Singleton Classes are Equal iff Sets are Equal
Jump to navigation
Jump to search
Theorem
Let $a$ and $b$ be sets.
Let $\set a$ and $\set b$ denote the singleton classes of $a$ and $b$.
Then:
- $\set a = \set b \iff a = b$
Proof
Let $a = b$.
Then $\set a$ and $\set b$ contain the same elements.
Hence by the axiom of extension it follows that $\set a = \set b$.
Let $\set a = \set b$.
We have that $a \in \set a$.
As $\set a = \set b$ we also have that $a \in \set b$.
But $b$ is the only element of $\set b$.
Hence it must be the case that $a = b$.
$\blacksquare$
Sources
- 2010: Raymond M. Smullyan and Melvin Fitting: Set Theory and the Continuum Problem (revised ed.) ... (previous) ... (next): Chapter $2$: Some Basics of Class-Set Theory: $\S 4$ The pairing axiom: Note $3$.