In mathematics, a quasivariety is a class of algebraic structures generalizing the notion of variety by allowing equational conditions on the axioms defining the class.
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Definition
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. K is a pseudoelementary class closed under subalgebras and direct products.
2. K is the class of all models of a set of quasiidentities, that is, implications of the form
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.
Examples
Every variety is a quasivariety by virtue of an equation being a quasiidentity for which n = 0.
Every class of ordered algebras is a quasivariety, since the partial order axioms are quasiidentities.