Re: [微積] 函數的連續性
f is continuous <=> preimage of an open set is open
<=> preimage of a closed set is closed
choose any subinterval J with irrational end points, then
f^{-1}(J) is open and closed, so it is either empty or R.
So obviously f is a constant.
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※ 編輯: JASS0213 來自: 140.247.0.119 (09/30 05:13)
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