In mathematical logic, a definable set is an nary relation on the domain of a structure whose elements are precisely those elements satisfying some formula in the language of that structure. A set can be defined with or without parameters, which are elements of the domain that can be referenced in the formula defining the relation.
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Let be a firstorder language, an structure with domain M, X a fixed subset of , and m a natural number. Then:
Let be the structure consisting of the natural numbers with the usual ordering. Then every natural is definable in without parameters. The number 0 is defined by the formula stating that there exist no elements less than x:
and a natural n > 0 is defined by the formula stating there exist exactly n elements less than x:
In contrast, one cannot define any specific integer without parameters in the structure consisting of the integers with the usual ordering (see the section on automorphisms below).
Let be the firstorder structure consisting of the natural numbers and their usual arithmetic operations and order relation. The sets definable in this structure are known as the arithmetical sets, and are classified in the arithmetical hierarchy. If the structure is considered in secondorder logic instead of firstorder logic, the definable sets of natural numbers in the resulting structure are classified in the analytical hierarchy. These hierarchies reveal many relationships between definability in this structure and computability theory, and are also of interest in descriptive set theory.
Let be the structure consisting of the field of real numbers. Although the usual ordering relation is not directly included in the structure, there is a formula that defines the set of nonnegative reals, since these are the only reals that possess square roots:
Thus any is nonnegative if and only if . In conjunction with a formula that defines the additive inverse of a real number in , one can use to define the usual ordering in : for , set if and only if b − a is nonnegative. The enlarged structure s is called a definitional extension of the original structure. It has the same expressive power as the original structure, in the sense that a set is definable over the enlarged structure from a set of parameters if and only if it is definable over the original structure from that same set of parameters.
The theory of has quantifier elimination. Thus the definable sets are Boolean combinations of solutions to polynomial equalities and inequalities; these are called semialgebraic sets. Generalizing this property of the real line leads to the study of ominimality.
An important result about definable sets is that they are preserved under automorphisms.
This result can sometimes be used to classify the definable subsets of a given structure. For example, in the case of above, any translation of is an automorphism preserving the empty set of parameters, and thus it is impossible to define any particular integer in this structure without parameters in . In fact, since any two integers are carried to each other by a translation and its inverse, the only sets of integers definable in without parameters are the empty set and itself.
The Tarski–Vaught test is used to characterize the elementary substructures of a given structure.
