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Reference.org
Definable set
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<p>In <a href="/facts/Mathematical_logic/SaI7Y3wN">mathematical logic</a>, a definable set is an <i>n</i>-ary <a href="/facts/Relation_(mathematics)/HjblDaMy">relation</a> on the <a href="/facts/Structure_(mathematical_logic)/GsMx1fEb">domain</a> of a <a href="/facts/Structure_(mathematical_logic)/GsMx1fEb">structure</a> whose elements satisfy some <a href="/facts/Formula_(mathematical_logic)/YpTVlzuj">formula</a> in the <a href="/facts/First-order_language/lcISqWIg">first-order language</a> of that structure. A <a href="/facts/Set_(mathematics)/BfucfHMq">set</a> can be defined with or without parameters, which are elements of the domain that can be referenced in the formula defining the relation. </p>