Singleton is Finite
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Theorem
Let $x$ be arbitrary.
Then $\set x$ is a finite set.
Proof
Define a mapping $f: \set x \to \N_{< 1}$:
- $\map f x = 0$
By definition of singleton:
- $\forall y, z \in \set x: \map f y = \map f z \implies y = z$
By definition:
- $f$ is an injection.
By definition of initial segment of natural numbers:
- $\N_{< 1} = \set 0$
By definition of $f$:
- $\forall n \in N_{< 1}: \exists z \in \set x: \map f z = n$
By definition:
- $f$ is a surjection.
By definition:
- $f$ is a bijection.
By definition of set equivalence:
- $\set x \sim \N_{< 1}$
Thus by definition:
- $\set x$ is a finite set.
$\blacksquare$
Sources
- Mizar article FINSET_1:funcreg 1