Abbreviation: CloA

A \emph{closure algebra} is a modal algebra $\mathbf{A}=\langle A,\vee,0, \wedge,1,\neg,\diamond\rangle$ such that

$\diamond$ is \emph{closure operator}: $x\le \diamond x$, $\diamond\diamond x=\diamond x$

Remark: Closure algebras provide algebraic models for the modal logic S4. The operator $\diamond$ is the \emph{possibility operator}, and the \emph{necessity operator} $\Box$ is defined as $\Box x=\neg\diamond\neg x$.

Let $\mathbf{A}$ and $\mathbf{B}$ be closure algebras. A morphism from $\mathbf{A}$ to $\mathbf{B}$ is a function $h:A\to B$ that is a Boolean homomorphism and preserves $\diamond$:

$h(\diamond x)=\diamond h(x)$

Example 1: $\langle P(X),\cup,\emptyset,\cap,X,-,cl\rangle$, where $X$ is any topological space and $cl$ is the closure operator associated with $X$.

$\begin{array}{lr} f(1)= &1
f(2)= &
f(3)= &
f(4)= &
f(5)= &
f(6)= &
\end{array}$

Monadic algebras

Modal algebras