연습장

Ex. 1

Existence)
Let $w$ be any string over an $\mathrm{\Sigma }$.
By the definition, the domain of $w$ is a finite initial segment of $\mathbb{\mathbb{N}}$.
This means that $\mathrm{\exists }n\in \mathbb{\mathbb{N}}$ s.t. w is defined at $i\leftrightarrow i<n$

Uniquness)
Suppose that $\mathrm{\exists }n,m$ s.t. both satisfy the condition for the string $w$.
That means, w is defined at position $i$ if and only if $(i<n)\wedge (i<m)$.
By definition, $\{i\in \mathbb{\mathbb{N}}|i<n\}$ and $\{i\in \mathbb{\mathbb{N}}|i<m\}$ are both domain of $w$, which means they are same set.
So the only way that $\{i\in \mathbb{\mathbb{N}}|i<n\}$ and $\{i\in \mathbb{\mathbb{N}}|i<m\}$ are same is their endpoints are same. Therefore, $n=m$.

Hence, there exists a unique natural number $n$ with the desired property.

Ex. 2

Existence)
By definition, the domain of $w$ is the finite initial segment of $\mathbb{\mathbb{N}}$.
If the domain is $\mathrm{\varnothing }$, then there is no position $i\in \mathbb{\mathbb{N}}$ where the $w$ is defined.
In this situation, the length of the string $w$ is 0.
Hence, there is a string $w$ s.t. $|w|=0$.

Uniquness)
Suppose that there is two sets $u,v$ s.t. $|u|=0,|v|=0$.
Then $u,v$ are not defined at any index, so both are empty relation, subset of $\mathbb{\mathbb{N}}\times \mathrm{\Sigma }$.
So $u,v$ are both empty, which means $u=v$.

Hence, there is the a unique empty string $w$ s.t. $|w|=0$, and we can define $ϵ$ as the empty string $w$

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