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

Existence of integrals:

  1. Sep 6, 2006 #1
    Let be the integrals:

    [tex] \int_{-\infty}^{\infty}dx Cos(uf(x)) [/tex] (or the same but a sine) and

    [tex] \int_{-\infty}^{\infty}dxe^{-ag(x)} [/tex]

    Where "a" is a a>0 positive constant, u can be either positive or negative.. and g(x)>0 for every real x.. my question is will these integrals "always2 exist under these conditions?..what would happen if we take the limit a-->oo and u-->oo ? are in this case equal to 0?
  2. jcsd
  3. Sep 6, 2006 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    For the second case, set g(x)=1 (for all x). What do you see?
  4. Sep 6, 2006 #3
    Oh..then sorry "Arildno".. then for similar case with f(x) and g(x) different from f(x)=C (C a real constant) and g(x)>0 i think the integrals should tend to 0 for big u and a, and that they exist..for example for the WKB approach in Physics if you definte the "Action" S of the system the approximate wave function can be written as [tex] \psi(x)= ACos(S(x)/\hbar) [/tex] so in the "Semi-classical " limit (h-->0 ) the Wave function is 0 , A is a normalization constant.

    - for the case of real exponential if you take g(x)=h(x) so h(x) is the "inverse" of g(x) the function becomes a "Laplace transform" of h'(x) that tends to 0 for big a
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Existence of integrals:
  1. Integral of x^x. exists? (Replies: 16)