# What is the quantified definition for proper subset

We're thinking its:

$A \subset B \leftrightarrow \forall x [x \in A \rightarrow x \in B] \land \exists x [x \notin A \land x \in B]$

is this OK?

Thanks,

z.

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Yes, that works. –  Brian M. Scott Jul 4 '12 at 17:19

As Brian says: Yes, that works. ${}{}{}{}{}{}{}{}{}{}{}{}{}{}$
You could use the axiom of extensionality to make a simpler definition: $$A\subsetneq B =_\mathrm{def} (\forall x\in A. x\in B)\land \lnot(A=B).$$