Subsection 2.3.2 (0030): Singleton Sets

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Let $X$ be a set.

The singleton set containing $X$ is the set $\webleft\{ X\webright\} $ defined by

\[ \webleft\{ X\webright\} \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\webleft\{ X,X\webright\} , \]

where $\webleft\{ X,X\webright\} $ is the pairing of $X$ with itself (Definition 2.3.3.1.1).

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