for clearly defining the concept of uniform convergence and fully investigating its implications. Compactness Compactness is a concept from general topology that plays an important role in many of the theorems of real analysis. The property of compactness is a generalization of the notion of a set being closed and bounded. (In the context of real analysis, these notions are equivalent: a set in Euclidean space is compact if and only if it is closed and bounded.) Briefly, a closed set contains all of its boundary points, while a set is bounded if there exists a real number such that the distance between any two points of the set is less than that number. In , sets that are closed and bounded, and therefore compact, include the empty set, any finite number of points, closed intervals, and their finite unions. However, this list is not exhaustive; for instance, the set is a compact set; the Cantor ternary set is another example of a compact set. On the other hand, the set is not compact because it is bounded but not closed, as the boundary point 0 is not a member of the set. The set is also not compact because it is closed but not bounded. For subsets of the real numbers, there are several equivalent definitions of compactness. Definition. A set is compact if it is closed and bounded. This definition also holds for Euclidean space of any finite dimension, , but it is not valid for metric spaces in general. The equivalence of the definition with the definition of compactness based on subcovers, given later in this section, is known as the Heine-Borel theorem. A more general definition that applies to all metric spaces uses the notion of a subsequence (see above). Definition. A set in a metric space is compact if every sequence in has a convergent subsequence. This particular property is known as subsequential compactness. In , a set is subsequentially compact if and only if it is closed and bounded, making this definition equivalent to the one given above. Subsequential compactness is equivalent to the definition of compactness based on subcovers for metric spaces, but not for topological spaces in general. The most general definition of compactness relies on the notion of open covers and subcovers, which is applicable to topological spaces (and thus to metric spaces and as special cases). In brief, a collection