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单词 LimitPointsAndClosureForConnectedSets
释义

limit points and closure for connected sets


The below theorem shows that adding limit pointsPlanetmathPlanetmath to a connectedset preserves connectedness.

Theorem 1.

Suppose A is a connected set in a topological spaceMathworldPlanetmath.If ABA¯, then B is connected.In particular, A¯ is connected.

Thus, one way to prove that a space X is connected is to find a densesubspace in X which is connected.

Two touching closed ballsPlanetmathPlanetmath in 2 shows that this theorem does not holdfor the interior. Along the same lines, taking the closureMathworldPlanetmathPlanetmath does notpreserve separatedness.

Proof.

Let X be the ambient topological space.By assumptionPlanetmathPlanetmath, if U,VA are open and UV=A, thenUV.To prove that B is connected, let U,V be open sets inB such that UV=B and for acontradition, suppose that UV=.Then there are open sets R,SX such that

U=RB,V=SB.

It follows that (RS)B=Band (RS)B=.Next, let U~,V~ be open sets in A defined as

U~=RA,V~=SA.

Now

A=BA(RS)AA

and as (RS)A=U~V~, it follows thatU~V~=(RS)A.Then, by the properties of the closure operator,

(RS)A¯(RS)A¯(RS)B=.

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更新时间:2025/5/4 20:44:29