Constructible set (topology)

In topology, a constructible set in a topological space is a finite union of locally closed sets. (A set is locally closed if it is the intersection of an open set and closed set, or equivalently, if it is open in its closure.) Constructible sets form a Boolean algebra (i.e., it is closed under finite union and complementation.) In fact, the constructible sets are precisely the Boolean algebra generated by open sets and closed sets; hence, the name "constructible". The notion appears in classical algebraic geometry.

Chevalley's theorem (EGA IV, 1.8.4.) states: Let f : X → Y be a morphism of finite presentation of schemes. Then the image of any constructible set under f is constructible. In particular, the image of a variety need not be a variety, but is (under the assumptions) always a constructible set. For example, the map A 2 → A 2 that sends ( x , y ) to ( x , x y ) has image the set { x ≠ 0 } ∪ { x = y = 0 } , which is not a variety, but is constructible.

In a topological space, every constructible set contains a dense open subset of its closure.