Math 2.0

General topology Definition

Topology of pointwise convergence

Builds on Topological spaceProduct topologySubbasisOpen setPointwise convergenceSubspace topology

Definition

Let XX be a set, let YY be a topological space, and let YXY^X be the set of all maps from XX to YY. For x∈Xx \in X and an open set U⊆YU \subseteq Y, put W(x,U)={f∈YX:f(x)∈U}.W(x, U) = \set{f \in Y^X : f(x) \in U}. The topology of pointwise convergence on YXY^X is the topology generated by the subbasis of all sets W(x,U)W(x, U). On a subset F⊆YXF \subseteq Y^X, such as the set of continuous maps when XX is a space, it is the subspace topology.

Remarks

Viewing YXY^X as the product ∏x∈XY\prod_{x \in X} Y, this topology is exactly the product topology. A sequence (fn)(f_n) converges to ff in it if and only if fn(x)→f(x)f_n(x) \to f(x) in YY for every x∈Xx \in X, which is pointwise convergence. The same holds for nets.

Example: on ℝ[0,1]\R^{[0, 1]}, let fn(x)=xnf_n(x) = x^n, and let f(x)=0f(x) = 0 for x<1x < 1 and f(1)=1f(1) = 1. For each fixed xx, the numbers xnx^n tend to f(x)f(x), so fn→ff_n \to f in this topology.

Non-example: the discrete topology on ℝ[0,1]\R^{[0, 1]} is not the topology of pointwise convergence. In the discrete topology, the sequence fnf_n above does not converge, because {f}\set{f} is open and contains no fnf_n.

For an uncountable set XX and Y=ℝY = \R, the space ℝX\R^X with this topology is not first-countable, so it is not metrizable.

Reading path

To understand this page from the ground up, read these 7 pages in order. The foundations come first.

Show the path as a graph
Each page uses the pages above it. The colour of the bar on a box shows how strong the evidence is, and the box also gives it as text. The list has the same pages.
  • no evidence level
  1. Open set concept
  2. Pointwise convergence concept
  3. Product topology concept
  4. Subbasis concept
  5. Subspace topology concept
  6. Topological space concept
  7. Topology of pointwise convergence definition

Further reading

  1. Web https://en.wikipedia.org/wiki/Pointwise_convergence
About this page
TagD8GHJN61MW
URIm2://D8GHJN61MW
Kinddefinition
WritingW0 stub
MSC54C35, 54B10
DefinesTopology of pointwise convergence
UsesTopological space, Product topology, Subbasis, Open set, Pointwise convergence, Subspace topology
Changed2 November 2026

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