Simple and sub-directly irreducible double Boolean algebras

Abstract: Double Boolean algebras are algebras $\underline{D}=(D;\sqcap,\sqcup,\neg,\lrcorner,\bot,\top)$ of type $(2,2,1,1,0,0)$ introduced by Rudolf Wille to capture the equational theory of the algebra of protoconcepts. Every double Boolean algebra $\underline{D}$ contains two Boolean algebras denoted by $\underline{D}{\sqcap}$ and $\underline{D}{\sqcup}$. A double Boolean algebra $\underline{D}$ is said pure if $D=D_{\sqcap}\cup D_{\sqcup}$, and trivial if $\bot\sqcup\bot=\top\sqcap\top$. In this work, we first show that a double Boolean algebra is pure and trivial if and only if it is a glued sum of two Boolean algebras; secondly, we characterize simple double Boolean algebras; and finally, we determine up to isomorphism all sub-directly irreducible algebras of some sub-classes of the variety of double Boolean algebras.