Algebra

Let $X$ be a nonempty set. An algebra $A\subseteq 2^{X}$ is a nonempty collection of subsets of $X$ that is closed under finite unions and complements.

By DeMorgan's laws, an algebra is also closed under finite intersections, and also contains the empty set and $X$ itself.

Examples of -algebras

Assume that $X$ is nonempty.

• Given a set $X$, then $2^{X}$ and $\{\emptyset ,X\}$ are algebras.
• Given a set $X$, the collection of all finite and cofinite (having finite complement) subsets of $X$ is an algebra.
• A $\sigma$-algebra is a particular example of an algebra.

Non-examples

• Given a topological space $(X,\tau )$, the topology $\tau$ is in general not an algebra: for $X=\mathbb {R}$ with the standard topology, the open interval $(0,1)$ is open, but its complement $(0,1)^{c}=(-\infty ,0]\cup [1,+\infty )$ is not.