Symmetric Difference with Intersection forms Ring/Proof 1
Theorem
Let $S$ be a set.
Let:
- $\symdif$ denote the symmetric difference operation
- $\cap$ denote the set intersection operation
- $\powerset S$ denote the power set of $S$.
Then $\struct {\powerset S, \symdif, \cap}$ is a commutative ring with unity, in which the unity is $S$.
This ring is not an integral domain.
Proof
From Symmetric Difference on Power Set forms Abelian Group, $\struct {\powerset S, \symdif}$ is an abelian group, where $\O$ is the identity and each element is self-inverse.
From Power Set with Intersection is Monoid, $\struct {\powerset S, \cap}$ is a commutative monoid whose identity is $S$.
Also Intersection Distributes over Symmetric Difference.
Thus $\struct {\powerset S, \cap}$ is a commutative ring with a unity which is $S$.
From Intersection with Empty Set:
- $\forall A \in \powerset S: A \cap \O = \O = \O \cap A$
Thus $\O$ is indeed the zero.
However, from Set Intersection Not Cancellable, it follows that $\struct {\powerset S, \symdif, \cap}$ is not an integral domain.
$\blacksquare$
Sources
- 1970: B. Hartley and T.O. Hawkes: Rings, Modules and Linear Algebra ... (previous) ... (next): Chapter $1$: Rings - Definitions and Examples: $2$: Some examples of rings: Ring Example $6$