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单词 LimitPointsOfUncountableSubsetsOfRn
释义

limit points of uncountable subsets of R^n


PropositionPlanetmathPlanetmath. Let n be an n-dimensional, real normed space and let An. If A is uncountable, then there exists limit pointPlanetmathPlanetmath of A in n.

Proof. For any k let

𝔹k={vn|||v||k},

i.e. 𝔹k is a closed ballPlanetmathPlanetmath centered in 0 with radius k. Assume, that for any k the set

Vk=𝔹kA

is finite. Then Vk=A would be at most countableMathworldPlanetmath. ContradictionMathworldPlanetmathPlanetmath, since A is uncountable. Thus, there exists k0 such that Vk0 is infiniteMathworldPlanetmath. But Vk0𝔹k0 and since 𝔹k0 is compactPlanetmathPlanetmath (and Vk0 is infinite), then there exists limit point of Vk0 in n. This completesPlanetmathPlanetmathPlanetmathPlanetmathPlanetmath the proof.

Corollary. If An is uncountable, then there exist infinitely many limit points of A in n.

Proof. Assume, that there are finitely many limit points of A, namely x1,,xkn. For ε>0 define

Aε={vn|i||v-xi||>ε}.

Briefly speaking, Aε is a complementPlanetmathPlanetmath of a union of closed balls centered at xi with radii ε. Of course Aε since there are finitely many limit points. Let

Vε=AAε.

Assume, that Vε is countable for every ε. Then

AnV1n{x1,,xk}

would be at most countable (of course under assumptionPlanetmathPlanetmath of Axiom of ChoiceMathworldPlanetmath). Contradiction. Thus, there is γ>0 such that Vγ is uncountable. Then (due to proposition) there is a limit point xn of Vγ. Note, that

xVγ¯Vγ

for some 0<γ<γ. Thus x is different from any xi. Contradiction, since x is also a limit point of A.

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