Interesting integral property

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Discussion Overview

The discussion revolves around an integral property related to the Schrödinger equation and its Fourier transform. Participants explore the implications of a specific integral involving a function and its derivatives, seeking clarification and assistance in understanding its application in quantum mechanics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant presents an integral property: ∫g du = c ∫g d²u, where the integral is from -infinity to positive infinity, and relates it to solving the Schrödinger equation.
  • Another participant questions the notation "d²u" and seeks clarification on its meaning within the context of the integral.
  • A participant explains that "d²u" refers to the second derivative of u, indicating that the constant c arises from integrating the function g.
  • Concerns are raised about the validity of the proposed property, with a participant suggesting that it may not hold for all functions, using u = x as a counterexample.
  • The original poster clarifies that the property is specific to their integral involving g = e^(ikx) and an unknown function u, which approaches zero at the boundaries of integration.
  • There is a mention of the hope that this property might simplify the problem related to Fourier transforms in the context of the Schrödinger equation.

Areas of Agreement / Disagreement

Participants express differing views on the validity of the integral property, with some questioning its general applicability while others maintain it is specific to the context of the discussion. The discussion remains unresolved regarding the broader implications of the property.

Contextual Notes

The discussion includes assumptions about the behavior of the function u at infinity and the conditions under which the integral property holds. There are unresolved questions about the generality of the property and its dependence on the specific forms of g and u.

CPL.Luke
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so I'm having trouble with an integral. However I noticed an interesting property to it.int{gdu}=c*int{gd^2u}

with the integral being from -infinity to positive infinity.

The problem arose while I was trying to solve the schroedinger equation in the presence of a potential, I'm trying to solve it through the use of a Fourier transform, but in order to do that I need to know the Fourier transform of the potential.

can anybody lend a hand?
 
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I don't understand what you mean by "d^2 u" inside the integral.
 
I found the property from integration by parts, the d^2u represents the differential of du.techincally the c came out of the g function when I integrated it, however I don't know whether or not it can be used to develop other properties of the integral.

if it helps at all g is equal to e^ikx and u is an unknown function which is the product of some known potential, and an unknown function psi.
 
The differential of du is zero. Did you mean something like u''(x) dx?
 
yes exactly
 
I'd be interested in knowing HOW you got that.

You are saying
[tex]\int_{-\infty}^\infty g(x)u'(x)dx= c \int_{-\infty}^\infty g(x) u"(x)dx[/tex]
for any g(x) and u(x)? I doubt that is true. In particular, if you take u= x, that turns into
[tex]\int_{-\infty}^\infty g(x)dx= 0[/tex]
 
no, as I said it was a property of a specific integral I was doing, where u is an unknown function, and g is equal to e^ikx, however running it through integration by parts I was able to deduce that that property exhisted, (because my function u goes to zero at -infinity and positive infinity, which is known because of other known properies of u)

the integral came up as I was experimenting with Fourier transforms of the schroedinger equation, I'm hoping that there may be some trick that would simplify the problem.
 

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