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Compact convergence

In mathematics compact convergence (or uniform convergence on compact sets) is a type of convergence which generalizes the idea of uniform convergence. It is associated with the compact-open topology.

Contents

  • Definition 1
  • Examples 2
  • Properties 3
  • See also 4
  • References 5

Definition

Let (X, \mathcal{T}) be a topological space and (Y,d_{Y}) be a metric space. A sequence of functions

f_{n} : X \to Y, n \in \mathbb{N},

is said to converge compactly as n \to \infty to some function f : X \to Y if, for every compact set K \subseteq X,

(f_{n})|_{K} \to f|_{K}

converges uniformly on K as n \to \infty. This means that for all compact K \subseteq X,

\lim_{n \to \infty} \sup_{x \in K} d_{Y} \left( f_{n} (x), f(x) \right) = 0.

Examples

  • If X = (0, 1) \subset \mathbb{R} and Y = \mathbb{R} with their usual topologies, with f_{n} (x) := x^{n}, then f_{n} converges compactly to the constant function with value 0, but not uniformly.
  • If X=(0,1], Y=\R and f_n(x)=x^n, then f_n converges pointwise to the function that is zero on (0,1) and one at 1, but the sequence does not converge compactly.
  • 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 which converges compactly to some continuous map.

Properties

  • If f_{n} \to f uniformly, then f_{n} \to f compactly.
  • If (X, \mathcal{T}) is a compact space and f_{n} \to f compactly, then f_{n} \to f uniformly.
  • If (X, \mathcal{T}) is locally compact, then f_{n} \to f compactly if and only if f_{n} \to f locally uniformly.
  • If (X, \mathcal{T}) is a compactly generated space, f_n\to f compactly, and each f_n is continuous, then f is continuous.

See also

References

  • R. Remmert Theory of complex functions (1991 Springer) p. 95
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