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Functional analysis: Shoe set is not dense in C([a,b])

  1. Apr 28, 2014 #1
    1. The problem statement, all variables and given/known data

    Let [itex] [a,b] \subset \mathbb{R} [/itex] be a compact interval and t0 [itex]\in [a,b] [/itex] fixed. Show that the set [itex] S = {f \in C[a,b] | f(t_0) = 0} [/itex] is not dense in the space [itex]C[a,b][/itex] (with the sup-norm).

    2. Relevant equations

    Dense set: http://en.wikipedia.org/wiki/Dense_set

    sup - norm: http://mathworld.wolfram.com/SupremumNorm.html


    3. The attempt at a solution

    I tried to take function f from S and function g from C[a,b] and calculate the sup-norm of the difference of the functions and make it bigger than some number. However I am not able to do so.. I'm not even sure if my approach is correct here. What should be my strategy?
     
  2. jcsd
  3. Apr 28, 2014 #2

    HallsofIvy

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    For a given number Y, take g(x)= Y+ 1, a constant function. What is d(f, g)?
     
  4. Apr 28, 2014 #3
    Thank you for reply!

    So are you saying that I should take g(t) = t0 + 1 for all t. Then

    [itex]\left\|f - g\right\| = sup_{t \in [a,b]} |f(t) - g(t)| \geq |f(t_0) - g(t)| = |0 - t_0 -1| = |t_0 + 1| [/itex]

    Therefore set S is not dense in C[a,b] ?
     
  5. Apr 28, 2014 #4

    LCKurtz

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    I don't think that is quite what he is saying. And you could accidentally have ##t_0=-1## which would wreck your argument. Why don't you just use a similar argument to show that if ##f(x)
    \equiv 1## that no ##g## in your subset gets close to it in sup norm?
     
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