# Homework Help: Integration involving spherical harmonics

1. Dec 7, 2011

### lordkelvin

1. The problem statement, all variables and given/known data

Evaluate the integral ∫∫dΩ V(Ω)Yml(Ω) for V(Ω) = +V0 for 0<θ<π/2 ; -V0 for π/2<θ<π

2. Relevant equations
I was hoping to apply the orthonormality properties of the spherical harmonics but this is a little more difficult since the integral breaks into two integrals over half spheres instead of one integral over a full sphere.

I guess it may be useful to rewrite the V(Ω) as ±√(4π)Y00(Ω)V0 in order to invoke the orthogonality properties of the spherical harmonics when integrated on a sphere.

3. The attempt at a solution

Basically I've split the integral into two integrals over the upper and lower halves of the sphere. Now what? Surely I'm not to use the definition of the spherical harmonics; there must be a cleaner way to do this by invoking the properties of Ylm(omega). There is nothing I can say about ∫∫half of spheredΩYlm(Y00)* as far as I know. Is there some way to combine the two integrals again so that I can use the orthonormality property?

2. Dec 8, 2011

### vela

Staff Emeritus
Orthonormality won't really help here because you're not integrating the product of two spherical harmonics. The integral should be pretty straightforward though. Start by integrating φ out first. Then you'll be left with an integral involving a Legendre polynomial.

3. Dec 8, 2011

### lordkelvin

I get zero. I think this is so since Ylm(θ,$\phi$) = const* e^(i*$\phi$)*Plm(cos($\theta$) and so the integral over phi yields zero.

4. Dec 8, 2011

### vela

Staff Emeritus
It's $e^{im\phi}$, not $e^{i\phi}$. It makes a difference.

Last edited: Dec 8, 2011