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Math Help - [Continuity Problem] Is f(x) which is continuous at x=a necessarily...

  1. #1
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    [Continuity Problem] Is f(x) which is continuous at x=a necessarily...

    First thanks for taking time to view my question.

    Is f(x) which is continuous at x=A necessarily continuous at a certain neighborhood of A?

    I am quite confused since many people say yes. But if f(x) is also continuous at say, B, which is in the neighborhood of A, then this process would be infinite, and so f(x) should be continuous everywhere.

    Sorry for my English. I really need your help.
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  2. #2
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    Quote Originally Posted by kevin_chn View Post
    Is f(x) which is continuous at x=A necessarily continuous at a certain neighborhood of A?
    The answer is No. The standard counterexample is the function defined by f(x) = \begin{cases}x&\text{if }x\text{ is rational,}\\0&\text{if }x\text{ is irrational.}\end{cases}
    This function is continuous at 0 but nowhere else.
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    A simpler example f(x):\{ 0 \} \mapsto \mathbb{R} defined as f(0)=0.
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