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§ Motivation for the Compact-open Topology

created 2021-10-31
  • If XXX is a compact space and YYY is a metric space, consider two functions f,g:X→Yf, g: X \to Yf,g:X→Y.
  • We can define a distance d(f,g)≡min⁡x∈Xd(f(x),g(x))d(f, g) \equiv \min_{x \in X} d(f(x), g(x))d(f,g)≡minx∈X​d(f(x),g(x)).
  • The min⁡x∈X\min_{x \in X}minx∈X​ has a maximum because XXX is compact.
  • Thus this is a real metric on the function space Map(X,Y)Map(X, Y)Map(X,Y).
  • Now suppose YYY is no longer a metric space, but is Haussdorf. Can we still define a topology on Map(X,Y)Map(X, Y)Map(X,Y)?
  • Let K⊆XK \subseteq XK⊆X be compact, and let U⊆YU \subseteq YU⊆Y be open such that f(K)⊆Uf(K) \subseteq Uf(K)⊆U.
  • Since YYY is Hausdorff, K⊆XK \subseteq XK⊆X
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