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 be a morphism of finite presentations 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 variety (the hyperbola) has image under projecting to the x-axis of the x-axis minus the origin: this is constructible but not a variety, and neither open nor closed in the plane.
In a topological space, every constructible set contains a dense open subset of its closure.
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