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Continuous functions

  1. Feb 17, 2012 #1
    A function defined on ℝ is continuous at x if given ε, there is a δ such that |f(x)-f(y)|<ε whenever |x-y|<δ. Does this imply that f(x+δ)-f(x)=ε? The definition only deals with open intervals so i am not sure about this. If this is not true could someone please show me a counter example for it?

    Any help would be appreciated. Thanks.
  2. jcsd
  3. Feb 17, 2012 #2
    No. You have to learn to think differently. Draw a lot of pictures and think about limiting processes, not equalities. A real function is continuous at x if I can draw a rectangular box around the point (x, f(x)), shrink the box arbitrarily small, and the function remains in the box. Consider f(x)=1 for all x. It is continuous everywhere, but for any epsilon>0 there is no delta which satisfies your statement. In fact, f(y)-f(x)=0 for all x,y, yet for any epsilon, no matter how small, I can choose delta arbitrarily large.
  4. Feb 17, 2012 #3


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    Another approach is :

    The description can also be interpreted as saying that one can find, for any ε>0, a value of δ>0 every point x in the interval:

    (y-δ,y+δ) on the x-axis

    Is mapped into the interval (f(y)-ε,f(y)+ε )

    on the y-axis.

    Try playing with relatively-simple functions like x2, and see what happens with
    the expression f(x+δ)-f(x), for different values of δ, and how you can choose δ to make the difference be within ε.
  5. Feb 17, 2012 #4
    Thanks for your replies. I understand now.
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