text: All exact differential forms can be described by this word, since their second exterior derivative is zero.
full_question: All exact differential forms can be described by this word, since their second exterior derivative is zero. A setis Hausdorff if and only if the diagonal can be described by this word. The union of a set and its boundaryalways has this property. Arbitrary intersections, but not arbitrary unions, of sets of this type produce anotherset of this type. A topological set is described by this word if it contains all of its limit points. If an operationapplied to two members of a set always remains in that set, the set is said to have this property with respect tothat operation. For 10 points, give this word which describes intervals that contain both of their endpoints. 
first_sentence: All exact differential forms can be described by this word, since their second exterior derivative is zero.
raw_answer: closed [or word forms, such as closure]