Quasivariety

A trivial algebra contains just one element. A quasivariety is a class K of algebras with a specified signature satisfying any of the following equivalent conditions.[1]

1. K is a pseudoelementary class closed under subalgebras and direct products.

2. K is the class of all models of a set of quasi-identities, that is, implications of the form ${\displaystyle s_{1}\approx t_{1}\land \ldots \land s_{n}\approx t_{n}\rightarrow s\approx t}$, where ${\displaystyle s,s_{1},\ldots ,s_{n},t,t_{1},\ldots ,t_{n}}$ are terms built up from variables using the operation symbols of the specified signature.

3. K contains a trivial algebra and is closed under isomorphisms, subalgebras, and reduced products.

4. K contains a trivial algebra and is closed under isomorphisms, subalgebras, direct products, and ultraproducts.

Every variety is a quasivariety by virtue of an equation being a quasi-identity for which n = 0.

The cancellative semigroups form a quasivariety.

Let K be a quasivariety. Then the class of orderable algebras from K forms a quasivariety, since the preservation-of-order axioms are Horn clauses.[2]