# Is this the correct way to solve this QM integral problem?

1. Apr 3, 2016

### Azruine

< Mentor Note -- thread moved to HH from the technical physics forums, so no HH Template is shown >

Problem is:

If the behavior of ψ( r,t ) as r->inf is dominated by r-n, what values can n assume if the integral
A(ψ*∇ψ-ψ∇ψ*)⋅nda
taken over the surface at infinity is to vanish.

I considered ψ as ar-n calculate like below
ψ*∇ψ≈ar-n⋅a*(-nr-n-1)=-naa*r-2n-1
ψ∇ψ*≈a*r-n⋅a(-nr-n-1)=-naa*r-2n-1
So... ψ*∇ψ-ψ∇ψ*=0 at anywhere. Thus, n does not affect to integration.

Well, this result is so ridiculous :/

2. Apr 3, 2016

### TSny

Welcome to PF!

Suppose $\psi(x) = \Large{\frac{e^{ikr}}{r^n}}$. Would this be considered a function that is dominated by r-n? (I think so, but I don't know the precise definition of "dominated by".)

3. Apr 3, 2016

### Azruine

I just tried and got the following result
$\psi^{*}\nabla\psi - \psi\nabla\psi^{*} = \Large{\frac{2ki}{r^{2n}}}$
So, now $n$ must be larger than 0. Quite acceptable result :)

4. Apr 3, 2016

### TSny

Are there any factors of r in the area element da?

5. Apr 3, 2016

### Azruine

Oh. r^2 dependency... So n>1

OMG I've submitted my homework lol

Last edited: Apr 3, 2016