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Continuity of functions

  1. Apr 1, 2008 #1
    Hey guys, I'm in a little bit of a jam here:
    I managed to miss a really important lecture on continuity the other day, and there were a few examples that the professor provided to the class that I just got, but would love it if someone could explain them to me.

    First, f(x)=x3 is continuous on all of R. Everyone went through this, but I'm not really sure how it works.

    Second, we used the fact that "a function [tex]f:A \rightarrow[/tex] R fails to be uniformly continuous on A if and only if there exists a particular [tex]\epsilon_{0} > 0[/tex] and two sequences (xn) and (yn) in A satisfying

    [tex]\left| x_n - y_n\right| \rightarrow 0 [/tex] but [tex]\left| f(x_n) - f(y_n)\right| \rightarrow \epsilon_0 [/tex]

    to show fis not uniformly continuous on R.

    Thirdly, we showed that f is uniformly continuous on any bounded subset of R
  2. jcsd
  3. Apr 1, 2008 #2
    I studied continuity in the last semester. I can't remember all the theorems ATM.
    Any way, as far as I can remember that a function, in plain words, can be continuous
    1) at a point or,
    2) can be continuous on a certain interval.

    For continuity at a point it must have a value at that point
    For an interval, it must be continuous at every points in the interval.

    I can remember upto this.
  4. Apr 1, 2008 #3
    For the first quesiton, you can use the contunuity of limit. As I remember ıt is something like that. lim(x --> a) f(x) = f(a). and for f(x^3), this statement is correct.
  5. Apr 1, 2008 #4
    This is incorrect.

    One counter example is the following function

    Let [tex]f(x)=\left\{\begin{array}{cc}0,&\mbox{ if }
    x\neq 0\\1, & \mbox{ if } x=0\end{array}\right.[/tex]

    This function has a value at x = 0, but is certainly not continuous at that point.

    From Wiki,
    To be more precise, we say that the function f is continuous at some point c when the following two requirements are satisfied:

    f(c) must be defined (i.e. c must be an element of the domain of f).

    The limit of f(x) as x approaches c must exist and be equal to f(c).
    Last edited: Apr 1, 2008
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