$, $s_4 = 01$, $\cdots$ in lexicographic order. The standard enumeration of \lbrace 0,1\rbrace ^{*} is the sequence of strings $s_0 =\lamda$, $s_1 = 0$, $s_2 = 1$, $s_3 = 00$, $s_4 = 01$, $\cdots$ in lexicographic order.
The characteristic function of a language $A$ is $\mathcal{X}_{A}:\mathbb{N}\rightarrow \lbrace 0, 1\rbrace$ such that The characteristic function of a language $A$ is $\mathcal{X}_{A}:\mathbb{N}\rightarrow \lbrace 0, 1\rbrace$ such that $\mathcal{X}(n)=\begin{cases}
\[\mathcal{X}(n)=\begin{cases}
0,\text{ if }s_n \in A\\ 0,\text{ if }s_n \in A\\
1,\text{ if }s_n \notin A 1,\text{ if }s_n \notin A
\end{cases}.\] \end{cases}$