I'm trying to do some Mathematica calculations with spherical harmonics. However, even testing the simple orthogonality relation does not work. code: Integrate[ SphericalHarmonicY[l, m, theta, phi] SphericalHarmonicY[lp, mp, theta, phi] Sin[theta], {theta, 0, Pi}, {phi, 0, 2 Pi}]

adding the assumptions that l, m, lp and mp are integers does not help.