In abstract algebra, a Robbins algebra is an algebra containing a single binary operation, usually denoted by ∨ {\displaystyle \lor }, and a single unary operation usually denoted by ¬ {\displaystyle \neg } satisfying the following axioms: For all elements a, b, and c: Associativity: a ∨ = ∨ c {\displaystyle a\lor \left=\left\lor c} Commutativity: a ∨ b = b ∨ a {\displaystyle a\lor b=b\lor a} Robbins equation: ¬ = a {\displaystyle \neg \left=a} For many years, it was conjectured, but unproven, that all Robbins algebras are Boolean algebras. Wikipedia