User:Jshflynn/Empty Word is Unique
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Theorem
The empty word over an alphabet is unique.
Proof
Let $\Sigma$ be an alphabet.
The empty word is defined to be any word of length $0$.
Supposing there is more than one empty word and we choose any two of them labelling them $\lambda$ and $\lambda '$.
Then by assumption:
- $\operatorname{len}(\lambda) = \operatorname{len}(\lambda ') = 0$ .
And:
- $\forall i: \lambda_i = \lambda'_i$ (Holds vacuously)
So by the definition of word equality:
- $\lambda = \lambda'$.
Hence there is only one empty word over $\Sigma$
$\blacksquare$