Definition:Relation of Set Inclusion

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Definition

The relation of set inclusion, written $\subseteq$, is the class of all ordered pairs $\left({x, y}\right)$ such that $x$ is a subset of $y$.


Also see


Sources

2010: Raymond M. Smullyan and Melvin Fitting: Set Theory and the Continuum Problem (revised ed.): $\S 4.1$