Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Denseness of bounded funtions in L^2?

  1. Oct 19, 2013 #1
    Let [itex]C_b^\infty(\mathbb{R}^n)[/itex] be the space of infinitely differentiable functions f, such that f and all its partial derivatives are bounded.

    Is [itex]C_b^\infty(\mathbb{R}^n)[/itex] dense in [itex]L^2(\mathbb{R}^n)[/itex]? I think the answer is yes, because [itex]C_b^\infty(\mathbb{R}^n)[/itex] contains [itex]C_0^\infty(\mathbb{R}^n)[/itex], the space of all infinitely differentiable functions with compact support, as a subset. And it's well known that [itex]C_0^\infty(\mathbb{R}^n)[/itex] is dense in [itex]L^p(\mathbb{R}^n)[/itex].

    However, there appear to be functions in [itex]C_b^\infty(\mathbb{R}^n)[/itex] but are not in [itex]L^2(\mathbb{R}^n)[/itex], for example the function f(x)=1. So this means that instead, we have [itex]C_b^\infty(\mathbb{R}^n)\cap L^2(\mathbb{R}^n)[/itex] dense in [itex]L^2(\mathbb{R}^n)[/itex]?
     
  2. jcsd
  3. Oct 19, 2013 #2

    mathman

    User Avatar
    Science Advisor
    Gold Member

    You seem to have answered your own question.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Denseness of bounded funtions in L^2?
Loading...