# Cardinality of Singleton

## Theorem

Let $A$ be a set.

Then $\card A = 1$ if and only if $\exists a: A = \set a$

where $\card A$ denotes the cardinality of $A$.

## Proof

### Sufficient Condition

Assume that

$\card A = 1$

By definition of cardinality of finite set:

$A \sim \N_{< 1} = \set 0$

where $\sim$ denotes set equivalence.

$\set 0 \sim A$

By definition of set equivalence there exists a bijection:

$f: \set 0 \to A$

By definition of bijection:

$f$ is s surjection.

Thus

 $\ds A$ $=$ $\ds \map {f^\to} {\set 0}$ Definition of Surjection $\ds$ $=$ $\ds \set {\map f 0}$ Image of Singleton under Mapping

$\Box$

### Necessary Condition

Assume that:

$\exists a: A = \set a$

Define a mapping $f: A \to \set 0$:

$\map f a = 0$

It is easy to see by definition that

$f$ is an injection and a surjection.

By definition

$f$ is bijection.

By definition of set equivalence:

$A \sim \set 0 = \N_{< 1}$

Thus by definition of cardinality of finite set:

$\card A = 1$

$\blacksquare$