Imaginary element

In model theory, a branch of mathematics, an imaginary element of a structure is roughly a definable equivalence class. These were introduced by, and elimination of imaginaries was introduced by.

Definitions

 * M is a model of some theory.
 * x and y stand for n-tuples of variables, for some natural number n.
 * An equivalence formula is a formula φ(x,&thinsp;y) that is a symmetric and transitive relation. Its domain is the set of elements a of Mn such that φ(a,&thinsp;a); it is an equivalence relation on its domain.
 * An imaginary element a/φ of M is an equivalence formula φ together with an equivalence class a.
 * M has elimination of imaginaries if for every imaginary element a/φ there is a formula θ(x,&thinsp;y) such that there is a unique tuple b so that the equivalence class of a consists of the tuples x such that θ(x,&thinsp;b).
 * A model has uniform elimination of imaginaries if the formula θ can be chosen independently of a.
 * A theory has elimination of imaginaries if every model of that theory does (and similarly for uniform elimination).

Examples

 * ZFC set theory has elimination of imaginaries.
 * Peano arithmetic has uniform elimination of imaginaries.
 * A vector space of dimension at least 2 over a finite field with at least 3 elements does not have elimination of imaginaries.