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March 30, 2017

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Suppose that a<b are extended real numbers and that f is differentiable on (a,b). If f' is bound on (a,b) prove f is uniformly continuous.

  • math - ,

    We know that (f(y)-f(x))/(y-x) = f'(a-x,y) for all x < y in the domain
    where a is in (x,y), from the Mean Value Theorem.
    So if we assume that |f'| is boudned by M, then
    (f(y) - f(x))/(y-x) <= M for all x < y.

    Or |f(x)-f(y)| <= M*|x-y|.

    Now if we are given e>0, we can choose d = e/M, and if
    |x-y| < d, then |f(x)-f(y)| < e, as required.

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