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单词 GraphTheoremsForTopologicalSpaces
释义

graph theorems for topological spaces


We wish to show the relationMathworldPlanetmath between continuous mapsMathworldPlanetmath and their graphs is closer that it may look. Recall, that if f:XY is a function between sets, then the set Γ(f)={(x,f(x))X×Y} is called the graph of f.

PropositionPlanetmathPlanetmath 1. If f:XY is a continuous map between topological spacesMathworldPlanetmath such that Y is HausdorffPlanetmathPlanetmath, then the graph Γ(f) is a closed subset of X×Y in product topology.

Proof. Indeed, we will show, that Z=(X×Y)\\Γ(f) is open. Let (x,y)Z. Then f(x)y and thus (since Y is Hausdorff) there exist open subsetes V1,V2Y such that f(x)V1, yV2 and V1V2=. Since f is continuousMathworldPlanetmath, then U=f-1(V1) is open in X.

Note, that the condition V1V2= implies, that f(U)V2=. Therefore U×V2 is a subset of Z. On the other hand this subset is open (since it is a productPlanetmathPlanetmath of two open sets) in product topology and (x,y)U×V2. This shows, that every point in Z belongs to Z together with a small neighbourhood, which completesPlanetmathPlanetmathPlanetmathPlanetmathPlanetmath the proof.

Unfortunetly, the converseMathworldPlanetmath of this theorem is not true as we will see later. Nevertheless we can achieve similar result, if we assume a bit more about spaces:

Proposition 2. Let f:XY be a function, where X,Y are Hausdorff spaces with Y compactPlanetmathPlanetmath. If Γ(f) is a closed subset of X×Y in product topology, then f is continuous.

Proof. Let FY be a closed set. We will show that f-1(F) is also closed. Consider projections

πY:X×YY;πX:X×YX.

They are both continuous and thus πY-1(F) is closed in X×Y. Since Γ(f) is also closed, then

Z=πY-1(F)Γ(f)

is closed in X×Y. It is well known, that since Y is compact, then πX is a closed map (this is easily seen to be equivalentMathworldPlanetmathPlanetmathPlanetmathPlanetmath to the tube lemma). Furthermore it is easy to see, that πX(Z)=f-1(F) and the proof is complete.

Counterexample. Let denote the set of reals (with standard topology). Consider function f: given by f(x)=1/x and f(0)=0. It is obvious, that f is discontinuousMathworldPlanetmath at x=0, but also it can be easily checked, that Γ(f) is closed in 2. Note, that is not compact.

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更新时间:2025/5/4 15:16:50