Equivalence of Definitions of Algebra of Sets
Theorem
The following definitions of the concept of Algebra of Sets are equivalent:
Definition 1
Let $S$ be a set.
Let $\powerset S$ be the power set of $S$.
Let $\RR \subseteq \powerset S$ be a set of subsets of $S$.
$\RR$ is an algebra of sets over $S$ if and only if $\RR$ satisfies the algebra of sets axioms:
\((\text {AS} 1)\) | $:$ | Unit: | \(\ds S \in \RR \) | ||||||
\((\text {AS} 2)\) | $:$ | Closure under Union: | \(\ds \forall A, B \in \RR:\) | \(\ds A \cup B \in \RR \) | |||||
\((\text {AS} 3)\) | $:$ | Closure under Complement Relative to $S$: | \(\ds \forall A \in \RR:\) | \(\ds \relcomp S A \in \RR \) |
Definition 2
An algebra of sets is a ring of sets with a unit.
Proof
Definition 1 implies Definition 2
Let $\RR$ be a system of sets such that $\forall A, B \in \RR$:
- $(1): \quad A \cup B \in \RR$
- $(2): \quad \relcomp X A \in \RR$
Let $A, B \in \RR$.
From the definition:
- $\forall A \in \RR: A \subseteq X$.
Hence from Intersection with Subset is Subset:
- $\forall A \in \RR: A \cap X = A$
Hence $X$ is the unit of $\RR$.
From Properties of Algebras of Sets, we have that $\RR$ is closed under set intersection.
From the definition of symmetric difference:
- $A \symdif B = \paren {A \setminus B} \cup \paren {B \setminus A}$
Since both set union and set difference are closed operations, it follows that symmetric difference is also closed.
So by definition 1 of ring of sets, it follows that $\RR$ is indeed a ring of sets.
$\Box$
Definition 2 implies Definition 1
Let $\RR$ be a ring of sets with a unit $X$.
By definition, $X \in \RR$.
From definition 2 of ring of sets, $\RR$ is:
- $(1) \quad$ closed under set union
- $(2) \quad$ closed under set difference.
From Unit of System of Sets is Unique, we have that:
- $\forall A \in \RR: A \subseteq X$
from which we have that $X \setminus A = \relcomp X A$.
So $\RR$ is an algebra of sets by definition 1.
$\blacksquare$
Sources
- 1970: Avner Friedman: Foundations of Modern Analysis ... (previous) ... (next): $\S 1.1$: Rings and Algebras