Loosely speaking, a Jónsson-Tarski algebra is an isomorphism $2^{\aleph_0}\cong 2^{\aleph_0}\times 2^{\aleph_0}$ gone algebra.

Definition

A Jónsson-Tarski algebra, also called a Cantor algebra, is a set$A$ together with an isomorphism$A\cong A\times A$.

More generally, an object $A$ in a symmetric monoidal category$\mathcal{M}$ together with an isomorphism $\alpha:A\otimes A\rightarrow A$ is called a Jónsson-Tarski object, or an idempotent object (Fiore&Leinster 2010).

The structure of a Jónsson-Tarski algebra can be described by an algebraic theory, with one binary operation $\mu$ and two unary operations $\lambda$ and $\rho$ such that $\mu(\lambda(x),\rho(x)) = x$, $\lambda(\mu(x,y))=x$, and $\rho(\mu(x,y))=y$.

Any two Jónsson-Tarski algebras freely generated from finite non empty sets are isomorphic. It was this property they owe their introduction to (Jónsson&Tarski 1961).

The category of Jónsson-Tarski algebras is a topos, the so called Jónsson-Tarski topos$\mathcal{J}_2$, and hence is an example for an algebraic variety that is also a topos (cf. Johnstone 1985).

The Thompson Group? F is the group of order-preserving automorphisms of the free Jónsson-Tarski algebra on one generator.