We begin by recalling the basic notions of set theory, which should be familiar to everybody:
Let be subsets of some universal set .
- The union of and is .
- The intersection of and is .
- The sum of and is .
We will follow the convention to denote the intersection of and just by juxtaposition: .
Given a universal set , the subsets of have the algebraic structure of a ring:
Let be a universal set. Let , the power set of .