Definition:Subclass
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Definition
Let $A$ and $B$ be classes.
Then $A$ is a subclass of $B$, and we write $A \subseteq B$, if and only if:
- $\forall x: \paren {x \in A \implies x \in B}$
where $x \in A$ denotes that $x$ is an element of $A$.
Proper Subclass
Let $A$ and $B$ be classes.
Let $B$ be a subclass of $A$.
Then $B$ is a proper subclass of $A$ if and only if $B \ne A$.
Also see
- Results about subclasses can be found here.
Sources
- 2002: Thomas Jech: Set Theory (3rd ed.) ... (previous) ... (next): Chapter $1$: Classes
- 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: Definition $0.1$