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

Homework Help: Ramp function, Dirac delta function and distributions

  1. Jun 13, 2010 #1
    [tex]r(x) = x[/tex] if [tex]x \geq 0[/tex] and [tex]r(x) = 0 [/tex] if [tex] x<0[/tex]

    I have to show that:

    1-[tex]\[ \int_{- \infty}^{+ \infty} r(x) \varphi ''(x) dx = \varphi(0) \][/tex]

    And 2- that the second derivative of r is the Dirac delta.

    And I managed to do this by integrating by parts. Howver, I don't get why I can't just write:

    [tex]\[ \int_{- \infty}^{+ \infty} r''(x) \varphi (x) dx = \varphi(0) \][/tex]

    Wouldn't that be correct considering distributions (I actually used this to show the second point)? I guess my question is, why can't I write the second derivatives of the ramp function (the derivative of the Heaviside function) simply as

    [tex]r(x) = 0[/tex] if [tex]x \geq 0[/tex] and [tex]r(x) = 0 [/tex] if [tex] x<0[/tex]

    i.e. 0

    Which would make the integral = 0

    Does it only have a second derivative in the distribution sense? Why?

    EDIT: I don't get why my message is being displayed like this...
    Last edited: Jun 14, 2010
  2. jcsd
  3. Jun 13, 2010 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Education Advisor

    Change the backslash in your closing tex tags to a forward slash, i.e. /tex instead of \tex.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook