# The relationship between logical implication and the subset relation

From Maths

This page is a

*dire page*and is in desperate need of an update.## Definition

[math]A\subseteq B[/math] (and we say "A is a subset of B") if and only if every element of [math]A[/math] also belongs to [math]B[/math]

That is: [math][A\subseteq B]\iff\forall x[x\in A\implies x\in B][/math]^{[1]}

### Note: 16/1/2017 by Alec (talk) 17:36, 16 January 2017 (UTC)

We may often write:

- [ilmath]\forall x\in A[x\in B][/ilmath] instead.

This is easily seen to be equivalent as if [ilmath]A[/ilmath] is empty (so there is no [ilmath]x\in A[/ilmath] to speak of) the implication is semantically true, and the forall is vacuously true.

## References

- ↑ Definition 3.10 (p10) - Introduction to Set Theory, Third Edition (Revised and Expanded) - Karel Hrbacek and Thomas Jech