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A very powerful tool for showing compact convergence is the Arzelà–Ascoli theorem. There are several versions of this theorem, roughly speaking it states that every sequence of equicontinuous and uniformly bounded maps has a subsequence that converges compactly to some continuous map.
A sequence of functions () converges uniformly to when for arbitrary small there is an index such that the graph of is in the -tube around f whenever . The limit of a sequence of continuous functions does not have to be continuous: the sequence of functions () = (marked in green and blue) converges pointwise over the entire domain, but the limit function is discontinuous (marked in red).
If u n (y) is a Cauchy sequence for any particular value of y, then the Harnack inequality applied to the harmonic function u m − u n implies, for an arbitrary compact set D containing y, that sup D |u m − u n | is arbitrarily small for sufficiently large m and n. This is exactly the definition of uniform convergence on compact sets.
If the domain of the functions is a topological space and the codomain is a uniform space, local uniform convergence (i.e. uniform convergence on a neighborhood of each point) and compact (uniform) convergence (i.e. uniform convergence on all compact subsets) may be defined. "Compact convergence" is always short for "compact uniform convergence ...
If the codomain of the functions under consideration has a uniform structure or a metric structure then the compact-open topology is the "topology of uniform convergence on compact sets." That is to say, a sequence of functions converges in the compact-open topology precisely when it converges uniformly on every compact subset of the domain. [2]
Hurwitz's theorem is used in the proof of the Riemann mapping theorem, [2] and also has the following two corollaries as an immediate consequence: . Let G be a connected, open set and {f n} a sequence of holomorphic functions which converge uniformly on compact subsets of G to a holomorphic function f.
By letting be the set of all convex balanced weakly compact subsets of , ′ will have the Mackey topology on ′ or the topology of uniform convergence on convex balanced weakly compact sets, which is denoted by (′,) and ′ with this topology is denoted by (′,) ′.
Local uniform convergence (i.e. uniform convergence on a neighborhood of each point) Compact (uniform) convergence (i.e. uniform convergence on all compact subsets) further instances of this pattern below. Implications: - "Global" modes of convergence imply the corresponding "local" and "compact" modes of convergence. E.g.: