TheoremBase

Continuity and Uniform Continuity of a Uniform Limit of Real-Valued Maps

A uniform limit of continuous real-valued maps on a subset of a metric space is continuous, and a uniform limit of uniformly continuous ones is uniformly continuous; continuity at one point needs uniform convergence only near that point.

Statement

In the setting of The Real Line: Standing Notation and Background for Calculus on Intervals, let (X,dX)(X,d_{X}) be a metric space, let A⊆XA\subseteq X, let (fk)k∈N(f_{k})_{k\in\mathbb{N}} be a sequence in the set RA\mathbb{R}^{A} of maps from AA to R\mathbb{R} of Sets and Maps: Ordinary Notation (v), and let f:A→Rf:A\to\mathbb{R}. For maps from AA to R\mathbb{R}, continuity at a point relative to AA and continuity on AA are those of Continuous Map Between Metric Spaces, and uniform continuity on AA that of Uniformly Continuous Map Between Metric Spaces, formed with dXd_{X} and the distance dRd_{\mathbb{R}} of Metric Spaces: Balls, the Metric Topology, and the Real and Complex Numbers as Metric Spaces (iv). Pointwise convergence and uniform convergence on a subset of AA are those of Pointwise and Uniform Convergence of a Sequence of Real-Valued Maps, read with D=E=AD=E=A.

Let T⊆S⊆AT\subseteq S\subseteq A and suppose that (fk)k∈N(f_{k})_{k\in\mathbb{N}} converges uniformly to ff on SS. Then (fk)k∈N(f_{k})_{k\in\mathbb{N}} converges uniformly to ff on TT, and converges pointwise to ff on SS.

Let x∈Ax\in A, let rr be a real number with r>0r>0, and let S⊆AS\subseteq A contain every point of the open ball BdX(x,r)B_{d_{X}}(x,r) that lies in AA. Suppose that (fk)k∈N(f_{k})_{k\in\mathbb{N}} converges uniformly to ff on SS and that fkf_{k} is continuous at xx relative to AA for every k∈Nk\in\mathbb{N}. Then ff is continuous at xx relative to AA.

Suppose that (fk)k∈N(f_{k})_{k\in\mathbb{N}} converges uniformly to ff on AA and that fkf_{k} is continuous on AA for every k∈Nk\in\mathbb{N}. Then ff is continuous on AA.

Suppose that (fk)k∈N(f_{k})_{k\in\mathbb{N}} converges uniformly to ff on AA and that fkf_{k} is uniformly continuous on AA for every k∈Nk\in\mathbb{N}. Then ff is uniformly continuous on AA.

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