## Quick Reference

Let *A* be a subset of *B*. Then *A* is a proper subset of *B* if *A* is not equal to *B* itself. Thus there is some element of *B* not in *A*. The subset *A* is then said to be properly (or strictly) included in *B*, and this is written *A* ⊂ *B*. Some authors use *A* ⊂ *B* to mean *A* ⊆ *B* (see subset), but they then have no easy means of indicating proper inclusion.

*Subjects:*
Mathematics.

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